Journal of Ecology 20191ndash13 wileyonlinelibrarycomjournaljec emsp|emsp1copy 2019 The Authors Journal of Ecology copy 2019 British Ecological Society

vReceived29October2018emsp |emsp Accepted18January2019DOI 1011111365-274513140

R E S E A R C H A R T I C L E

A thorny issue Woody plant defence and growth in an East African savanna

Benjamin J Wigley12 emsp| Corli Coetsee23emsp| David J Augustine4emsp| Jayashree Ratnam1emsp| Dawood Hattas5emsp| Mahesh Sankaran16

1National Centre for Biological Sciences Tata Institute of Fundamental Research Bangalore India 2School of Natural Resource Management Nelson Mandela UniversityGeorgeSouthAfrica3ScientificServicesSkukuzaSouthAfrica4RangelandResourcesResearchUnitUSDAAgriculturalResearchServiceFort Collins Colorado 5DepartmentofBiologicalSciencesUniversityofCapeTownCapeTownSouthAfricaand6Faculty of Biological Sciences School of Biology University of Leeds Leeds UK

CorrespondenceBenjaminJWigleyEmail benwigleygmailcom

Funding informationNationalGeographicSocietyGrantAwardNumber982815NERCGrantAwardNumber NE-E017436-1

Handling Editor Giselda Durigan

Abstract1 Recent work suggests that savanna woody plant species utilise two different

strategies based on their defences against herbivory a low nutrienthigh chemical defencestrategyandanutritionpairedwithmostlyarchitecturaldefencesstrat-egyTheconceptthatchemicalandstructuraldefencescanaugmenteachotherand do not necessarily trade-off has emanated from this work In this study we examinewoodyplantdefencestrategieshowtheserespondtoherbivoreremovalandhowtheyaffectplantgrowthinanEastAfricansavanna

2 At three paired long‐term exclosure siteswith high browser andmixed‐feederdensitiesatMpalaRanchKenyaweinvestigated(a)whetherdefencesemployedby the dominant fine‐ and broad‐leavedwoody savanna species form defencestrategiesandifthesealignwithpreviouslyproposedstrategies(b)howninekeyplantdefencetraitsrespondtoherbivoreremovaland(c)howeffectivethediffer-entdefencestrategiesareatprotectingagainstintenseherbivory(bymeasuringplantgrowthwithandwithoutherbivorespresent)

3 WeidentifiedthreedefencestrategiesWefoundagroup(a)withhighNshortspinesandhighN‐freesecondarymetabolitesagroup(b)withhighNlongspinesandlowN‐freesecondarymetabolitesandagroup(c)withmoderateNnospinesand low N‐free secondary metabolites (most likely defended by unmeasuredchemicaldefences)Structuraldefences(spinelengthbranching)weregenerallyfoundtobeinducedbyherbivoryleafavailableNincreasedordidnotrespondandN‐free secondarymetabolites decreased or did not respond to herbivorySpecieswithlongspinescombinedwithincreasedldquocaginessrdquo(densecanopyarchi-tecturearisingfromcomplexarrangementofnumerouswoodyandspinyaxiscat-egories) of branches maintained the highest growth under intense browsingcomparedtospecieswithshortspinesandhighN‐freesecondarymetabolitesandspecieswithnospinesandlowN‐freesecondarymetabolites

4 SynthesisAtourstudysitestructuraltraits(iespinesincreasedcaginess)werethe most inducible and effective defences against intense mammalian herbivory

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1emsp |emspINTRODUC TION

Along‐standingconundruminecology ishowplantspersist in theface of intense herbivory when resources are limited Plants growing inAfricansavannashavealonghistoryofco‐evolutionwithadiversearray of mammalian herbivores and have evolved a range of different strategies todealwithherbivory (Charles‐Dominiqueet al 2016)Classical defence theory suggests that plants can respond to her-bivoryintwowaysieeithertolerateorresistit(HermsampMattson1992Nuacutentildeez‐FarfaacutenFornoniampValverde2007StraussampAgrawal1999)Defencetheoriesmakepredictionsastohowplantsallocateresourcestotraitsthatconfertoleranceversusresistancedepend-ingontheenvironmentalconditionsandavailableresourcesWhilemuchworkhasfocussedongeneratingtheoriesthatpredictalloca-tiontodefence‐relatedplantsecondarymetabolites(egseeStamp2003forareviewofthedifferenttheories)itislessclearunderwhatconditions (ie resource and herbivory levels) plants should investinstructuraldefencesandmorespecificallyinspinesasnoneoftheexistingdefencetheoriesareabletoclearlypredicttheincidenceofspinescence (Grubb 1992Hanley Lamont Fairbanks amp Rafferty2007HermsampMattson1992Tomlinsonetal2016)

More recently studiesofplant investments in traits related toherbivore tolerance or resistance have increasingly recognised that defencemustbeconsideredintermsofco‐adaptedtraitcomplexesie ldquosyndromesrdquo or ldquostrategiesrdquo rather than simple trade‐offs be-tween allocation to growth versus defence (Agrawal amp Fishbein2006 Barton 2016 Cornelissen et al 1998 Koricheva Nykaumlnen ampGianoli 2004Moleset al 2013Readet al 2008 StewardampKeeler 1988 Twigg amp Socha 1996) For example Da Silva andBatalha(2011)categorisedplantsgrowinginSouthAmericansavan-nas into two defence syndromes a low nutrienthigh chemical de-fencesyndrome(lownitrogenlowspecificleafareaandpresenceofsecondarymetabolites)andanutritionanddefencesyndrome(highnitrogenthickerleavesandhigherdensityoftrichomes)Tomlinsonetal (2016)foundthatratherthantrade‐offsbetweentraitsde-fencesofsavannajuveniletreespeciescouldbegroupedintotwostrategies a low nutrienthigh chemical defence strategy that may includelownutrientcontentphysicaltoughnessandleafchemicals(eitherdigestionretardantssuchastanninsorpoisonssuchasalka-loids)thatmakeplantsunattractivetoherbivoresandastructuralor architectural defence strategywhichmay include spinescencebranchingandpubescenceLikewiseinastudyofsouthernAfricansavannas spanning a range of resource levels Wigley Fritz and

Coetsee (2018)founda lownutrienthighchemicaldefencestrat-egy with low N and high N-free secondary metabolites levels as well as a nutrition and defence strategy which included variable combi-nations of chemical and structural defences

Whiletheclassificationofsavannatreesintothesetwocatego-riesmdashldquolownutrienthighchemicaldefencerdquoandldquonutritioncombinedwitharchitecturaldefencerdquomdashprovidesafirstapproximationoftheiroverall defence strategies defence strategies in savanna trees are likely to be more nuanced than these broad categories suggest Previous studies have often found several ldquoclustersrdquo or ldquogroupsrdquowithinoverarchingstrategiesparticularlyforthenutritionandde-fencestrategy(AgrawalampFishbein2006DaSilvaampBatalha2011Wigleyetal2018)ForexampleWigleyetal(2018)proposedthatthenutritionanddefencestrategyinsouthernAfricansavannatreescanbesubdividedintotwogroupsastrategythatincludeshighleafN and high structural defence investment but low investment in N-free secondary metabolites as well as an additional strategy that in-cludesplantswithhighleafNandahighincidenceofbothstructuraldefencesandN‐freesecondarymetabolitesAgrawalandFishbein(2006)similarlyfoundtwoclusterswithinthenutritionanddefencesyndromeinonespeciesweredefendedthroughacombinationoflatex and trichomes and in the other through cardenolides

There are good reasons why multiple traits should evolve asstrategies or syndromes including the need for plants to defendthemselves against a wide range of herbivores (Agrawal 2011)However there are only a few studies that have investigated how plant physical and chemical defences that comprise these strate-giesorsyndromesareinducedorrelaxedinresponsetoherbivorepresenceorexclusion(iedoplantsinducedifferentdefencetraitssimilarlyinresponsetoherbivoryorarechangesgreaterforonevstheother)ultimatelylimitingourunderstandingoftheldquosyndromerdquoconcept(Barton2016)Furthermoreitisverydifficulttoevaluatehowdifferent defence strategies affect plant performance as thistypicallyrequireslong‐termherbivoreexclusionexperiments

For syndromes to manifest plants should be able to employdifferentcombinationsofphysicalandchemicaldefences(Barton2016) Plant defence theories predict that slow‐growing plants(whichareusuallyassociatedwithlimitedresources)shouldinvestheavily in defences andmore specifically in lowmaintenance de-fencessuchastanninsandlignin(BryantChapinIIIampKlein1983Coley1988ColeyBryantampChapin III1985CraineBondLeeReich ampOllinger 2003HermsampMattson 1992) Depending onthe theorygrowth‐dominatedplantsarepredicted toeitherhave

Wepropose thathigh levelsofvariability in theway thatnutrientanddefencetraitscombinemaycontributetothecoexistenceofcloselyrelatedspeciescom-prisingsavannawoodycommunities

K E Y W O R D S

herbivore exclosures induced defences mammal browsers N-free secondary metabolites plantdefencestrategiesplantndashherbivoreinteractionsstructuraldefences

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lowinvestmentsindefences(Bryantetal1983HermsampMattson1992) or invest in qualitative defences (Coley 1988 Coley et al1985) Architectural defences which include spines are generallypredictedtobemoreprevalentonnutrient‐richsoilsespecially insavannaecosystems(Craineetal2003Grubb1992Hanleyetal2007Scholesetal2002)Wigleyetal (2018) foundsomesup-portforthiswithsignificantlyhigherbranchingandspinedensitiesatnutrient‐richcomparedtonutrient‐poorsitesinSouthernAfricansavannas Architectural defences (spines and branching densityorldquocaginessrdquo)aregenerally inducedbyherbivory insavannas (egMilewskiYoungampMadden1991WigleyBondFritzampCoetsee2015)Chemicaldefencesontheotherhavebeenfoundtobedown‐regulated by herbivory in savannas most likely due to C-limitation imposedbytherepeatedremovalofphotosynthesising leafmate-rialbybrowsingherbivores(egScogingsHjaumllteacutenampSkarpe20112013ScogingsMamashelaampZobolo2013Wigleyetal2015)It is however still not clear if similar plant functional types (egfine‐vsbroad‐leaved)orcloselyrelatedspecieswithinacommu-nitygrowingonthesamesoils(iesamesoilfertilitystatus)employsimilarsuitesorcombinationsofdefences(defencesyndromes)

Wenotethatldquosyndromerdquoorldquostrategyrdquoandldquoclusterrdquoorldquogrouprdquohaveoftenbeenusedinterchangeablyintheplantdefenceliterature(AgrawalampFishbein2006DaSilvaampBatalha2011Tomlinsonetal2016)Forthesakeofexpediencyweconsistentlyuseldquostrategyrdquoand ldquogrouprdquo hereafter to encompass the definitions of syndromeandclusterrespectivelyWeexaminedefencestrategiesinsavannatreesbyquantifyingtheexpressionofplantphysicalandchemicaldefencesaswellasplantgrowthinthepresenceversusabsenceofintense browsing by mammalian herbivores using a set of long-term herbivoreexclosuresinanEastAfricansavannainLaikipiaCountyKenyaSpecificallyweaskedthefollowingquestions(a)Arethede-fence strategies observed in the dominant fine- and broad-leaved woodyspeciesof thisEastAfricansavannasimilar to thosedocu-mented in other savanna ecosystemsmdasheg low nutrienthigh chem-icaldefencestrategy(sensuTomlinsonetal2016)versusnutritionanddefencestrategy(Tomlinsonetal2016Wigleyetal2018)(b)Aretheredifferencesintheextenttowhichalternativedefencestrategies are successful at resisting herbivory ie as indexed by dif-ferencesingrowthinthepresenceandabsenceofherbivory(c)Aretheredifferencesinhowphysicalandchemicaltraitsassociatedwithdifferentdefencestrategies respond toherbivory iewhich traitsaremore inducible Based on previouswork (Da Silva amp Batalha2011Tomlinsonetal2016Wigleyetal2018)weexpectedtofinddifferentdefencestrategiesbeingemployedbywoodyplantsin our study system with various combinations of chemical and structural defences We expected that these different strategieswilldeterminehowsuccessfulplantsareatresistingherbivory ieweexpecttofinddifferencesingrowthbetweenstrategiesFinallybasedonprevious findingswepredicted that structuraldefenceswillbehigher (induced)withherbivorespresentbutchemicalde-fences will not be up‐regulated at high browsing intensities (egScogings Hjaumllteacuten et al 2013 Scogings Mamashela et al 2013Wigleyetal2015)

2emsp |emspMATERIAL S AND METHODS

21emsp|emspStudy sites

Our study was carried out at the Mpala Research Centre (MRC)and Mpala Ranch (190km2) in Laikipia County in central Kenya(37deg53primeE0deg17primeN)Weusedthreesetsofherbivoreexclosurescon-structed at MRC in 1999 The exclosures consisted of an 11-strand 3-m tall electrified fence with additional mesh and electrified wires from 0 to 05 m height and excluded herbivores larger than 2 kg for 17years (Augustine amp McNaughton 2004 Sankaran AugustineampRatnam2013)Thesavannasat thesesitesoccuronredsandyloamsoilsdevelopedfrombasementmetamorphicparentmaterials(Augustine 2003 Pringle Prior Palmer YoungampGoheen 2016)Topography consists of gently rolling hills interspersed with oc-casionalgranitic inselbergs (AugustineampMcNaughton2006)Thelong‐termmeanannualrainfall(1972ndash2009)is514mm(Sankaranetal2013)VegetationischaracterisedbyanAcacia‐dominatedtreeandshrubcommunityandadiscontinuouslayerofperennialgrasses(Augustine2003)Themostcommonnativebrowsersandmixed‐feeders found include impala Aepyceros melampus (c 20kmminus2)Guumlnthers dik-dik Madoqua guentheri (c 140kmminus2) and elephantLoxodonta africana(c17kmminus2Augustine2010)Impalaanddik‐dikare present throughout the yearwhile elephants tend tomigrateinto the area during the wet season and are less abundant during dry seasons(Augustine2010Thouless1995)ElandTaurotragus oryx giraffe Giraffa camelopardalis reticulata and greater kudu Tragelaphus strepsiceros alsooccur atMpala at lowerdensities (seeAugustine2010)Previousstudieshaveshownthatthehighdensitiesofbrows-ers and mixed-feeders at the study sites have major effects on the dynamicspopulationdemographyandstructureofthewoodyveg-etation(AugustineampMcNaughton2004Sankaranetal2013)

22emsp|emspTrait sampling

In2016wesampledthesixmostabundantwoodyspeciesforkeyplanttraitsconstitutingthearchitecturaldefenceandlownutrienthighchemicaldefencestrategiesThreeofthesixspecieswerefine‐leavedspecies(Acacia mellifera Acacia etbaica and Acacia brevispica)andthreewerebroad‐leaved(Balanites pedicellaris Grewia tenax and Croton dichogamus)BothB pedicellaris and C dichogamus are ever-green species Speciesnomenclature isbasedonNoadandBirnie(1990)

Measured traits that influence leaf quality included concen-trations of leaf condensed tannins and total polyphenolics leaftotal and available N specific leaf area and acid detergent fibreArchitectural traits included spine length spine density bite sizeindex and a branching index For trait measurements we randomly selectedfive individualsofeachspeciesateachsite inthesaplingsizeclass(typicallybetween1and2minheight) insideexclosures(h‐)and inadjacentcontrolplotswithherbivorespresent (h+)Wecollectedfullyexpandedsun‐exposedleafmaterialfromeachplantduring the peak of the growing season for nutrient and chemical

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analysesAll leafmaterialwasair‐drieduntilsamplesreachedcon-stantweightatMpalaResearchStationSamplesweremilledusingaMF10basicIKAgrinderfittedwitha1mmsieve

Weanalysedleafmaterialfornitrogen(N)usingaLecoTruSpecCNAnalyser(LECOCorporationStJosephMI)Driedleafdigest-ibility and leaf available N were determined as outlined in DeGabriel Wallis Moore and Foley (2008) but with some improvementsBriefly800plusmn10mgofdry leafmaterialwas transferred intopre‐weighed filterbags (ANKOMF57ANKOMTechnology) andheat‐sealedtolockinplantmaterialAmaximumof20bagswereplacedina1‐litrepolypropylenescrewcapcontainertowhich35mlofpepsinsolutionperbagwasaddedBagswereincubatedat37degCfor24hrwithcircular shaking inanorbital rotator (GFL3040Gesellschaftfuumlr LabortechnikmbHGermany) at 14rpmThe additionof rota-tional shaking far better simulates ruminal contraction or gut motil-itywhich is an importantphysiological adaptationwhichensuresconstant mixing of ingested food and probably aids in digestion(ClaussampHummel2005)Afterincubationbagswerewashedfivetimes with distilled water after which 35 ml of cellulose solution was addedtoeachbagandincubatedasoutlinedabovefor48hrAfterincubation bags were again washed 10 times with distilled water anddriedat60degCtoconstantweightToaccountforanylosstothebagintheincubationprocessablankbagwasincludedDrymatterdigestibility was calculated from the amount of material lost in the incubationprocessandleafavailableNwascalculatedbysubtract-ingNremainingintheresiduefromtheinitialtotalleafNWethencalculated how much of the total leaf N was available to herbivores onadryweightbasis andnamed this total availableN (ie leafN(mggminus1)timesproportionavailableN)Wemeasuredtheconcentrationoftotalcondensedtannins(CT)ineachleafsamplefollowingHattasandJulkunen‐Tiitto(2012)andtheconcentrationoftotalpolyphe-noliccompounds(TP)followingHattasStockMabuselaandGreen(2005)WeusedanAnkomfibreanalysertomeasureaciddetergentfibre (ADF)contentofall leafsampleswhichprovidesan indexoftheunpalatable fibrecomponentof leafmaterial and includes thecell‐wallcomponentsofcelluloseand lignin (CooperOwen‐SmithampBryant1988)

Tomeasurespecificleafareawecollected10ndash20healthyfullyexpanded sun‐exposed leaves from each plant and immediatelyscannedthemataresolutionof300dpiinthefieldusingaCanonCanoScanLiDE100flatbedscannerAfterscanningtheleaveswereovendried andweighedWemeasured the areaof the leaf scansusingBlackSpot(VarmaampOsuri2013)Wecalculatedspecificleafarea(SLA)bydividingleafareabydryweightforeachplant

We measured average spine lengths and average diameter atthespinebaseof10maturespinesoneachofthreebranchesperplantusingdigitalVerniercallipersWedeterminedspinedensitybycountingthetotalnumberofspinesonameasuredlengthofeachofthesamethreebranchesanddividingbythebranchlengthWecalculated branch density on three branches per tree by dividingthe number of lateral branches on each branch by the length of the terminalbranch(seePerez‐Harguindeguyetal2013)Thebitesizeindex(BSI)foragivenplantwasestimatedasthetotaldryweight

ofleavesremovedfrom10humanbitestakenfromeachplant(seeCharles‐DominiqueMidgleyampBond2015WigleyFritzCoetseeamp Bond 2014) An attempt was made to remove the maximumamountofleafmaterialwitheachbiteThesamepersonconductedallBSImeasurementstocontrolforpotentialdifferencesbetweenindividual recorders

23emsp|emspPlant growth measurements

At the timeof fenceconstruction in1999all individual treesandshrubs gt05m tall within a 50times50m area in each exclosure andpaired control site weremapped tagged and their basal area (at15 cm above-ground level including all stems on multi-stemmed individuals)canopydimensions(maximumlengthandwidthinthecardinaldirections)andplantheightweremeasuredAllplotswerefully censused again in 2002 2009 and 2016 During each cen-sustheheightbasalareaandcanopydimensionsofallpreviouslytaggedplantswereremeasuredandallnewrecruits(gt05m)werealsomeasuredandrecordedandallmortalitieswerenotedWecal-culatedthemeanchangeinplantheightandbasalareaforeachofthe six species using all individuals thatweremapped andmeas-uredin1999andthatwerestillpresentin2009ineachtreatmentat the three sites (A brevispica n = 359 A etbaica n = 349 A mel-lifera n = 326 B pedicellaris n = 24 G tenax n = 205 C Dichogamus n=54)By2009plantdensities inside theexclosureshadsignifi-cantlyincreasedtoalevelwherecompetitionbetweenplantswaslikely to have started affecting growth We therefore only usedgrowthdataupuntil2009toavoidtheeffectsofinterspecificandintraspecificcompetition

24emsp|emspStatistical analyses

AllanalyseswereperformedusingRversion331(RDevelopmentCore Team 2016) To testwhether species adopted different de-fence strategies and whether these strategies responded to her-bivore exclusionwe ran a principal component analysis (PCA) onthe nine measured defence traits using the function dudipca (ade4 packageforRDrayampDufour2007)WeranHornsParallelAnalysisusingthefunctionparan(paranpackageforR)toevaluatethenum-ber of components to be retained in the principal componentsanalysisWe first used theFlignerndashKilleen testofhomogeneityofvariance (flignertest in the stats package forR) to test if thedataused for treatment (h‐ vs h+) comparisonswere normally distrib-utedWhentheassumptionofnormalitywasmetweusedpairedt teststoevaluateeffectsofbrowserexclusiononthemeasuredplanttraitschangeinplantheightandwoodybasalareaforeachofthesixdominantwoodyspeciesWhentheassumptionofnormalitywasviolatedweused thenonparametricWilcoxon rank sum testWepooledtraitdatafromthethreeexclosuresitesforeachspeciesgiv-ing15individualssampledforeachtreatmentforfourofthespeciesA brevispica and C dichogamus only occurred at two of the three sitesandconsequentlyweonlyhad10individualssampledforeachtrait in each treatment

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3emsp |emspRESULTS

31emsp|emspDefence strategies

Results fromHornsParallelAnalysis for factor retentionbasedon 5000 iterations using the mean estimate showed that the first six components of the PCA should be retained The PCAbased on the measured plant traits showed that the six domi-nantspeciesatthestudysiteseparatedintothreebroadgroupsalong the first two PC axes which together accounted for al-most 60 of the total variance (Figure 1) This separationwasmost strongly driven by differences in spine length and fibre

onPC1 separating the twospecieswith long spinesand lowerfibre(A etbaica and B pedicellaris)fromthespecieswithhigherfibreandshortspines(A brevispica and A mellifera)ornospines(G tenax andC dichogamus (Table1)OnPC2 the groupsweremoststronglyseparatedbydifferencesinN‐freesecondaryme-tabolitesandspinedensityTwoofthebroad‐leavedspeciesG tenax and C dichogamus weresimilarintermsofhavingnospinesand lower concentrations of N-free secondary metabolites and weremoststronglyseparatedfromA brevispica and A mellifera whichhadhighN‐freesecondarymetabolitesandspinedensities(Figure1Table1)

F I G U R E 1 emsp (a)Plotofalineardiscriminant analysis of the measured defencetraitsshowingthegroupingofthesixMpalawoodyspeciesrelativetoWigleyetals(2018)dominantspeciesfromsouthernAfricansavannas(b)principalcomponentanalysis(PCA)basedonninemeasuredplantdefencetraitsofthesixwoodyplantspeciesgrowinginthepresence(black)andlong‐termabsence(grey)oflargemammalianherbivoresincentralLaikipiaCountyKenyaAxis1explained33ofthevarianceandwasprimarilyassociatedwithvariationinspinelengthandaciddetergentfibre(ADF)Axis2explained26ofthevarianceandwasprimarilyassociatedwithvariationincondensedtannins(CT)totalpolyphenols(TP)andspinedensity(seeTable1foreigenvectorscoresofthePCA)PointsrepresentthemeanlocationofeachspeciesonPCaxes1and2ateachofthethreepairedexclosureandcontrolsitesOvalsshowtheoverallmeans(centre)foreachspeciesandtreatmentanddispersion(ellipses)

TA B L E 1 emspEigenvectorscoresfortheprincipalcomponentanalysis(PCA)basedonninedefencetraits

PC1 (33) PC2 (26) PC3 (15) PC4 (10) PC5 (8) PC6 (4)

Spinelength(mm) minus053 minus005 019 minus007 010 025

Spinedensity(spinescmminus1) minus030 minus046 013 004 043 016

Total available N(mggminus1) 008 038 minus029 026 078 minus026

SLA(cm2 gminus1) 032 minus004 048 minus056 015 minus039

TP() 028 minus049 minus028 009 minus011 minus040

CT() 033 minus042 minus036 009 004 037

ADF() 048 013 007 minus030 024 061

BI(branchescmminus1) minus014 037 minus053 minus040 minus024 005

BSI(g) 028 028 037 059 minus023 012

NoteSLAspecificleafareaTPtotalpolyphenolsCTcondensedtanninsADFaciddetergentfibreBIbranchingindexBSIbitesizeindex

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32emsp|emspEffects of herbivore removal on plant defence traits and strategies

ThePCA(Figure1)illustratestheeffectsofherbivoreremovalandcontroltreatmentsontheplantdefencetraits(andstrategies)withherbivore removal treatments diverging from control treatments alongbothPCaxestovaryingdegrees(Figure1)Howevernoneofthespecieschangeddefencestrategiesasaresultofherbivorere-movalbothstructuralandchemicaldefencesrespondedtovariabledegrees both within and between strategies The long-term removal of large mammalian browsers from this savanna did not affect leaf totalNandaciddetergentfibreforanyofthesixofthewoodyspe-cies(Table2)Specificleafareadeclined(ieleavesbecamesmallerandthicker)withherbivoreexclusionforonefine‐leavedspecies(A mellifera p=005)andincreased(leavesbecamelargerandthinner)withherbivoreexclusionfortwobroad‐leavedspeciesB pedicellaris (p=003)andG tenax(p=009Figure2)HerbivoreremovalresultedinhighertotalpolyphenolsinA etbaica and A mellifera(p = 007 and p=001respectively)andsignificantlyhighercondensedtanninsinA brevispica(p=0003Figure2)andloweravailableNinA mellifera (plt01) andC dichogamus (plt001) Plant architectural defences(spinesandbranchingdensities)werefoundtorelaxwithherbivoreremoval ie became less structurally defended Herbivore removal decreasedthebranchingindexandbranchespervolume(plt0001)and increased the bite size index (iemore leaf biomass per bite

plt00001) for all speciesexceptC dichogamus (Figures3 and4) Herbivore removal resulted in significantly shorter spines inA et-baica(plt0001)andA mellifera (p=002)andasignificantlylowerdensityofspinesinA etbaica (p=001)(Figure3)

33emsp|emspGrowth defence strategies and responses to herbivore removal

Overthe10‐yearperiodbetween1999and2009wedocumentedsubstantialvariationingrowthwhichwepresent inrelationtothethreegroupsWhengrowing in thepresenceofherbivoresgroup1 (A brevispica and A mellifera) decreased in height (minus042plusmn007and minus010plusmn009m respectively) but increased in basal area(269plusmn116and 110plusmn656cm2) In this group herbivore removalresultedinsignificantlytallerplantswithhigherbasalareathanwithherbivorespresent(plt0001Table3Figure5)Group2(A etbaica and B pedicellaris)increasedinheight(031plusmn006and024plusmn044mrespectively)andbasalarea(209plusmn314and346plusmn124cm2)inthepresenceofherbivores Inthisgroupherbivoreremoval increasedplantheight(plt001)butnotbasalarea(Table3Figure5)Plantsingroup3(G tenax and C dichogamus)generallydecreasedinheightbut no consistent trend was evident for basal area when herbivores were present ForG tenax herbivore removal resulted in signifi-cantlytallerplantswithhigherbasalareas(plt0001)C dichogamus plantsshowedaslightdecreaseinheightandincreaseinbasalarea

TA B L E 2 emsp Mean plusmn SEforleaftotalavailablenitrogen(mggminus1)leaftotalnitrogen(mggminus1)specificleafarea(SLAcm2 gminus1)leaftotalpolyphenolics(TP)leafcondensedtannins(CT)leafaciddetergentfibre(ADF)branchingindex(BIbranchescm-1)bitesizeindex(BSIg)andaveragespinelength(ASLmm)

Plant trait treatment Aca bre (1) Aca mel (1) Aca etb (2) Bal ped (2) Gre ten (3) Cro dic (3)

Total available N

In(h‐) 234 plusmn 142 156 plusmn 106 156 plusmn 086 215 plusmn 24 226 plusmn 099 126 plusmn 024

Out(h+) 248 plusmn 111 185 plusmn 105 152 plusmn 114 235 plusmn 171 220 plusmn 093 167 plusmn 082

Leaf total N In(h‐) 361 plusmn 220 344 plusmn 088 329 plusmn 090 283 plusmn 190 340 plusmn 090 214 plusmn 030

Out(h+) 394 plusmn 130 352 plusmn 060 320 plusmn 080 312 plusmn 150 322 plusmn 090 234 plusmn 070

SLA In(h‐) 102 plusmn 734 107 plusmn 544 141 plusmn 786 546 plusmn 507 150 plusmn 514 147 plusmn 600

Out(h+) 119 plusmn 809 118 plusmn 302 139 plusmn 120 407 plusmn 252 136 plusmn 712 146 plusmn 600

TP In(h‐) 331 plusmn 046 626 plusmn 025 155 plusmn 009 055 plusmn 003 109 plusmn 014 126 plusmn 006

Out(h+) 278 plusmn 030 535 plusmn 022 136 plusmn 007 055 plusmn 005 129 plusmn 013 142 plusmn 005

CT In(h‐) 472 plusmn 057 658 plusmn 104 038 plusmn 004 008 plusmn 0007 142 plusmn 022 033 plusmn 003

Out(h+) 248 plusmn 037 715 plusmn 051 041 plusmn 004 009 plusmn 001 154 plusmn 022 037 plusmn 001

ADF In(h‐) 329 plusmn 239 371 plusmn 198 304 plusmn 129 246 plusmn 089 430 plusmn 084 338 plusmn 045

Out(h+) 350 plusmn 216 336 plusmn 183 288 plusmn 158 241 plusmn 060 420 plusmn 105 324 plusmn 116

BI In(h‐) 007 plusmn 003 009 plusmn 003 004 plusmn 0007 025 plusmn 003 016 plusmn 002 067 plusmn 016

Out(h+) 045 plusmn 005 038 plusmn 004 032 plusmn 004 125 plusmn 008 119 plusmn 018 056 plusmn 007

BSI In(h‐) 300 plusmn 023 201 plusmn 015 081 plusmn 006 249 plusmn 023 557 plusmn 048 534 plusmn 05

Out(h+) 087 plusmn 008 037 plusmn 003 011 plusmn 001 044 plusmn 004 108 plusmn 008 427 plusmn 049

ASL In(h‐) 346 plusmn 006 373 plusmn 004 529 plusmn 101 547 plusmn 125 mdash mdash

Out(h+) 347 plusmn 007 385 plusmn 003 586 plusmn 090 550 plusmn 086 mdash mdash

NoteTreatmentsareherbivoresexcluded(h‐)andherbivorespresent(h+)GroupmembershipisindicatedinparenthesesnexttoeachspeciesSignificance levels are indicated byp lt 01 p lt 005 p lt 001 p lt 0001 and are highlighted in bold

emspensp emsp | emsp7Journal of EcologyWIGLEY Et aL

duringthestudyperiodwithherbivorespresenthoweverneitherofthe measures of growth differed between the herbivore removal and herbivores present treatments (pgt005) Based on themeasuredchanges inplantheight andbasal area in thepresenceversus ab-sence of browsers over the course of a decade we ranked the three groupsfromlowtohighintermsoftheircapacitytopersistinthefaceofintensebrowsingpressurewheregroup1ltgroup3ltgroup2(Table3Figure5)

4emsp |emspDISCUSSION

OuroverarchinggoalwastoexaminehowwoodyspeciesinthisEastAfricansavannausesuitesoftraitstocontendwitharelativelyintensebrowsing regime imposedbyadiverseandabundantassemblageoflargemammalianherbivoresThehypotheseswhichunderpinclassicplant defence theory rely heavily on resource availability to predictwhetherplantsinvestingrowthordefence(HermsampMattson1992)and hence do not predict relative investments in structural versuschemicaldefences(Hanleyetal2007)orwhichdefenceswillbemosteffectiveunderfixedresourcelevelsbutvaryingherbivorypressureOurresultsindicatethat(a)woodyspeciesinthissystemcanbeclas-sifiedintothreedifferentgroupsorstrategiesbasedontheirdifferen-tialinvestmentinstructuralandchemicaldefences(b)structuralandchemical defences responded to varying degrees to the removal ofherbivoresbothwithinandacrossgroupsbutplantdefencestrategies

remainedqualitativelyunchangedeven after nearly twodecadesofherbivoreexclusion (c) structuralandchemicaldefencesdidnot re-spondinthesamewaytoherbivoreremovalwithstructuraltraits(es-peciallybranching)typicallyrespondingmorepositively(ieincreased)and (d) thespecies thatweremost resilient to intensebrowsing (ieachievedthehighestgrowth)werethosethatmaintainedhighspinelengthspinedensityandbranchinginthepresenceofherbivores

Noneofthespeciesinourstudysiteappeartobeadoptingtheldquolow nutrienthigh chemical defencerdquo strategy with most specieshavingmoderatetohighleafNcomparedtospeciespreviouslycat-egorised as low nutrienthigh chemical defence strategists in other savannasystems(Wigleyetal2018Figure1a)Thethreestrate-giesweidentifiedthusallfallwithinthebroadremitofldquonutritionanddefencerdquobutspeciesinthesedifferentgroupsappeartoinvestvari-ablyinstructuralandN‐freechemicaldefencesFine‐leavedspeciessuch as A brevispica and A mellifera (group1)havemoderatetohighleafNlowinvestmentinstructuraldefences(shortspines)andhighinvestmentinN‐freechemicaldefences(highCTandTP)A etbaica and B pedicellaris(group2)similarlyhadmoderatetohighleafNandbothspeciesinvestedmoreinphysicaldefences(denselongspinesthorns)thanchemicaldefences(lowN‐freesecondarymetabolites)Finally species suchasG tenax and C dichogamus (group3 bothbroad‐leaved)hadmoderatetohighleafNnospinesandlowN‐freesecondary metabolites

Basedonourmeasuredchanges inplantheightandbasalareainthepresencecomparedtoabsenceofbrowsersoverthecourse

F I G U R E 2 emsp Mean plusmn SE total available leafnitrogen(totalNxavailableNproportion(a)specificleafarea(b)totalpolyphenols(c)andcondensedtannins(d)insideandoutsideoftheexclosuresAcabreAcacia brevispicaAcamelAcacia melliferaAcaetbAcacia etbaica BalpedBalanites pedicellaris Gre ten Grewia tenax Cro dic Croton dichogamus Significance levels are p lt 01 p lt 005 p lt 001

8emsp |emsp emspenspJournal of Ecology WIGLEY Et aL

ofonedecadewerankedthesixwoodyspeciesfromlowtohighintermsoftheircapacitytocoexistwithintensebrowsingpressurewhere G tenaxltA brevispicaltA melliferaltC dichogamusltB pedi-cellarisltA etbaica (Figure5)Our results for the twobroad‐leavedspeciesthat lack investment inspines(group3G tenax and C di-chogamus)provideseveralkeyinsightsregardingstrategiesforldquolivingwithbrowsersrdquoBasedontheframeworkdevelopedfromsouthernAfrican savannas (Wigley et al 2018) we expected both speciesto invest heavily in N-free secondary metabolites as a trade-off to the lackof armamentContrary to thispredictionneither speciesinvestedinN‐freesecondarymetaboliteseitherinthepresenceorabsence of large herbivores

LowerleafN(bothtotalandavailable)wasoneconspicuoustraitdistinguishing Crotonfromtheotherspeciesinthisstudyandmaycontributetothefactthatthisspeciesisgenerallyunpalatableanduneatenbylargebrowsers(Kartzineletal2014)HoweverleafN(~22)wasnot so lowas tocompromise ruminantdigestioneffi-ciency(whichtypicallyoccurscloserto1NVanSoest1994)sug-gesting thatsomeotherpotentiallycostlyandasyetunmeasuredaspect ofCroton leaf chemistry is a key trait allowing this broad‐leavedspeciestocoexistwithlargebrowsersCroton sppingeneralarehighlyaromaticandknowntocontainmultipleuniquesecondarychemicals including cembranoids halimanes crotofolanes sesqui-terpenoids flavonoids and cyclohexanol derivatives (Langat et al2016)someofwhichareverysuccessfulinlimitingvertebrateand

invertebrate herbivory (Kaplan Halitschke Kessler Sardanelli ampDenno 2008 Levin 1976) Including these types of chemical de-fences in a generalised trait framework will be challenging as they arenotubiquitousacrossspeciesandtheirexpressioncandependonspecificherbivoreelicitors(Moreiraetal2013)

The lack of investment in N-free secondary metabolites by the secondbroad‐leavedspeciesG tenaxwasalsosurprisingbutcon-sistentwithourfindingthatgrowthofthisspeciesisseverelyneg-ativelyaffectedbylargebrowsersandwithpreviousworkshowingsubstantial declines in all size classes of G tenaxinthepresenceofbrowsersinthissavanna(AugustineampMcNaughton2004Sankaranetal2013)Ratherthanemployinganyformofcostlychemicalde-fence this species appears to coexist (uneasily)with browsers byincreasing the complexity of its branching architecture (BI) andgrowing in close association with other thorny species that cre-ate structural refugia where G tenax saplings are protected frombrowsers (personalobservationbyallauthorsseealsoCoverdaleGoheenPalmerampPringle2018)OnevalueofthisstrategyisthatwhenbrowsingpressureisremovedorlowthelackofinvestmentincostlydefencescombinedwithhighleafNallowsforrapidgrowth

TheremainingfourspeciesbelongingtotheothertwogroupsallhavesomeformofinvestmentinspinesTheycanbearrayedalongagradientofincreasingspineinvestmentfromgroup1togroup2withspeciesingroup1consistingofthosewithshortstraightspines(A brevispica) or short recurved spines (A mellifera) and those in

F I G U R E 3 emsp Mean plusmn SEspinelength(a)spinediameter(b)bitesizeindex(c)andbranchdensity(d)insideandoutsidetheexclosuresAcabreAcacia brevispica AcamelAcacia mellifera AcaetbAcacia etbaicaBalpedBalanites pedicellaris Gre ten Grewia tenax Cro dic Croton dichogamus Significance levels are p lt 005 p lt 001 p lt 0001

(a) (b)

(c) (d)

emspensp emsp | emsp9Journal of EcologyWIGLEY Et aL

F I G U R E 4 emspExamplesofAcacia brevispica and Balanites pedicellaris growing in the absence of large mammalianherbivores(ac)andthesamespeciesgrowinginthepresenceoflargemammalianherbivores(bd)incentralLaikipiaCountyKenyaNotethecleardifferences in architecture branching densities and browser damage between the treatments

(a) (b)

(c) (d)

TA B L E 3 emsp Mean plusmn SEforplantheightandbasalareain1999and2009forthesixspeciesgrowingwithandwithoutherbivorespresentnetchangesinplantheightandbasalareabetweenthe2yearsarealsoshown

Species Treatment Height 1999 (m) Height 2009 (m)Basal area 1999 (cm2)

Basal area 2009 (cm2) Δ height Δ basal area

Acacia brevispica Out(h+) 195 plusmn 007 152 plusmn 005 1379 plusmn 174 1649 plusmn 209 minus043plusmn007 269 plusmn 117

Acacia brevispica In(h‐) 201 plusmn 006 297 plusmn 008 991 plusmn 096 164 plusmn 147 095 plusmn 006 649 plusmn 088

Acacia mellifera Out(h+) 228 plusmn 011 218 plusmn 01 8368 plusmn 1598 9465 plusmn 134 minus010plusmn009 1097 plusmn 657

Acacia mellifera In(h‐) 219 plusmn 009 349 plusmn 01 8048 plusmn 1331 12236 plusmn 1492 13 plusmn 005 4188 plusmn 611

Acacia etbaica Out(h+) 168 plusmn 008 199 plusmn 01 3699 plusmn 616 5786 plusmn 715 031 plusmn 006 2087 plusmn 314

Acacia etbaica In(h‐) 157 plusmn 007 305 plusmn 007 3039 plusmn 754 5183 plusmn 77 148 plusmn 006 2117 plusmn 323

Balanaites pedicellaris Out(h+) 25 plusmn 07 286 plusmn 05 8823 plusmn 4682 12282 plusmn 5008 036 plusmn 047 3459 plusmn 1242

Balanaites pedicellaris In(h‐) 196 plusmn 028 401 plusmn 026 2542 plusmn 829 7193 plusmn 1645 205 plusmn 023 4650 plusmn 1283

Grewia tenax Out(h+) 131 plusmn 009 095 plusmn 024 336 plusmn 044 296 plusmn 048 minus036plusmn024 minus041plusmn035

Grewia tenax In(h‐) 116 plusmn 005 24 plusmn 008 194 plusmn 023 578 plusmn 05 124 plusmn 007 384 plusmn 044

Croton dichogamus Out(h+) 216 plusmn 022 21 plusmn 024 2393 plusmn 831 3323 plusmn 829 minus007plusmn014 929 plusmn 217

Croton dichogamus In(h‐) 159 plusmn 008 182 plusmn 012 932 plusmn 267 1346 plusmn 317 023 plusmn 011 414 plusmn 116

NoteAllmeasurementsarebasedonthesameindividualplantsofeachspeciesthatwerepresentintheplotsin1999andthatwerestillalivein2009

10emsp |emsp emspenspJournal of Ecology WIGLEY Et aL

group2havingeither long straight spines (B pedicellaris)orbothshort recurvedand long straight spines (A etbaica) Specieswiththelongestandmostdensespines(group2)showedlittletonoin-vestmentinCTorTPwhilethespecieswithlowspineinvestment(group 1) showed the greatest concentrations of bothCT and TP(Figure2)The latterstrategyexperiencesgreaterconsumptionbybrowsersinthislandscaperelativetotheformer(Fordetal2014)consistentwithpreviousstudieswhichhavereportedthatspecieswithhigherinvestmentinstructuraldefences(group2)aremostsuc-cessfulatcoexistingwithbrowsersinthissavannasystem(GoheenKeesingAllanOgadaampOstfeld2004Sankaranetal2013)Inthepresenceofbrowsersspeciesinbothgroups1and2maintainedanincreasedcomplexityoftheirbranchingarchitecturecombinedwithsmallerleaves(reflectedinsubstantialreductionsinbitesizeindex)Increased branching even in the absence of changes in thorn length anddensitywhichoccurredinsomespeciesbutnototherscanre-sultindramaticincreasesinthenumberofspinesperunitvolumeofcanopy(ArchibaldampBond2003StaverBondCramerampWakeling2012)servingasaneffectivedeterrentforbrowsers

To summarise the strategies expressed by spinescent speciesin termsof ldquolivingwithbrowsersrdquo canbecharacterisedas (a)veryhigh and inducible investment in spinesbutnotN‐free secondarymetabolites and low available N leading to greatest success under intensebrowsing(A etbaica)(b)moderateandinducibleinvestmentinstraightspinescombinedwith thickermore fibrous leavesbuthigh available N leading to sufficient success under intense brows-ing(B pedicellaris)(c)lowandnon‐inducibleinvestmentinrecurvedspines combinedwithhighN‐free secondarymetabolites and lowavailableNleadingtoreducedsuccessunderbrowsing(A mellifera)

and(d)lowandnon‐inducibleinvestmentinshortspinescombinedwith moderate investment in N-free secondary metabolites and high availableN leading to low success under browsing (A brevispica)Fornara and Du Toit (2008) suggested that physical defences to-getherwithmasscompensatorygrowthabilitiesarekeyadaptationstolivingwithhighbrowsingpressureOurfindingslendstrongsup-porttothisnotionasthemostheavilystructurallydefendedspecies(A etbaica and B pedicellaris)performedthebestinthepresenceofherbivoresdespitelowinvestmentsinN‐freesecondarymetabolitesHowever Scogingset al (2011)predicted that the concentrationsofN‐freesecondarycompoundswoulddependonbrowsinginten-sity with the highest concentrations occurring at intermediate levels ofbrowsingforcertainfunctionalgroupsForexampleScogingsetal(2011)predictalinearincreaseinN‐freesecondarycompoundswithbrowsingintensitywithapotentiallysuddendecreaseatveryhigh levelsofbrowsingpressure indeciduoussavannaspeciesAsourexperimentaldesign lackedan intermediatebrowsingpressuretreatmentwewereunabletotestthesepredictions

Our analyses of available Nprovidedsomeinterestingandvalu-ableinsightsFirstlyinterpretationsofbrowsequalitydifferdepend-ing on whether total N (also commonly converted to crudeproteinby multiplying by 625 eg Cooper et al 1988) or available N isevaluatedForexamplethethreeAcaciaspeciesinthisstudyA bre-vispica A mellifera and A etbaica all had similar and relatively high con-centrations of leaf total N (~3835and33respectively)buttheproportionofthetotalN available to herbivores in A brevispica was ca 20 higher than in A mellifera and A etbaica(~2417and15respectively seeTable2) Ifweonly lookedat totalN they would all appear tobeofhighqualityhowever ifwe lookat availableN

F I G U R E 5 emsp (a)MeanplusmnSE change in plantheightbetween1999and2009forthesixdominantspeciesgrowinginsideandoutsidetheexclosures(b)Mean plusmn SE change in stem basal area between 1999 and 2009 for the same six speciesSignificancelevelsarep lt 001 p lt 0001

emspensp emsp | emsp11Journal of EcologyWIGLEY Et aL

itappearsasifA brevispicaisofmuchhigherqualitythantheothertwospeciesSecondlyavailableNappearstobemoreresponsivetoherbivore removal than total leaf N No differences in total N were evident between herbivory treatments while available N was signifi-cantly higher in A mellifera and C dichogamus when herbivores were presentThirdlydifferencesinavailableNdonotseemtobelinkedtophylogeny(eghighinsomeAcaciaspecieslowinothers)functionaltype(egfine‐leavedvsbroad‐leaved)norconcentrationsofN‐freesecondarymetabolites (availableNfoundtobe low inspecieswithbothhighand lowN‐freesecondarymetabolites)Finally these re-sults raise some importantquestions regarding the commonlyheldview (especially in African savannas) that fine‐leaved species (egAcacia)havehigherqualityleavesandarestructurallydefendedwhilebroad‐leavedspeciesarelessnutritiousandchemicallydefendedForexampleG tenax and B pedicellarisbothbroad‐leavedspecieshadhigher available Nthanthetwofine‐leavedspeciesA mellifera and A etbaica These findings highlight the need for further studies to eluci-date the determinants of leaf available N and that caution needs to be taken when using total NasameasureofbrowsequalityFurthermoreour findings suggest that neither total N nor available N are sufficient bythemselvestopredictbrowserutilisationhighlightingtheneedtoconsideralltraitscollectively(iedefencestrategies)

5emsp |emspCONCLUSIONS

Allofthedominantspeciesatourstudysitehadmoderatetohighleaf N concentrations but differed in available N and in the way they combined chemical and structural defences thus fitting under thebroaderldquonutritionanddefencerdquosyndromepreviouslydescribedforsavannasStructuraldefenceshadmorepronouncedresponsesto herbivory through increased spine length and density andorincreased branching while N-free secondary metabolites did not increaseinresponsetoherbivoryHighinvestmentinstructuralde-fences was the most successful defence strategy at our study sites withmoderateresourcelevels(relativetootherAfricansavannas)andhighherbivorypressureresultinginthehighestgrowthinthissavanna system This work has shown that within one overarching defence syndrome (nutritionanddefence) specieswithina com-munity can widely diverge in their adopted defence strategiesEndaraetal(2015)reportedhighlevelsofdissimilarityinthede-fencesof closely relatedco‐occurring forest speciesand suggestthat this would be necessary for the coexistence of closely related neighboursandcouldaccountforthehighlocaldiversityoftropicalforests Similarly we suggest that diverse defence strategies en-able savanna species (which are often closely related) to occupydifferentnichesanddefendagainstdifferent typesofherbivoresresultinginmoreresilientandspeciesrichwoodycommunities

ACKNOWLEDG EMENTS

WethankJParsareforassistanceinthefieldandChengappaSKand Balavignesh B for their assistance with laboratory analyses

This work was funded by the National Centre for Biological Sciences TIFR India andbyNationalGeographic (GrantNumber982815)andNERC (GrantNE‐E017436‐1)Wearehugely grateful to theOfficeofthePresidentoftheRepublicofKenyaforpermissiontoconducttheresearchandtheMpalaResearchCentreforlogisti-calsupport

AUTHORSrsquo CONTRIBUTIONS

BJWCCDJAandMSco‐wrotethemanuscriptBJWandMS collected the data DJA MS and JR implemented theexclosure experiment DH conceptualised and performed theanalyticalapproacheswithrespecttofibreandavailableNdeter-mination andperformed condensed tannin and total polyphenolanalysesAllauthorscommentedandaddedtoearlierversionsofthemanuscript

DATA ACCE SSIBILIT Y

Data available from the Dryad Digital Repository httpsdoiorg105061dryadp2d00sf(Wigleyetal2019)

ORCID

Benjamin J Wigley httpsorcidorg0000‐0002‐6964‐3624

Mahesh Sankaran httpsorcidorg0000‐0002‐1661‐6542

R E FE R E N C E S

Agrawal A A (2011) Current trends in the evolutionary ecology ofplant defence Functional Ecology 25(2) 420ndash432 httpsdoiorg101111j1365-2435201001796x

Agrawal A A amp Fishbein M (2006) Plant defense syndromesEcology 87(sp7) 132ndash149 httpsdoiorg1018900012‐9658 (2006)87[132PDS]20CO2

Archibald S ampBond W J (2003) Growing tall vs growing wideTree architecture and allometry of Acacia karroo in forest sa-vanna and arid environments Oikos 102(1) 3ndash14 httpsdoiorg101034j1600-0706200312181x

AugustineDJ(2003)Long‐termlivestock‐mediatedredistributionofnitro-genandphosphorusinanEastAfricansavannaJournal of Applied Ecology 40(1)137ndash149httpsdoiorg101046j1365‐2664200300778x

AugustineDJ(2010)Responseofnativeungulatestodroughtinsemi‐arid Kenyan rangeland African Journal of Ecology 48(4)1009ndash1020httpsdoiorg101111j1365‐2028201001207x

AugustineDJampMcNaughtonSJ(2004)RegulationofshrubdynamicsbynativebrowsingungulatesonEastAfricanrangelandJournal of Applied Ecology 41(1)45ndash58httpsdoiorg101111j1365‐2664200400864x

AugustineDJampMcNaughtonSJ(2006)Interactiveeffectsofungu-lateherbivoressoilfertilityandvariablerainfallonecosystempro-cesses in a semi-arid savanna Ecosystems 9(8)1242ndash1256httpsdoiorg101007s10021-005-0020-y

BartonKE (2016)TougherandthornierGeneralpatterns in the in-ductionofphysicaldefencetraitsFunctional Ecology 30(2)181ndash187httpsdoiorg1011111365‐243512495

BryantJPChapinFSIIIampKleinDR(1983)Carbonnutrientbal-anceofborealplantsinrelationtovertebrateherbivoryOikos 40(3)357ndash368httpsdoiorg1023073544308

emspensp emsp | emsp3Journal of EcologyWIGLEY Et aL

lowinvestmentsindefences(Bryantetal1983HermsampMattson1992) or invest in qualitative defences (Coley 1988 Coley et al1985) Architectural defences which include spines are generallypredictedtobemoreprevalentonnutrient‐richsoilsespecially insavannaecosystems(Craineetal2003Grubb1992Hanleyetal2007Scholesetal2002)Wigleyetal (2018) foundsomesup-portforthiswithsignificantlyhigherbranchingandspinedensitiesatnutrient‐richcomparedtonutrient‐poorsitesinSouthernAfricansavannas Architectural defences (spines and branching densityorldquocaginessrdquo)aregenerally inducedbyherbivory insavannas (egMilewskiYoungampMadden1991WigleyBondFritzampCoetsee2015)Chemicaldefencesontheotherhavebeenfoundtobedown‐regulated by herbivory in savannas most likely due to C-limitation imposedbytherepeatedremovalofphotosynthesising leafmate-rialbybrowsingherbivores(egScogingsHjaumllteacutenampSkarpe20112013ScogingsMamashelaampZobolo2013Wigleyetal2015)It is however still not clear if similar plant functional types (egfine‐vsbroad‐leaved)orcloselyrelatedspecieswithinacommu-nitygrowingonthesamesoils(iesamesoilfertilitystatus)employsimilarsuitesorcombinationsofdefences(defencesyndromes)

Wenotethatldquosyndromerdquoorldquostrategyrdquoandldquoclusterrdquoorldquogrouprdquohaveoftenbeenusedinterchangeablyintheplantdefenceliterature(AgrawalampFishbein2006DaSilvaampBatalha2011Tomlinsonetal2016)Forthesakeofexpediencyweconsistentlyuseldquostrategyrdquoand ldquogrouprdquo hereafter to encompass the definitions of syndromeandclusterrespectivelyWeexaminedefencestrategiesinsavannatreesbyquantifyingtheexpressionofplantphysicalandchemicaldefencesaswellasplantgrowthinthepresenceversusabsenceofintense browsing by mammalian herbivores using a set of long-term herbivoreexclosuresinanEastAfricansavannainLaikipiaCountyKenyaSpecificallyweaskedthefollowingquestions(a)Arethede-fence strategies observed in the dominant fine- and broad-leaved woodyspeciesof thisEastAfricansavannasimilar to thosedocu-mented in other savanna ecosystemsmdasheg low nutrienthigh chem-icaldefencestrategy(sensuTomlinsonetal2016)versusnutritionanddefencestrategy(Tomlinsonetal2016Wigleyetal2018)(b)Aretheredifferencesintheextenttowhichalternativedefencestrategies are successful at resisting herbivory ie as indexed by dif-ferencesingrowthinthepresenceandabsenceofherbivory(c)Aretheredifferencesinhowphysicalandchemicaltraitsassociatedwithdifferentdefencestrategies respond toherbivory iewhich traitsaremore inducible Based on previouswork (Da Silva amp Batalha2011Tomlinsonetal2016Wigleyetal2018)weexpectedtofinddifferentdefencestrategiesbeingemployedbywoodyplantsin our study system with various combinations of chemical and structural defences We expected that these different strategieswilldeterminehowsuccessfulplantsareatresistingherbivory ieweexpecttofinddifferencesingrowthbetweenstrategiesFinallybasedonprevious findingswepredicted that structuraldefenceswillbehigher (induced)withherbivorespresentbutchemicalde-fences will not be up‐regulated at high browsing intensities (egScogings Hjaumllteacuten et al 2013 Scogings Mamashela et al 2013Wigleyetal2015)

2emsp |emspMATERIAL S AND METHODS

21emsp|emspStudy sites

Our study was carried out at the Mpala Research Centre (MRC)and Mpala Ranch (190km2) in Laikipia County in central Kenya(37deg53primeE0deg17primeN)Weusedthreesetsofherbivoreexclosurescon-structed at MRC in 1999 The exclosures consisted of an 11-strand 3-m tall electrified fence with additional mesh and electrified wires from 0 to 05 m height and excluded herbivores larger than 2 kg for 17years (Augustine amp McNaughton 2004 Sankaran AugustineampRatnam2013)Thesavannasat thesesitesoccuronredsandyloamsoilsdevelopedfrombasementmetamorphicparentmaterials(Augustine 2003 Pringle Prior Palmer YoungampGoheen 2016)Topography consists of gently rolling hills interspersed with oc-casionalgranitic inselbergs (AugustineampMcNaughton2006)Thelong‐termmeanannualrainfall(1972ndash2009)is514mm(Sankaranetal2013)VegetationischaracterisedbyanAcacia‐dominatedtreeandshrubcommunityandadiscontinuouslayerofperennialgrasses(Augustine2003)Themostcommonnativebrowsersandmixed‐feeders found include impala Aepyceros melampus (c 20kmminus2)Guumlnthers dik-dik Madoqua guentheri (c 140kmminus2) and elephantLoxodonta africana(c17kmminus2Augustine2010)Impalaanddik‐dikare present throughout the yearwhile elephants tend tomigrateinto the area during the wet season and are less abundant during dry seasons(Augustine2010Thouless1995)ElandTaurotragus oryx giraffe Giraffa camelopardalis reticulata and greater kudu Tragelaphus strepsiceros alsooccur atMpala at lowerdensities (seeAugustine2010)Previousstudieshaveshownthatthehighdensitiesofbrows-ers and mixed-feeders at the study sites have major effects on the dynamicspopulationdemographyandstructureofthewoodyveg-etation(AugustineampMcNaughton2004Sankaranetal2013)

22emsp|emspTrait sampling

In2016wesampledthesixmostabundantwoodyspeciesforkeyplanttraitsconstitutingthearchitecturaldefenceandlownutrienthighchemicaldefencestrategiesThreeofthesixspecieswerefine‐leavedspecies(Acacia mellifera Acacia etbaica and Acacia brevispica)andthreewerebroad‐leaved(Balanites pedicellaris Grewia tenax and Croton dichogamus)BothB pedicellaris and C dichogamus are ever-green species Speciesnomenclature isbasedonNoadandBirnie(1990)

Measured traits that influence leaf quality included concen-trations of leaf condensed tannins and total polyphenolics leaftotal and available N specific leaf area and acid detergent fibreArchitectural traits included spine length spine density bite sizeindex and a branching index For trait measurements we randomly selectedfive individualsofeachspeciesateachsite inthesaplingsizeclass(typicallybetween1and2minheight) insideexclosures(h‐)and inadjacentcontrolplotswithherbivorespresent (h+)Wecollectedfullyexpandedsun‐exposedleafmaterialfromeachplantduring the peak of the growing season for nutrient and chemical

4emsp |emsp emspenspJournal of Ecology WIGLEY Et aL

analysesAll leafmaterialwasair‐drieduntilsamplesreachedcon-stantweightatMpalaResearchStationSamplesweremilledusingaMF10basicIKAgrinderfittedwitha1mmsieve

Weanalysedleafmaterialfornitrogen(N)usingaLecoTruSpecCNAnalyser(LECOCorporationStJosephMI)Driedleafdigest-ibility and leaf available N were determined as outlined in DeGabriel Wallis Moore and Foley (2008) but with some improvementsBriefly800plusmn10mgofdry leafmaterialwas transferred intopre‐weighed filterbags (ANKOMF57ANKOMTechnology) andheat‐sealedtolockinplantmaterialAmaximumof20bagswereplacedina1‐litrepolypropylenescrewcapcontainertowhich35mlofpepsinsolutionperbagwasaddedBagswereincubatedat37degCfor24hrwithcircular shaking inanorbital rotator (GFL3040Gesellschaftfuumlr LabortechnikmbHGermany) at 14rpmThe additionof rota-tional shaking far better simulates ruminal contraction or gut motil-itywhich is an importantphysiological adaptationwhichensuresconstant mixing of ingested food and probably aids in digestion(ClaussampHummel2005)Afterincubationbagswerewashedfivetimes with distilled water after which 35 ml of cellulose solution was addedtoeachbagandincubatedasoutlinedabovefor48hrAfterincubation bags were again washed 10 times with distilled water anddriedat60degCtoconstantweightToaccountforanylosstothebagintheincubationprocessablankbagwasincludedDrymatterdigestibility was calculated from the amount of material lost in the incubationprocessandleafavailableNwascalculatedbysubtract-ingNremainingintheresiduefromtheinitialtotalleafNWethencalculated how much of the total leaf N was available to herbivores onadryweightbasis andnamed this total availableN (ie leafN(mggminus1)timesproportionavailableN)Wemeasuredtheconcentrationoftotalcondensedtannins(CT)ineachleafsamplefollowingHattasandJulkunen‐Tiitto(2012)andtheconcentrationoftotalpolyphe-noliccompounds(TP)followingHattasStockMabuselaandGreen(2005)WeusedanAnkomfibreanalysertomeasureaciddetergentfibre (ADF)contentofall leafsampleswhichprovidesan indexoftheunpalatable fibrecomponentof leafmaterial and includes thecell‐wallcomponentsofcelluloseand lignin (CooperOwen‐SmithampBryant1988)

Tomeasurespecificleafareawecollected10ndash20healthyfullyexpanded sun‐exposed leaves from each plant and immediatelyscannedthemataresolutionof300dpiinthefieldusingaCanonCanoScanLiDE100flatbedscannerAfterscanningtheleaveswereovendried andweighedWemeasured the areaof the leaf scansusingBlackSpot(VarmaampOsuri2013)Wecalculatedspecificleafarea(SLA)bydividingleafareabydryweightforeachplant

We measured average spine lengths and average diameter atthespinebaseof10maturespinesoneachofthreebranchesperplantusingdigitalVerniercallipersWedeterminedspinedensitybycountingthetotalnumberofspinesonameasuredlengthofeachofthesamethreebranchesanddividingbythebranchlengthWecalculated branch density on three branches per tree by dividingthe number of lateral branches on each branch by the length of the terminalbranch(seePerez‐Harguindeguyetal2013)Thebitesizeindex(BSI)foragivenplantwasestimatedasthetotaldryweight

ofleavesremovedfrom10humanbitestakenfromeachplant(seeCharles‐DominiqueMidgleyampBond2015WigleyFritzCoetseeamp Bond 2014) An attempt was made to remove the maximumamountofleafmaterialwitheachbiteThesamepersonconductedallBSImeasurementstocontrolforpotentialdifferencesbetweenindividual recorders

23emsp|emspPlant growth measurements

At the timeof fenceconstruction in1999all individual treesandshrubs gt05m tall within a 50times50m area in each exclosure andpaired control site weremapped tagged and their basal area (at15 cm above-ground level including all stems on multi-stemmed individuals)canopydimensions(maximumlengthandwidthinthecardinaldirections)andplantheightweremeasuredAllplotswerefully censused again in 2002 2009 and 2016 During each cen-sustheheightbasalareaandcanopydimensionsofallpreviouslytaggedplantswereremeasuredandallnewrecruits(gt05m)werealsomeasuredandrecordedandallmortalitieswerenotedWecal-culatedthemeanchangeinplantheightandbasalareaforeachofthe six species using all individuals thatweremapped andmeas-uredin1999andthatwerestillpresentin2009ineachtreatmentat the three sites (A brevispica n = 359 A etbaica n = 349 A mel-lifera n = 326 B pedicellaris n = 24 G tenax n = 205 C Dichogamus n=54)By2009plantdensities inside theexclosureshadsignifi-cantlyincreasedtoalevelwherecompetitionbetweenplantswaslikely to have started affecting growth We therefore only usedgrowthdataupuntil2009toavoidtheeffectsofinterspecificandintraspecificcompetition

24emsp|emspStatistical analyses

AllanalyseswereperformedusingRversion331(RDevelopmentCore Team 2016) To testwhether species adopted different de-fence strategies and whether these strategies responded to her-bivore exclusionwe ran a principal component analysis (PCA) onthe nine measured defence traits using the function dudipca (ade4 packageforRDrayampDufour2007)WeranHornsParallelAnalysisusingthefunctionparan(paranpackageforR)toevaluatethenum-ber of components to be retained in the principal componentsanalysisWe first used theFlignerndashKilleen testofhomogeneityofvariance (flignertest in the stats package forR) to test if thedataused for treatment (h‐ vs h+) comparisonswere normally distrib-utedWhentheassumptionofnormalitywasmetweusedpairedt teststoevaluateeffectsofbrowserexclusiononthemeasuredplanttraitschangeinplantheightandwoodybasalareaforeachofthesixdominantwoodyspeciesWhentheassumptionofnormalitywasviolatedweused thenonparametricWilcoxon rank sum testWepooledtraitdatafromthethreeexclosuresitesforeachspeciesgiv-ing15individualssampledforeachtreatmentforfourofthespeciesA brevispica and C dichogamus only occurred at two of the three sitesandconsequentlyweonlyhad10individualssampledforeachtrait in each treatment

emspensp emsp | emsp5Journal of EcologyWIGLEY Et aL

3emsp |emspRESULTS

31emsp|emspDefence strategies

Results fromHornsParallelAnalysis for factor retentionbasedon 5000 iterations using the mean estimate showed that the first six components of the PCA should be retained The PCAbased on the measured plant traits showed that the six domi-nantspeciesatthestudysiteseparatedintothreebroadgroupsalong the first two PC axes which together accounted for al-most 60 of the total variance (Figure 1) This separationwasmost strongly driven by differences in spine length and fibre

onPC1 separating the twospecieswith long spinesand lowerfibre(A etbaica and B pedicellaris)fromthespecieswithhigherfibreandshortspines(A brevispica and A mellifera)ornospines(G tenax andC dichogamus (Table1)OnPC2 the groupsweremoststronglyseparatedbydifferencesinN‐freesecondaryme-tabolitesandspinedensityTwoofthebroad‐leavedspeciesG tenax and C dichogamus weresimilarintermsofhavingnospinesand lower concentrations of N-free secondary metabolites and weremoststronglyseparatedfromA brevispica and A mellifera whichhadhighN‐freesecondarymetabolitesandspinedensities(Figure1Table1)

F I G U R E 1 emsp (a)Plotofalineardiscriminant analysis of the measured defencetraitsshowingthegroupingofthesixMpalawoodyspeciesrelativetoWigleyetals(2018)dominantspeciesfromsouthernAfricansavannas(b)principalcomponentanalysis(PCA)basedonninemeasuredplantdefencetraitsofthesixwoodyplantspeciesgrowinginthepresence(black)andlong‐termabsence(grey)oflargemammalianherbivoresincentralLaikipiaCountyKenyaAxis1explained33ofthevarianceandwasprimarilyassociatedwithvariationinspinelengthandaciddetergentfibre(ADF)Axis2explained26ofthevarianceandwasprimarilyassociatedwithvariationincondensedtannins(CT)totalpolyphenols(TP)andspinedensity(seeTable1foreigenvectorscoresofthePCA)PointsrepresentthemeanlocationofeachspeciesonPCaxes1and2ateachofthethreepairedexclosureandcontrolsitesOvalsshowtheoverallmeans(centre)foreachspeciesandtreatmentanddispersion(ellipses)

TA B L E 1 emspEigenvectorscoresfortheprincipalcomponentanalysis(PCA)basedonninedefencetraits

PC1 (33) PC2 (26) PC3 (15) PC4 (10) PC5 (8) PC6 (4)

Spinelength(mm) minus053 minus005 019 minus007 010 025

Spinedensity(spinescmminus1) minus030 minus046 013 004 043 016

Total available N(mggminus1) 008 038 minus029 026 078 minus026

SLA(cm2 gminus1) 032 minus004 048 minus056 015 minus039

TP() 028 minus049 minus028 009 minus011 minus040

CT() 033 minus042 minus036 009 004 037

ADF() 048 013 007 minus030 024 061

BI(branchescmminus1) minus014 037 minus053 minus040 minus024 005

BSI(g) 028 028 037 059 minus023 012

NoteSLAspecificleafareaTPtotalpolyphenolsCTcondensedtanninsADFaciddetergentfibreBIbranchingindexBSIbitesizeindex

6emsp |emsp emspenspJournal of Ecology WIGLEY Et aL

32emsp|emspEffects of herbivore removal on plant defence traits and strategies

ThePCA(Figure1)illustratestheeffectsofherbivoreremovalandcontroltreatmentsontheplantdefencetraits(andstrategies)withherbivore removal treatments diverging from control treatments alongbothPCaxestovaryingdegrees(Figure1)Howevernoneofthespecieschangeddefencestrategiesasaresultofherbivorere-movalbothstructuralandchemicaldefencesrespondedtovariabledegrees both within and between strategies The long-term removal of large mammalian browsers from this savanna did not affect leaf totalNandaciddetergentfibreforanyofthesixofthewoodyspe-cies(Table2)Specificleafareadeclined(ieleavesbecamesmallerandthicker)withherbivoreexclusionforonefine‐leavedspecies(A mellifera p=005)andincreased(leavesbecamelargerandthinner)withherbivoreexclusionfortwobroad‐leavedspeciesB pedicellaris (p=003)andG tenax(p=009Figure2)HerbivoreremovalresultedinhighertotalpolyphenolsinA etbaica and A mellifera(p = 007 and p=001respectively)andsignificantlyhighercondensedtanninsinA brevispica(p=0003Figure2)andloweravailableNinA mellifera (plt01) andC dichogamus (plt001) Plant architectural defences(spinesandbranchingdensities)werefoundtorelaxwithherbivoreremoval ie became less structurally defended Herbivore removal decreasedthebranchingindexandbranchespervolume(plt0001)and increased the bite size index (iemore leaf biomass per bite

plt00001) for all speciesexceptC dichogamus (Figures3 and4) Herbivore removal resulted in significantly shorter spines inA et-baica(plt0001)andA mellifera (p=002)andasignificantlylowerdensityofspinesinA etbaica (p=001)(Figure3)

33emsp|emspGrowth defence strategies and responses to herbivore removal

Overthe10‐yearperiodbetween1999and2009wedocumentedsubstantialvariationingrowthwhichwepresent inrelationtothethreegroupsWhengrowing in thepresenceofherbivoresgroup1 (A brevispica and A mellifera) decreased in height (minus042plusmn007and minus010plusmn009m respectively) but increased in basal area(269plusmn116and 110plusmn656cm2) In this group herbivore removalresultedinsignificantlytallerplantswithhigherbasalareathanwithherbivorespresent(plt0001Table3Figure5)Group2(A etbaica and B pedicellaris)increasedinheight(031plusmn006and024plusmn044mrespectively)andbasalarea(209plusmn314and346plusmn124cm2)inthepresenceofherbivores Inthisgroupherbivoreremoval increasedplantheight(plt001)butnotbasalarea(Table3Figure5)Plantsingroup3(G tenax and C dichogamus)generallydecreasedinheightbut no consistent trend was evident for basal area when herbivores were present ForG tenax herbivore removal resulted in signifi-cantlytallerplantswithhigherbasalareas(plt0001)C dichogamus plantsshowedaslightdecreaseinheightandincreaseinbasalarea

TA B L E 2 emsp Mean plusmn SEforleaftotalavailablenitrogen(mggminus1)leaftotalnitrogen(mggminus1)specificleafarea(SLAcm2 gminus1)leaftotalpolyphenolics(TP)leafcondensedtannins(CT)leafaciddetergentfibre(ADF)branchingindex(BIbranchescm-1)bitesizeindex(BSIg)andaveragespinelength(ASLmm)

Plant trait treatment Aca bre (1) Aca mel (1) Aca etb (2) Bal ped (2) Gre ten (3) Cro dic (3)

Total available N

In(h‐) 234 plusmn 142 156 plusmn 106 156 plusmn 086 215 plusmn 24 226 plusmn 099 126 plusmn 024

Out(h+) 248 plusmn 111 185 plusmn 105 152 plusmn 114 235 plusmn 171 220 plusmn 093 167 plusmn 082

Leaf total N In(h‐) 361 plusmn 220 344 plusmn 088 329 plusmn 090 283 plusmn 190 340 plusmn 090 214 plusmn 030

Out(h+) 394 plusmn 130 352 plusmn 060 320 plusmn 080 312 plusmn 150 322 plusmn 090 234 plusmn 070

SLA In(h‐) 102 plusmn 734 107 plusmn 544 141 plusmn 786 546 plusmn 507 150 plusmn 514 147 plusmn 600

Out(h+) 119 plusmn 809 118 plusmn 302 139 plusmn 120 407 plusmn 252 136 plusmn 712 146 plusmn 600

TP In(h‐) 331 plusmn 046 626 plusmn 025 155 plusmn 009 055 plusmn 003 109 plusmn 014 126 plusmn 006

Out(h+) 278 plusmn 030 535 plusmn 022 136 plusmn 007 055 plusmn 005 129 plusmn 013 142 plusmn 005

CT In(h‐) 472 plusmn 057 658 plusmn 104 038 plusmn 004 008 plusmn 0007 142 plusmn 022 033 plusmn 003

Out(h+) 248 plusmn 037 715 plusmn 051 041 plusmn 004 009 plusmn 001 154 plusmn 022 037 plusmn 001

ADF In(h‐) 329 plusmn 239 371 plusmn 198 304 plusmn 129 246 plusmn 089 430 plusmn 084 338 plusmn 045

Out(h+) 350 plusmn 216 336 plusmn 183 288 plusmn 158 241 plusmn 060 420 plusmn 105 324 plusmn 116

BI In(h‐) 007 plusmn 003 009 plusmn 003 004 plusmn 0007 025 plusmn 003 016 plusmn 002 067 plusmn 016

Out(h+) 045 plusmn 005 038 plusmn 004 032 plusmn 004 125 plusmn 008 119 plusmn 018 056 plusmn 007

BSI In(h‐) 300 plusmn 023 201 plusmn 015 081 plusmn 006 249 plusmn 023 557 plusmn 048 534 plusmn 05

Out(h+) 087 plusmn 008 037 plusmn 003 011 plusmn 001 044 plusmn 004 108 plusmn 008 427 plusmn 049

ASL In(h‐) 346 plusmn 006 373 plusmn 004 529 plusmn 101 547 plusmn 125 mdash mdash

Out(h+) 347 plusmn 007 385 plusmn 003 586 plusmn 090 550 plusmn 086 mdash mdash

NoteTreatmentsareherbivoresexcluded(h‐)andherbivorespresent(h+)GroupmembershipisindicatedinparenthesesnexttoeachspeciesSignificance levels are indicated byp lt 01 p lt 005 p lt 001 p lt 0001 and are highlighted in bold

emspensp emsp | emsp7Journal of EcologyWIGLEY Et aL

duringthestudyperiodwithherbivorespresenthoweverneitherofthe measures of growth differed between the herbivore removal and herbivores present treatments (pgt005) Based on themeasuredchanges inplantheight andbasal area in thepresenceversus ab-sence of browsers over the course of a decade we ranked the three groupsfromlowtohighintermsoftheircapacitytopersistinthefaceofintensebrowsingpressurewheregroup1ltgroup3ltgroup2(Table3Figure5)

4emsp |emspDISCUSSION

OuroverarchinggoalwastoexaminehowwoodyspeciesinthisEastAfricansavannausesuitesoftraitstocontendwitharelativelyintensebrowsing regime imposedbyadiverseandabundantassemblageoflargemammalianherbivoresThehypotheseswhichunderpinclassicplant defence theory rely heavily on resource availability to predictwhetherplantsinvestingrowthordefence(HermsampMattson1992)and hence do not predict relative investments in structural versuschemicaldefences(Hanleyetal2007)orwhichdefenceswillbemosteffectiveunderfixedresourcelevelsbutvaryingherbivorypressureOurresultsindicatethat(a)woodyspeciesinthissystemcanbeclas-sifiedintothreedifferentgroupsorstrategiesbasedontheirdifferen-tialinvestmentinstructuralandchemicaldefences(b)structuralandchemical defences responded to varying degrees to the removal ofherbivoresbothwithinandacrossgroupsbutplantdefencestrategies

remainedqualitativelyunchangedeven after nearly twodecadesofherbivoreexclusion (c) structuralandchemicaldefencesdidnot re-spondinthesamewaytoherbivoreremovalwithstructuraltraits(es-peciallybranching)typicallyrespondingmorepositively(ieincreased)and (d) thespecies thatweremost resilient to intensebrowsing (ieachievedthehighestgrowth)werethosethatmaintainedhighspinelengthspinedensityandbranchinginthepresenceofherbivores

Noneofthespeciesinourstudysiteappeartobeadoptingtheldquolow nutrienthigh chemical defencerdquo strategy with most specieshavingmoderatetohighleafNcomparedtospeciespreviouslycat-egorised as low nutrienthigh chemical defence strategists in other savannasystems(Wigleyetal2018Figure1a)Thethreestrate-giesweidentifiedthusallfallwithinthebroadremitofldquonutritionanddefencerdquobutspeciesinthesedifferentgroupsappeartoinvestvari-ablyinstructuralandN‐freechemicaldefencesFine‐leavedspeciessuch as A brevispica and A mellifera (group1)havemoderatetohighleafNlowinvestmentinstructuraldefences(shortspines)andhighinvestmentinN‐freechemicaldefences(highCTandTP)A etbaica and B pedicellaris(group2)similarlyhadmoderatetohighleafNandbothspeciesinvestedmoreinphysicaldefences(denselongspinesthorns)thanchemicaldefences(lowN‐freesecondarymetabolites)Finally species suchasG tenax and C dichogamus (group3 bothbroad‐leaved)hadmoderatetohighleafNnospinesandlowN‐freesecondary metabolites

Basedonourmeasuredchanges inplantheightandbasalareainthepresencecomparedtoabsenceofbrowsersoverthecourse

F I G U R E 2 emsp Mean plusmn SE total available leafnitrogen(totalNxavailableNproportion(a)specificleafarea(b)totalpolyphenols(c)andcondensedtannins(d)insideandoutsideoftheexclosuresAcabreAcacia brevispicaAcamelAcacia melliferaAcaetbAcacia etbaica BalpedBalanites pedicellaris Gre ten Grewia tenax Cro dic Croton dichogamus Significance levels are p lt 01 p lt 005 p lt 001

8emsp |emsp emspenspJournal of Ecology WIGLEY Et aL

ofonedecadewerankedthesixwoodyspeciesfromlowtohighintermsoftheircapacitytocoexistwithintensebrowsingpressurewhere G tenaxltA brevispicaltA melliferaltC dichogamusltB pedi-cellarisltA etbaica (Figure5)Our results for the twobroad‐leavedspeciesthat lack investment inspines(group3G tenax and C di-chogamus)provideseveralkeyinsightsregardingstrategiesforldquolivingwithbrowsersrdquoBasedontheframeworkdevelopedfromsouthernAfrican savannas (Wigley et al 2018) we expected both speciesto invest heavily in N-free secondary metabolites as a trade-off to the lackof armamentContrary to thispredictionneither speciesinvestedinN‐freesecondarymetaboliteseitherinthepresenceorabsence of large herbivores

LowerleafN(bothtotalandavailable)wasoneconspicuoustraitdistinguishing Crotonfromtheotherspeciesinthisstudyandmaycontributetothefactthatthisspeciesisgenerallyunpalatableanduneatenbylargebrowsers(Kartzineletal2014)HoweverleafN(~22)wasnot so lowas tocompromise ruminantdigestioneffi-ciency(whichtypicallyoccurscloserto1NVanSoest1994)sug-gesting thatsomeotherpotentiallycostlyandasyetunmeasuredaspect ofCroton leaf chemistry is a key trait allowing this broad‐leavedspeciestocoexistwithlargebrowsersCroton sppingeneralarehighlyaromaticandknowntocontainmultipleuniquesecondarychemicals including cembranoids halimanes crotofolanes sesqui-terpenoids flavonoids and cyclohexanol derivatives (Langat et al2016)someofwhichareverysuccessfulinlimitingvertebrateand

invertebrate herbivory (Kaplan Halitschke Kessler Sardanelli ampDenno 2008 Levin 1976) Including these types of chemical de-fences in a generalised trait framework will be challenging as they arenotubiquitousacrossspeciesandtheirexpressioncandependonspecificherbivoreelicitors(Moreiraetal2013)

The lack of investment in N-free secondary metabolites by the secondbroad‐leavedspeciesG tenaxwasalsosurprisingbutcon-sistentwithourfindingthatgrowthofthisspeciesisseverelyneg-ativelyaffectedbylargebrowsersandwithpreviousworkshowingsubstantial declines in all size classes of G tenaxinthepresenceofbrowsersinthissavanna(AugustineampMcNaughton2004Sankaranetal2013)Ratherthanemployinganyformofcostlychemicalde-fence this species appears to coexist (uneasily)with browsers byincreasing the complexity of its branching architecture (BI) andgrowing in close association with other thorny species that cre-ate structural refugia where G tenax saplings are protected frombrowsers (personalobservationbyallauthorsseealsoCoverdaleGoheenPalmerampPringle2018)OnevalueofthisstrategyisthatwhenbrowsingpressureisremovedorlowthelackofinvestmentincostlydefencescombinedwithhighleafNallowsforrapidgrowth

TheremainingfourspeciesbelongingtotheothertwogroupsallhavesomeformofinvestmentinspinesTheycanbearrayedalongagradientofincreasingspineinvestmentfromgroup1togroup2withspeciesingroup1consistingofthosewithshortstraightspines(A brevispica) or short recurved spines (A mellifera) and those in

F I G U R E 3 emsp Mean plusmn SEspinelength(a)spinediameter(b)bitesizeindex(c)andbranchdensity(d)insideandoutsidetheexclosuresAcabreAcacia brevispica AcamelAcacia mellifera AcaetbAcacia etbaicaBalpedBalanites pedicellaris Gre ten Grewia tenax Cro dic Croton dichogamus Significance levels are p lt 005 p lt 001 p lt 0001

(a) (b)

(c) (d)

emspensp emsp | emsp9Journal of EcologyWIGLEY Et aL

F I G U R E 4 emspExamplesofAcacia brevispica and Balanites pedicellaris growing in the absence of large mammalianherbivores(ac)andthesamespeciesgrowinginthepresenceoflargemammalianherbivores(bd)incentralLaikipiaCountyKenyaNotethecleardifferences in architecture branching densities and browser damage between the treatments

(a) (b)

(c) (d)

TA B L E 3 emsp Mean plusmn SEforplantheightandbasalareain1999and2009forthesixspeciesgrowingwithandwithoutherbivorespresentnetchangesinplantheightandbasalareabetweenthe2yearsarealsoshown

Species Treatment Height 1999 (m) Height 2009 (m)Basal area 1999 (cm2)

Basal area 2009 (cm2) Δ height Δ basal area

Acacia brevispica Out(h+) 195 plusmn 007 152 plusmn 005 1379 plusmn 174 1649 plusmn 209 minus043plusmn007 269 plusmn 117

Acacia brevispica In(h‐) 201 plusmn 006 297 plusmn 008 991 plusmn 096 164 plusmn 147 095 plusmn 006 649 plusmn 088

Acacia mellifera Out(h+) 228 plusmn 011 218 plusmn 01 8368 plusmn 1598 9465 plusmn 134 minus010plusmn009 1097 plusmn 657

Acacia mellifera In(h‐) 219 plusmn 009 349 plusmn 01 8048 plusmn 1331 12236 plusmn 1492 13 plusmn 005 4188 plusmn 611

Acacia etbaica Out(h+) 168 plusmn 008 199 plusmn 01 3699 plusmn 616 5786 plusmn 715 031 plusmn 006 2087 plusmn 314

Acacia etbaica In(h‐) 157 plusmn 007 305 plusmn 007 3039 plusmn 754 5183 plusmn 77 148 plusmn 006 2117 plusmn 323

Balanaites pedicellaris Out(h+) 25 plusmn 07 286 plusmn 05 8823 plusmn 4682 12282 plusmn 5008 036 plusmn 047 3459 plusmn 1242

Balanaites pedicellaris In(h‐) 196 plusmn 028 401 plusmn 026 2542 plusmn 829 7193 plusmn 1645 205 plusmn 023 4650 plusmn 1283

Grewia tenax Out(h+) 131 plusmn 009 095 plusmn 024 336 plusmn 044 296 plusmn 048 minus036plusmn024 minus041plusmn035

Grewia tenax In(h‐) 116 plusmn 005 24 plusmn 008 194 plusmn 023 578 plusmn 05 124 plusmn 007 384 plusmn 044

Croton dichogamus Out(h+) 216 plusmn 022 21 plusmn 024 2393 plusmn 831 3323 plusmn 829 minus007plusmn014 929 plusmn 217

Croton dichogamus In(h‐) 159 plusmn 008 182 plusmn 012 932 plusmn 267 1346 plusmn 317 023 plusmn 011 414 plusmn 116

NoteAllmeasurementsarebasedonthesameindividualplantsofeachspeciesthatwerepresentintheplotsin1999andthatwerestillalivein2009

10emsp |emsp emspenspJournal of Ecology WIGLEY Et aL

group2havingeither long straight spines (B pedicellaris)orbothshort recurvedand long straight spines (A etbaica) Specieswiththelongestandmostdensespines(group2)showedlittletonoin-vestmentinCTorTPwhilethespecieswithlowspineinvestment(group 1) showed the greatest concentrations of bothCT and TP(Figure2)The latterstrategyexperiencesgreaterconsumptionbybrowsersinthislandscaperelativetotheformer(Fordetal2014)consistentwithpreviousstudieswhichhavereportedthatspecieswithhigherinvestmentinstructuraldefences(group2)aremostsuc-cessfulatcoexistingwithbrowsersinthissavannasystem(GoheenKeesingAllanOgadaampOstfeld2004Sankaranetal2013)Inthepresenceofbrowsersspeciesinbothgroups1and2maintainedanincreasedcomplexityoftheirbranchingarchitecturecombinedwithsmallerleaves(reflectedinsubstantialreductionsinbitesizeindex)Increased branching even in the absence of changes in thorn length anddensitywhichoccurredinsomespeciesbutnototherscanre-sultindramaticincreasesinthenumberofspinesperunitvolumeofcanopy(ArchibaldampBond2003StaverBondCramerampWakeling2012)servingasaneffectivedeterrentforbrowsers

To summarise the strategies expressed by spinescent speciesin termsof ldquolivingwithbrowsersrdquo canbecharacterisedas (a)veryhigh and inducible investment in spinesbutnotN‐free secondarymetabolites and low available N leading to greatest success under intensebrowsing(A etbaica)(b)moderateandinducibleinvestmentinstraightspinescombinedwith thickermore fibrous leavesbuthigh available N leading to sufficient success under intense brows-ing(B pedicellaris)(c)lowandnon‐inducibleinvestmentinrecurvedspines combinedwithhighN‐free secondarymetabolites and lowavailableNleadingtoreducedsuccessunderbrowsing(A mellifera)

and(d)lowandnon‐inducibleinvestmentinshortspinescombinedwith moderate investment in N-free secondary metabolites and high availableN leading to low success under browsing (A brevispica)Fornara and Du Toit (2008) suggested that physical defences to-getherwithmasscompensatorygrowthabilitiesarekeyadaptationstolivingwithhighbrowsingpressureOurfindingslendstrongsup-porttothisnotionasthemostheavilystructurallydefendedspecies(A etbaica and B pedicellaris)performedthebestinthepresenceofherbivoresdespitelowinvestmentsinN‐freesecondarymetabolitesHowever Scogingset al (2011)predicted that the concentrationsofN‐freesecondarycompoundswoulddependonbrowsinginten-sity with the highest concentrations occurring at intermediate levels ofbrowsingforcertainfunctionalgroupsForexampleScogingsetal(2011)predictalinearincreaseinN‐freesecondarycompoundswithbrowsingintensitywithapotentiallysuddendecreaseatveryhigh levelsofbrowsingpressure indeciduoussavannaspeciesAsourexperimentaldesign lackedan intermediatebrowsingpressuretreatmentwewereunabletotestthesepredictions

Our analyses of available Nprovidedsomeinterestingandvalu-ableinsightsFirstlyinterpretationsofbrowsequalitydifferdepend-ing on whether total N (also commonly converted to crudeproteinby multiplying by 625 eg Cooper et al 1988) or available N isevaluatedForexamplethethreeAcaciaspeciesinthisstudyA bre-vispica A mellifera and A etbaica all had similar and relatively high con-centrations of leaf total N (~3835and33respectively)buttheproportionofthetotalN available to herbivores in A brevispica was ca 20 higher than in A mellifera and A etbaica(~2417and15respectively seeTable2) Ifweonly lookedat totalN they would all appear tobeofhighqualityhowever ifwe lookat availableN

F I G U R E 5 emsp (a)MeanplusmnSE change in plantheightbetween1999and2009forthesixdominantspeciesgrowinginsideandoutsidetheexclosures(b)Mean plusmn SE change in stem basal area between 1999 and 2009 for the same six speciesSignificancelevelsarep lt 001 p lt 0001

emspensp emsp | emsp11Journal of EcologyWIGLEY Et aL

itappearsasifA brevispicaisofmuchhigherqualitythantheothertwospeciesSecondlyavailableNappearstobemoreresponsivetoherbivore removal than total leaf N No differences in total N were evident between herbivory treatments while available N was signifi-cantly higher in A mellifera and C dichogamus when herbivores were presentThirdlydifferencesinavailableNdonotseemtobelinkedtophylogeny(eghighinsomeAcaciaspecieslowinothers)functionaltype(egfine‐leavedvsbroad‐leaved)norconcentrationsofN‐freesecondarymetabolites (availableNfoundtobe low inspecieswithbothhighand lowN‐freesecondarymetabolites)Finally these re-sults raise some importantquestions regarding the commonlyheldview (especially in African savannas) that fine‐leaved species (egAcacia)havehigherqualityleavesandarestructurallydefendedwhilebroad‐leavedspeciesarelessnutritiousandchemicallydefendedForexampleG tenax and B pedicellarisbothbroad‐leavedspecieshadhigher available Nthanthetwofine‐leavedspeciesA mellifera and A etbaica These findings highlight the need for further studies to eluci-date the determinants of leaf available N and that caution needs to be taken when using total NasameasureofbrowsequalityFurthermoreour findings suggest that neither total N nor available N are sufficient bythemselvestopredictbrowserutilisationhighlightingtheneedtoconsideralltraitscollectively(iedefencestrategies)

5emsp |emspCONCLUSIONS

Allofthedominantspeciesatourstudysitehadmoderatetohighleaf N concentrations but differed in available N and in the way they combined chemical and structural defences thus fitting under thebroaderldquonutritionanddefencerdquosyndromepreviouslydescribedforsavannasStructuraldefenceshadmorepronouncedresponsesto herbivory through increased spine length and density andorincreased branching while N-free secondary metabolites did not increaseinresponsetoherbivoryHighinvestmentinstructuralde-fences was the most successful defence strategy at our study sites withmoderateresourcelevels(relativetootherAfricansavannas)andhighherbivorypressureresultinginthehighestgrowthinthissavanna system This work has shown that within one overarching defence syndrome (nutritionanddefence) specieswithina com-munity can widely diverge in their adopted defence strategiesEndaraetal(2015)reportedhighlevelsofdissimilarityinthede-fencesof closely relatedco‐occurring forest speciesand suggestthat this would be necessary for the coexistence of closely related neighboursandcouldaccountforthehighlocaldiversityoftropicalforests Similarly we suggest that diverse defence strategies en-able savanna species (which are often closely related) to occupydifferentnichesanddefendagainstdifferent typesofherbivoresresultinginmoreresilientandspeciesrichwoodycommunities

ACKNOWLEDG EMENTS

WethankJParsareforassistanceinthefieldandChengappaSKand Balavignesh B for their assistance with laboratory analyses

This work was funded by the National Centre for Biological Sciences TIFR India andbyNationalGeographic (GrantNumber982815)andNERC (GrantNE‐E017436‐1)Wearehugely grateful to theOfficeofthePresidentoftheRepublicofKenyaforpermissiontoconducttheresearchandtheMpalaResearchCentreforlogisti-calsupport

AUTHORSrsquo CONTRIBUTIONS

BJWCCDJAandMSco‐wrotethemanuscriptBJWandMS collected the data DJA MS and JR implemented theexclosure experiment DH conceptualised and performed theanalyticalapproacheswithrespecttofibreandavailableNdeter-mination andperformed condensed tannin and total polyphenolanalysesAllauthorscommentedandaddedtoearlierversionsofthemanuscript

DATA ACCE SSIBILIT Y

Data available from the Dryad Digital Repository httpsdoiorg105061dryadp2d00sf(Wigleyetal2019)

ORCID

Benjamin J Wigley httpsorcidorg0000‐0002‐6964‐3624

Mahesh Sankaran httpsorcidorg0000‐0002‐1661‐6542

R E FE R E N C E S

Agrawal A A (2011) Current trends in the evolutionary ecology ofplant defence Functional Ecology 25(2) 420ndash432 httpsdoiorg101111j1365-2435201001796x

Agrawal A A amp Fishbein M (2006) Plant defense syndromesEcology 87(sp7) 132ndash149 httpsdoiorg1018900012‐9658 (2006)87[132PDS]20CO2

Archibald S ampBond W J (2003) Growing tall vs growing wideTree architecture and allometry of Acacia karroo in forest sa-vanna and arid environments Oikos 102(1) 3ndash14 httpsdoiorg101034j1600-0706200312181x

AugustineDJ(2003)Long‐termlivestock‐mediatedredistributionofnitro-genandphosphorusinanEastAfricansavannaJournal of Applied Ecology 40(1)137ndash149httpsdoiorg101046j1365‐2664200300778x

AugustineDJ(2010)Responseofnativeungulatestodroughtinsemi‐arid Kenyan rangeland African Journal of Ecology 48(4)1009ndash1020httpsdoiorg101111j1365‐2028201001207x

AugustineDJampMcNaughtonSJ(2004)RegulationofshrubdynamicsbynativebrowsingungulatesonEastAfricanrangelandJournal of Applied Ecology 41(1)45ndash58httpsdoiorg101111j1365‐2664200400864x

AugustineDJampMcNaughtonSJ(2006)Interactiveeffectsofungu-lateherbivoressoilfertilityandvariablerainfallonecosystempro-cesses in a semi-arid savanna Ecosystems 9(8)1242ndash1256httpsdoiorg101007s10021-005-0020-y

BartonKE (2016)TougherandthornierGeneralpatterns in the in-ductionofphysicaldefencetraitsFunctional Ecology 30(2)181ndash187httpsdoiorg1011111365‐243512495

BryantJPChapinFSIIIampKleinDR(1983)Carbonnutrientbal-anceofborealplantsinrelationtovertebrateherbivoryOikos 40(3)357ndash368httpsdoiorg1023073544308

4emsp |emsp emspenspJournal of Ecology WIGLEY Et aL

analysesAll leafmaterialwasair‐drieduntilsamplesreachedcon-stantweightatMpalaResearchStationSamplesweremilledusingaMF10basicIKAgrinderfittedwitha1mmsieve

Weanalysedleafmaterialfornitrogen(N)usingaLecoTruSpecCNAnalyser(LECOCorporationStJosephMI)Driedleafdigest-ibility and leaf available N were determined as outlined in DeGabriel Wallis Moore and Foley (2008) but with some improvementsBriefly800plusmn10mgofdry leafmaterialwas transferred intopre‐weighed filterbags (ANKOMF57ANKOMTechnology) andheat‐sealedtolockinplantmaterialAmaximumof20bagswereplacedina1‐litrepolypropylenescrewcapcontainertowhich35mlofpepsinsolutionperbagwasaddedBagswereincubatedat37degCfor24hrwithcircular shaking inanorbital rotator (GFL3040Gesellschaftfuumlr LabortechnikmbHGermany) at 14rpmThe additionof rota-tional shaking far better simulates ruminal contraction or gut motil-itywhich is an importantphysiological adaptationwhichensuresconstant mixing of ingested food and probably aids in digestion(ClaussampHummel2005)Afterincubationbagswerewashedfivetimes with distilled water after which 35 ml of cellulose solution was addedtoeachbagandincubatedasoutlinedabovefor48hrAfterincubation bags were again washed 10 times with distilled water anddriedat60degCtoconstantweightToaccountforanylosstothebagintheincubationprocessablankbagwasincludedDrymatterdigestibility was calculated from the amount of material lost in the incubationprocessandleafavailableNwascalculatedbysubtract-ingNremainingintheresiduefromtheinitialtotalleafNWethencalculated how much of the total leaf N was available to herbivores onadryweightbasis andnamed this total availableN (ie leafN(mggminus1)timesproportionavailableN)Wemeasuredtheconcentrationoftotalcondensedtannins(CT)ineachleafsamplefollowingHattasandJulkunen‐Tiitto(2012)andtheconcentrationoftotalpolyphe-noliccompounds(TP)followingHattasStockMabuselaandGreen(2005)WeusedanAnkomfibreanalysertomeasureaciddetergentfibre (ADF)contentofall leafsampleswhichprovidesan indexoftheunpalatable fibrecomponentof leafmaterial and includes thecell‐wallcomponentsofcelluloseand lignin (CooperOwen‐SmithampBryant1988)

Tomeasurespecificleafareawecollected10ndash20healthyfullyexpanded sun‐exposed leaves from each plant and immediatelyscannedthemataresolutionof300dpiinthefieldusingaCanonCanoScanLiDE100flatbedscannerAfterscanningtheleaveswereovendried andweighedWemeasured the areaof the leaf scansusingBlackSpot(VarmaampOsuri2013)Wecalculatedspecificleafarea(SLA)bydividingleafareabydryweightforeachplant

We measured average spine lengths and average diameter atthespinebaseof10maturespinesoneachofthreebranchesperplantusingdigitalVerniercallipersWedeterminedspinedensitybycountingthetotalnumberofspinesonameasuredlengthofeachofthesamethreebranchesanddividingbythebranchlengthWecalculated branch density on three branches per tree by dividingthe number of lateral branches on each branch by the length of the terminalbranch(seePerez‐Harguindeguyetal2013)Thebitesizeindex(BSI)foragivenplantwasestimatedasthetotaldryweight

ofleavesremovedfrom10humanbitestakenfromeachplant(seeCharles‐DominiqueMidgleyampBond2015WigleyFritzCoetseeamp Bond 2014) An attempt was made to remove the maximumamountofleafmaterialwitheachbiteThesamepersonconductedallBSImeasurementstocontrolforpotentialdifferencesbetweenindividual recorders

23emsp|emspPlant growth measurements

At the timeof fenceconstruction in1999all individual treesandshrubs gt05m tall within a 50times50m area in each exclosure andpaired control site weremapped tagged and their basal area (at15 cm above-ground level including all stems on multi-stemmed individuals)canopydimensions(maximumlengthandwidthinthecardinaldirections)andplantheightweremeasuredAllplotswerefully censused again in 2002 2009 and 2016 During each cen-sustheheightbasalareaandcanopydimensionsofallpreviouslytaggedplantswereremeasuredandallnewrecruits(gt05m)werealsomeasuredandrecordedandallmortalitieswerenotedWecal-culatedthemeanchangeinplantheightandbasalareaforeachofthe six species using all individuals thatweremapped andmeas-uredin1999andthatwerestillpresentin2009ineachtreatmentat the three sites (A brevispica n = 359 A etbaica n = 349 A mel-lifera n = 326 B pedicellaris n = 24 G tenax n = 205 C Dichogamus n=54)By2009plantdensities inside theexclosureshadsignifi-cantlyincreasedtoalevelwherecompetitionbetweenplantswaslikely to have started affecting growth We therefore only usedgrowthdataupuntil2009toavoidtheeffectsofinterspecificandintraspecificcompetition

24emsp|emspStatistical analyses

AllanalyseswereperformedusingRversion331(RDevelopmentCore Team 2016) To testwhether species adopted different de-fence strategies and whether these strategies responded to her-bivore exclusionwe ran a principal component analysis (PCA) onthe nine measured defence traits using the function dudipca (ade4 packageforRDrayampDufour2007)WeranHornsParallelAnalysisusingthefunctionparan(paranpackageforR)toevaluatethenum-ber of components to be retained in the principal componentsanalysisWe first used theFlignerndashKilleen testofhomogeneityofvariance (flignertest in the stats package forR) to test if thedataused for treatment (h‐ vs h+) comparisonswere normally distrib-utedWhentheassumptionofnormalitywasmetweusedpairedt teststoevaluateeffectsofbrowserexclusiononthemeasuredplanttraitschangeinplantheightandwoodybasalareaforeachofthesixdominantwoodyspeciesWhentheassumptionofnormalitywasviolatedweused thenonparametricWilcoxon rank sum testWepooledtraitdatafromthethreeexclosuresitesforeachspeciesgiv-ing15individualssampledforeachtreatmentforfourofthespeciesA brevispica and C dichogamus only occurred at two of the three sitesandconsequentlyweonlyhad10individualssampledforeachtrait in each treatment

emspensp emsp | emsp5Journal of EcologyWIGLEY Et aL

3emsp |emspRESULTS

31emsp|emspDefence strategies

Results fromHornsParallelAnalysis for factor retentionbasedon 5000 iterations using the mean estimate showed that the first six components of the PCA should be retained The PCAbased on the measured plant traits showed that the six domi-nantspeciesatthestudysiteseparatedintothreebroadgroupsalong the first two PC axes which together accounted for al-most 60 of the total variance (Figure 1) This separationwasmost strongly driven by differences in spine length and fibre

onPC1 separating the twospecieswith long spinesand lowerfibre(A etbaica and B pedicellaris)fromthespecieswithhigherfibreandshortspines(A brevispica and A mellifera)ornospines(G tenax andC dichogamus (Table1)OnPC2 the groupsweremoststronglyseparatedbydifferencesinN‐freesecondaryme-tabolitesandspinedensityTwoofthebroad‐leavedspeciesG tenax and C dichogamus weresimilarintermsofhavingnospinesand lower concentrations of N-free secondary metabolites and weremoststronglyseparatedfromA brevispica and A mellifera whichhadhighN‐freesecondarymetabolitesandspinedensities(Figure1Table1)

F I G U R E 1 emsp (a)Plotofalineardiscriminant analysis of the measured defencetraitsshowingthegroupingofthesixMpalawoodyspeciesrelativetoWigleyetals(2018)dominantspeciesfromsouthernAfricansavannas(b)principalcomponentanalysis(PCA)basedonninemeasuredplantdefencetraitsofthesixwoodyplantspeciesgrowinginthepresence(black)andlong‐termabsence(grey)oflargemammalianherbivoresincentralLaikipiaCountyKenyaAxis1explained33ofthevarianceandwasprimarilyassociatedwithvariationinspinelengthandaciddetergentfibre(ADF)Axis2explained26ofthevarianceandwasprimarilyassociatedwithvariationincondensedtannins(CT)totalpolyphenols(TP)andspinedensity(seeTable1foreigenvectorscoresofthePCA)PointsrepresentthemeanlocationofeachspeciesonPCaxes1and2ateachofthethreepairedexclosureandcontrolsitesOvalsshowtheoverallmeans(centre)foreachspeciesandtreatmentanddispersion(ellipses)

TA B L E 1 emspEigenvectorscoresfortheprincipalcomponentanalysis(PCA)basedonninedefencetraits

PC1 (33) PC2 (26) PC3 (15) PC4 (10) PC5 (8) PC6 (4)

Spinelength(mm) minus053 minus005 019 minus007 010 025

Spinedensity(spinescmminus1) minus030 minus046 013 004 043 016

Total available N(mggminus1) 008 038 minus029 026 078 minus026

SLA(cm2 gminus1) 032 minus004 048 minus056 015 minus039

TP() 028 minus049 minus028 009 minus011 minus040

CT() 033 minus042 minus036 009 004 037

ADF() 048 013 007 minus030 024 061

BI(branchescmminus1) minus014 037 minus053 minus040 minus024 005

BSI(g) 028 028 037 059 minus023 012

NoteSLAspecificleafareaTPtotalpolyphenolsCTcondensedtanninsADFaciddetergentfibreBIbranchingindexBSIbitesizeindex

6emsp |emsp emspenspJournal of Ecology WIGLEY Et aL

32emsp|emspEffects of herbivore removal on plant defence traits and strategies

ThePCA(Figure1)illustratestheeffectsofherbivoreremovalandcontroltreatmentsontheplantdefencetraits(andstrategies)withherbivore removal treatments diverging from control treatments alongbothPCaxestovaryingdegrees(Figure1)Howevernoneofthespecieschangeddefencestrategiesasaresultofherbivorere-movalbothstructuralandchemicaldefencesrespondedtovariabledegrees both within and between strategies The long-term removal of large mammalian browsers from this savanna did not affect leaf totalNandaciddetergentfibreforanyofthesixofthewoodyspe-cies(Table2)Specificleafareadeclined(ieleavesbecamesmallerandthicker)withherbivoreexclusionforonefine‐leavedspecies(A mellifera p=005)andincreased(leavesbecamelargerandthinner)withherbivoreexclusionfortwobroad‐leavedspeciesB pedicellaris (p=003)andG tenax(p=009Figure2)HerbivoreremovalresultedinhighertotalpolyphenolsinA etbaica and A mellifera(p = 007 and p=001respectively)andsignificantlyhighercondensedtanninsinA brevispica(p=0003Figure2)andloweravailableNinA mellifera (plt01) andC dichogamus (plt001) Plant architectural defences(spinesandbranchingdensities)werefoundtorelaxwithherbivoreremoval ie became less structurally defended Herbivore removal decreasedthebranchingindexandbranchespervolume(plt0001)and increased the bite size index (iemore leaf biomass per bite

plt00001) for all speciesexceptC dichogamus (Figures3 and4) Herbivore removal resulted in significantly shorter spines inA et-baica(plt0001)andA mellifera (p=002)andasignificantlylowerdensityofspinesinA etbaica (p=001)(Figure3)

33emsp|emspGrowth defence strategies and responses to herbivore removal

Overthe10‐yearperiodbetween1999and2009wedocumentedsubstantialvariationingrowthwhichwepresent inrelationtothethreegroupsWhengrowing in thepresenceofherbivoresgroup1 (A brevispica and A mellifera) decreased in height (minus042plusmn007and minus010plusmn009m respectively) but increased in basal area(269plusmn116and 110plusmn656cm2) In this group herbivore removalresultedinsignificantlytallerplantswithhigherbasalareathanwithherbivorespresent(plt0001Table3Figure5)Group2(A etbaica and B pedicellaris)increasedinheight(031plusmn006and024plusmn044mrespectively)andbasalarea(209plusmn314and346plusmn124cm2)inthepresenceofherbivores Inthisgroupherbivoreremoval increasedplantheight(plt001)butnotbasalarea(Table3Figure5)Plantsingroup3(G tenax and C dichogamus)generallydecreasedinheightbut no consistent trend was evident for basal area when herbivores were present ForG tenax herbivore removal resulted in signifi-cantlytallerplantswithhigherbasalareas(plt0001)C dichogamus plantsshowedaslightdecreaseinheightandincreaseinbasalarea

TA B L E 2 emsp Mean plusmn SEforleaftotalavailablenitrogen(mggminus1)leaftotalnitrogen(mggminus1)specificleafarea(SLAcm2 gminus1)leaftotalpolyphenolics(TP)leafcondensedtannins(CT)leafaciddetergentfibre(ADF)branchingindex(BIbranchescm-1)bitesizeindex(BSIg)andaveragespinelength(ASLmm)

Plant trait treatment Aca bre (1) Aca mel (1) Aca etb (2) Bal ped (2) Gre ten (3) Cro dic (3)

Total available N

In(h‐) 234 plusmn 142 156 plusmn 106 156 plusmn 086 215 plusmn 24 226 plusmn 099 126 plusmn 024

Out(h+) 248 plusmn 111 185 plusmn 105 152 plusmn 114 235 plusmn 171 220 plusmn 093 167 plusmn 082

Leaf total N In(h‐) 361 plusmn 220 344 plusmn 088 329 plusmn 090 283 plusmn 190 340 plusmn 090 214 plusmn 030

Out(h+) 394 plusmn 130 352 plusmn 060 320 plusmn 080 312 plusmn 150 322 plusmn 090 234 plusmn 070

SLA In(h‐) 102 plusmn 734 107 plusmn 544 141 plusmn 786 546 plusmn 507 150 plusmn 514 147 plusmn 600

Out(h+) 119 plusmn 809 118 plusmn 302 139 plusmn 120 407 plusmn 252 136 plusmn 712 146 plusmn 600

TP In(h‐) 331 plusmn 046 626 plusmn 025 155 plusmn 009 055 plusmn 003 109 plusmn 014 126 plusmn 006

Out(h+) 278 plusmn 030 535 plusmn 022 136 plusmn 007 055 plusmn 005 129 plusmn 013 142 plusmn 005

CT In(h‐) 472 plusmn 057 658 plusmn 104 038 plusmn 004 008 plusmn 0007 142 plusmn 022 033 plusmn 003

Out(h+) 248 plusmn 037 715 plusmn 051 041 plusmn 004 009 plusmn 001 154 plusmn 022 037 plusmn 001

ADF In(h‐) 329 plusmn 239 371 plusmn 198 304 plusmn 129 246 plusmn 089 430 plusmn 084 338 plusmn 045

Out(h+) 350 plusmn 216 336 plusmn 183 288 plusmn 158 241 plusmn 060 420 plusmn 105 324 plusmn 116

BI In(h‐) 007 plusmn 003 009 plusmn 003 004 plusmn 0007 025 plusmn 003 016 plusmn 002 067 plusmn 016

Out(h+) 045 plusmn 005 038 plusmn 004 032 plusmn 004 125 plusmn 008 119 plusmn 018 056 plusmn 007

BSI In(h‐) 300 plusmn 023 201 plusmn 015 081 plusmn 006 249 plusmn 023 557 plusmn 048 534 plusmn 05

Out(h+) 087 plusmn 008 037 plusmn 003 011 plusmn 001 044 plusmn 004 108 plusmn 008 427 plusmn 049

ASL In(h‐) 346 plusmn 006 373 plusmn 004 529 plusmn 101 547 plusmn 125 mdash mdash

Out(h+) 347 plusmn 007 385 plusmn 003 586 plusmn 090 550 plusmn 086 mdash mdash

NoteTreatmentsareherbivoresexcluded(h‐)andherbivorespresent(h+)GroupmembershipisindicatedinparenthesesnexttoeachspeciesSignificance levels are indicated byp lt 01 p lt 005 p lt 001 p lt 0001 and are highlighted in bold

emspensp emsp | emsp7Journal of EcologyWIGLEY Et aL

duringthestudyperiodwithherbivorespresenthoweverneitherofthe measures of growth differed between the herbivore removal and herbivores present treatments (pgt005) Based on themeasuredchanges inplantheight andbasal area in thepresenceversus ab-sence of browsers over the course of a decade we ranked the three groupsfromlowtohighintermsoftheircapacitytopersistinthefaceofintensebrowsingpressurewheregroup1ltgroup3ltgroup2(Table3Figure5)

4emsp |emspDISCUSSION

OuroverarchinggoalwastoexaminehowwoodyspeciesinthisEastAfricansavannausesuitesoftraitstocontendwitharelativelyintensebrowsing regime imposedbyadiverseandabundantassemblageoflargemammalianherbivoresThehypotheseswhichunderpinclassicplant defence theory rely heavily on resource availability to predictwhetherplantsinvestingrowthordefence(HermsampMattson1992)and hence do not predict relative investments in structural versuschemicaldefences(Hanleyetal2007)orwhichdefenceswillbemosteffectiveunderfixedresourcelevelsbutvaryingherbivorypressureOurresultsindicatethat(a)woodyspeciesinthissystemcanbeclas-sifiedintothreedifferentgroupsorstrategiesbasedontheirdifferen-tialinvestmentinstructuralandchemicaldefences(b)structuralandchemical defences responded to varying degrees to the removal ofherbivoresbothwithinandacrossgroupsbutplantdefencestrategies

remainedqualitativelyunchangedeven after nearly twodecadesofherbivoreexclusion (c) structuralandchemicaldefencesdidnot re-spondinthesamewaytoherbivoreremovalwithstructuraltraits(es-peciallybranching)typicallyrespondingmorepositively(ieincreased)and (d) thespecies thatweremost resilient to intensebrowsing (ieachievedthehighestgrowth)werethosethatmaintainedhighspinelengthspinedensityandbranchinginthepresenceofherbivores

Noneofthespeciesinourstudysiteappeartobeadoptingtheldquolow nutrienthigh chemical defencerdquo strategy with most specieshavingmoderatetohighleafNcomparedtospeciespreviouslycat-egorised as low nutrienthigh chemical defence strategists in other savannasystems(Wigleyetal2018Figure1a)Thethreestrate-giesweidentifiedthusallfallwithinthebroadremitofldquonutritionanddefencerdquobutspeciesinthesedifferentgroupsappeartoinvestvari-ablyinstructuralandN‐freechemicaldefencesFine‐leavedspeciessuch as A brevispica and A mellifera (group1)havemoderatetohighleafNlowinvestmentinstructuraldefences(shortspines)andhighinvestmentinN‐freechemicaldefences(highCTandTP)A etbaica and B pedicellaris(group2)similarlyhadmoderatetohighleafNandbothspeciesinvestedmoreinphysicaldefences(denselongspinesthorns)thanchemicaldefences(lowN‐freesecondarymetabolites)Finally species suchasG tenax and C dichogamus (group3 bothbroad‐leaved)hadmoderatetohighleafNnospinesandlowN‐freesecondary metabolites

Basedonourmeasuredchanges inplantheightandbasalareainthepresencecomparedtoabsenceofbrowsersoverthecourse

F I G U R E 2 emsp Mean plusmn SE total available leafnitrogen(totalNxavailableNproportion(a)specificleafarea(b)totalpolyphenols(c)andcondensedtannins(d)insideandoutsideoftheexclosuresAcabreAcacia brevispicaAcamelAcacia melliferaAcaetbAcacia etbaica BalpedBalanites pedicellaris Gre ten Grewia tenax Cro dic Croton dichogamus Significance levels are p lt 01 p lt 005 p lt 001

8emsp |emsp emspenspJournal of Ecology WIGLEY Et aL

ofonedecadewerankedthesixwoodyspeciesfromlowtohighintermsoftheircapacitytocoexistwithintensebrowsingpressurewhere G tenaxltA brevispicaltA melliferaltC dichogamusltB pedi-cellarisltA etbaica (Figure5)Our results for the twobroad‐leavedspeciesthat lack investment inspines(group3G tenax and C di-chogamus)provideseveralkeyinsightsregardingstrategiesforldquolivingwithbrowsersrdquoBasedontheframeworkdevelopedfromsouthernAfrican savannas (Wigley et al 2018) we expected both speciesto invest heavily in N-free secondary metabolites as a trade-off to the lackof armamentContrary to thispredictionneither speciesinvestedinN‐freesecondarymetaboliteseitherinthepresenceorabsence of large herbivores

LowerleafN(bothtotalandavailable)wasoneconspicuoustraitdistinguishing Crotonfromtheotherspeciesinthisstudyandmaycontributetothefactthatthisspeciesisgenerallyunpalatableanduneatenbylargebrowsers(Kartzineletal2014)HoweverleafN(~22)wasnot so lowas tocompromise ruminantdigestioneffi-ciency(whichtypicallyoccurscloserto1NVanSoest1994)sug-gesting thatsomeotherpotentiallycostlyandasyetunmeasuredaspect ofCroton leaf chemistry is a key trait allowing this broad‐leavedspeciestocoexistwithlargebrowsersCroton sppingeneralarehighlyaromaticandknowntocontainmultipleuniquesecondarychemicals including cembranoids halimanes crotofolanes sesqui-terpenoids flavonoids and cyclohexanol derivatives (Langat et al2016)someofwhichareverysuccessfulinlimitingvertebrateand

invertebrate herbivory (Kaplan Halitschke Kessler Sardanelli ampDenno 2008 Levin 1976) Including these types of chemical de-fences in a generalised trait framework will be challenging as they arenotubiquitousacrossspeciesandtheirexpressioncandependonspecificherbivoreelicitors(Moreiraetal2013)

The lack of investment in N-free secondary metabolites by the secondbroad‐leavedspeciesG tenaxwasalsosurprisingbutcon-sistentwithourfindingthatgrowthofthisspeciesisseverelyneg-ativelyaffectedbylargebrowsersandwithpreviousworkshowingsubstantial declines in all size classes of G tenaxinthepresenceofbrowsersinthissavanna(AugustineampMcNaughton2004Sankaranetal2013)Ratherthanemployinganyformofcostlychemicalde-fence this species appears to coexist (uneasily)with browsers byincreasing the complexity of its branching architecture (BI) andgrowing in close association with other thorny species that cre-ate structural refugia where G tenax saplings are protected frombrowsers (personalobservationbyallauthorsseealsoCoverdaleGoheenPalmerampPringle2018)OnevalueofthisstrategyisthatwhenbrowsingpressureisremovedorlowthelackofinvestmentincostlydefencescombinedwithhighleafNallowsforrapidgrowth

TheremainingfourspeciesbelongingtotheothertwogroupsallhavesomeformofinvestmentinspinesTheycanbearrayedalongagradientofincreasingspineinvestmentfromgroup1togroup2withspeciesingroup1consistingofthosewithshortstraightspines(A brevispica) or short recurved spines (A mellifera) and those in

F I G U R E 3 emsp Mean plusmn SEspinelength(a)spinediameter(b)bitesizeindex(c)andbranchdensity(d)insideandoutsidetheexclosuresAcabreAcacia brevispica AcamelAcacia mellifera AcaetbAcacia etbaicaBalpedBalanites pedicellaris Gre ten Grewia tenax Cro dic Croton dichogamus Significance levels are p lt 005 p lt 001 p lt 0001

(a) (b)

(c) (d)

emspensp emsp | emsp9Journal of EcologyWIGLEY Et aL

F I G U R E 4 emspExamplesofAcacia brevispica and Balanites pedicellaris growing in the absence of large mammalianherbivores(ac)andthesamespeciesgrowinginthepresenceoflargemammalianherbivores(bd)incentralLaikipiaCountyKenyaNotethecleardifferences in architecture branching densities and browser damage between the treatments

(a) (b)

(c) (d)

TA B L E 3 emsp Mean plusmn SEforplantheightandbasalareain1999and2009forthesixspeciesgrowingwithandwithoutherbivorespresentnetchangesinplantheightandbasalareabetweenthe2yearsarealsoshown

Species Treatment Height 1999 (m) Height 2009 (m)Basal area 1999 (cm2)

Basal area 2009 (cm2) Δ height Δ basal area

Acacia brevispica Out(h+) 195 plusmn 007 152 plusmn 005 1379 plusmn 174 1649 plusmn 209 minus043plusmn007 269 plusmn 117

Acacia brevispica In(h‐) 201 plusmn 006 297 plusmn 008 991 plusmn 096 164 plusmn 147 095 plusmn 006 649 plusmn 088

Acacia mellifera Out(h+) 228 plusmn 011 218 plusmn 01 8368 plusmn 1598 9465 plusmn 134 minus010plusmn009 1097 plusmn 657

Acacia mellifera In(h‐) 219 plusmn 009 349 plusmn 01 8048 plusmn 1331 12236 plusmn 1492 13 plusmn 005 4188 plusmn 611

Acacia etbaica Out(h+) 168 plusmn 008 199 plusmn 01 3699 plusmn 616 5786 plusmn 715 031 plusmn 006 2087 plusmn 314

Acacia etbaica In(h‐) 157 plusmn 007 305 plusmn 007 3039 plusmn 754 5183 plusmn 77 148 plusmn 006 2117 plusmn 323

Balanaites pedicellaris Out(h+) 25 plusmn 07 286 plusmn 05 8823 plusmn 4682 12282 plusmn 5008 036 plusmn 047 3459 plusmn 1242

Balanaites pedicellaris In(h‐) 196 plusmn 028 401 plusmn 026 2542 plusmn 829 7193 plusmn 1645 205 plusmn 023 4650 plusmn 1283

Grewia tenax Out(h+) 131 plusmn 009 095 plusmn 024 336 plusmn 044 296 plusmn 048 minus036plusmn024 minus041plusmn035

Grewia tenax In(h‐) 116 plusmn 005 24 plusmn 008 194 plusmn 023 578 plusmn 05 124 plusmn 007 384 plusmn 044

Croton dichogamus Out(h+) 216 plusmn 022 21 plusmn 024 2393 plusmn 831 3323 plusmn 829 minus007plusmn014 929 plusmn 217

Croton dichogamus In(h‐) 159 plusmn 008 182 plusmn 012 932 plusmn 267 1346 plusmn 317 023 plusmn 011 414 plusmn 116

NoteAllmeasurementsarebasedonthesameindividualplantsofeachspeciesthatwerepresentintheplotsin1999andthatwerestillalivein2009

10emsp |emsp emspenspJournal of Ecology WIGLEY Et aL

group2havingeither long straight spines (B pedicellaris)orbothshort recurvedand long straight spines (A etbaica) Specieswiththelongestandmostdensespines(group2)showedlittletonoin-vestmentinCTorTPwhilethespecieswithlowspineinvestment(group 1) showed the greatest concentrations of bothCT and TP(Figure2)The latterstrategyexperiencesgreaterconsumptionbybrowsersinthislandscaperelativetotheformer(Fordetal2014)consistentwithpreviousstudieswhichhavereportedthatspecieswithhigherinvestmentinstructuraldefences(group2)aremostsuc-cessfulatcoexistingwithbrowsersinthissavannasystem(GoheenKeesingAllanOgadaampOstfeld2004Sankaranetal2013)Inthepresenceofbrowsersspeciesinbothgroups1and2maintainedanincreasedcomplexityoftheirbranchingarchitecturecombinedwithsmallerleaves(reflectedinsubstantialreductionsinbitesizeindex)Increased branching even in the absence of changes in thorn length anddensitywhichoccurredinsomespeciesbutnototherscanre-sultindramaticincreasesinthenumberofspinesperunitvolumeofcanopy(ArchibaldampBond2003StaverBondCramerampWakeling2012)servingasaneffectivedeterrentforbrowsers

To summarise the strategies expressed by spinescent speciesin termsof ldquolivingwithbrowsersrdquo canbecharacterisedas (a)veryhigh and inducible investment in spinesbutnotN‐free secondarymetabolites and low available N leading to greatest success under intensebrowsing(A etbaica)(b)moderateandinducibleinvestmentinstraightspinescombinedwith thickermore fibrous leavesbuthigh available N leading to sufficient success under intense brows-ing(B pedicellaris)(c)lowandnon‐inducibleinvestmentinrecurvedspines combinedwithhighN‐free secondarymetabolites and lowavailableNleadingtoreducedsuccessunderbrowsing(A mellifera)

and(d)lowandnon‐inducibleinvestmentinshortspinescombinedwith moderate investment in N-free secondary metabolites and high availableN leading to low success under browsing (A brevispica)Fornara and Du Toit (2008) suggested that physical defences to-getherwithmasscompensatorygrowthabilitiesarekeyadaptationstolivingwithhighbrowsingpressureOurfindingslendstrongsup-porttothisnotionasthemostheavilystructurallydefendedspecies(A etbaica and B pedicellaris)performedthebestinthepresenceofherbivoresdespitelowinvestmentsinN‐freesecondarymetabolitesHowever Scogingset al (2011)predicted that the concentrationsofN‐freesecondarycompoundswoulddependonbrowsinginten-sity with the highest concentrations occurring at intermediate levels ofbrowsingforcertainfunctionalgroupsForexampleScogingsetal(2011)predictalinearincreaseinN‐freesecondarycompoundswithbrowsingintensitywithapotentiallysuddendecreaseatveryhigh levelsofbrowsingpressure indeciduoussavannaspeciesAsourexperimentaldesign lackedan intermediatebrowsingpressuretreatmentwewereunabletotestthesepredictions

Our analyses of available Nprovidedsomeinterestingandvalu-ableinsightsFirstlyinterpretationsofbrowsequalitydifferdepend-ing on whether total N (also commonly converted to crudeproteinby multiplying by 625 eg Cooper et al 1988) or available N isevaluatedForexamplethethreeAcaciaspeciesinthisstudyA bre-vispica A mellifera and A etbaica all had similar and relatively high con-centrations of leaf total N (~3835and33respectively)buttheproportionofthetotalN available to herbivores in A brevispica was ca 20 higher than in A mellifera and A etbaica(~2417and15respectively seeTable2) Ifweonly lookedat totalN they would all appear tobeofhighqualityhowever ifwe lookat availableN

F I G U R E 5 emsp (a)MeanplusmnSE change in plantheightbetween1999and2009forthesixdominantspeciesgrowinginsideandoutsidetheexclosures(b)Mean plusmn SE change in stem basal area between 1999 and 2009 for the same six speciesSignificancelevelsarep lt 001 p lt 0001

emspensp emsp | emsp11Journal of EcologyWIGLEY Et aL

itappearsasifA brevispicaisofmuchhigherqualitythantheothertwospeciesSecondlyavailableNappearstobemoreresponsivetoherbivore removal than total leaf N No differences in total N were evident between herbivory treatments while available N was signifi-cantly higher in A mellifera and C dichogamus when herbivores were presentThirdlydifferencesinavailableNdonotseemtobelinkedtophylogeny(eghighinsomeAcaciaspecieslowinothers)functionaltype(egfine‐leavedvsbroad‐leaved)norconcentrationsofN‐freesecondarymetabolites (availableNfoundtobe low inspecieswithbothhighand lowN‐freesecondarymetabolites)Finally these re-sults raise some importantquestions regarding the commonlyheldview (especially in African savannas) that fine‐leaved species (egAcacia)havehigherqualityleavesandarestructurallydefendedwhilebroad‐leavedspeciesarelessnutritiousandchemicallydefendedForexampleG tenax and B pedicellarisbothbroad‐leavedspecieshadhigher available Nthanthetwofine‐leavedspeciesA mellifera and A etbaica These findings highlight the need for further studies to eluci-date the determinants of leaf available N and that caution needs to be taken when using total NasameasureofbrowsequalityFurthermoreour findings suggest that neither total N nor available N are sufficient bythemselvestopredictbrowserutilisationhighlightingtheneedtoconsideralltraitscollectively(iedefencestrategies)

5emsp |emspCONCLUSIONS

Allofthedominantspeciesatourstudysitehadmoderatetohighleaf N concentrations but differed in available N and in the way they combined chemical and structural defences thus fitting under thebroaderldquonutritionanddefencerdquosyndromepreviouslydescribedforsavannasStructuraldefenceshadmorepronouncedresponsesto herbivory through increased spine length and density andorincreased branching while N-free secondary metabolites did not increaseinresponsetoherbivoryHighinvestmentinstructuralde-fences was the most successful defence strategy at our study sites withmoderateresourcelevels(relativetootherAfricansavannas)andhighherbivorypressureresultinginthehighestgrowthinthissavanna system This work has shown that within one overarching defence syndrome (nutritionanddefence) specieswithina com-munity can widely diverge in their adopted defence strategiesEndaraetal(2015)reportedhighlevelsofdissimilarityinthede-fencesof closely relatedco‐occurring forest speciesand suggestthat this would be necessary for the coexistence of closely related neighboursandcouldaccountforthehighlocaldiversityoftropicalforests Similarly we suggest that diverse defence strategies en-able savanna species (which are often closely related) to occupydifferentnichesanddefendagainstdifferent typesofherbivoresresultinginmoreresilientandspeciesrichwoodycommunities

ACKNOWLEDG EMENTS

WethankJParsareforassistanceinthefieldandChengappaSKand Balavignesh B for their assistance with laboratory analyses

This work was funded by the National Centre for Biological Sciences TIFR India andbyNationalGeographic (GrantNumber982815)andNERC (GrantNE‐E017436‐1)Wearehugely grateful to theOfficeofthePresidentoftheRepublicofKenyaforpermissiontoconducttheresearchandtheMpalaResearchCentreforlogisti-calsupport

AUTHORSrsquo CONTRIBUTIONS

BJWCCDJAandMSco‐wrotethemanuscriptBJWandMS collected the data DJA MS and JR implemented theexclosure experiment DH conceptualised and performed theanalyticalapproacheswithrespecttofibreandavailableNdeter-mination andperformed condensed tannin and total polyphenolanalysesAllauthorscommentedandaddedtoearlierversionsofthemanuscript

DATA ACCE SSIBILIT Y

Data available from the Dryad Digital Repository httpsdoiorg105061dryadp2d00sf(Wigleyetal2019)

ORCID

Benjamin J Wigley httpsorcidorg0000‐0002‐6964‐3624

Mahesh Sankaran httpsorcidorg0000‐0002‐1661‐6542

R E FE R E N C E S

Agrawal A A (2011) Current trends in the evolutionary ecology ofplant defence Functional Ecology 25(2) 420ndash432 httpsdoiorg101111j1365-2435201001796x

Agrawal A A amp Fishbein M (2006) Plant defense syndromesEcology 87(sp7) 132ndash149 httpsdoiorg1018900012‐9658 (2006)87[132PDS]20CO2

Archibald S ampBond W J (2003) Growing tall vs growing wideTree architecture and allometry of Acacia karroo in forest sa-vanna and arid environments Oikos 102(1) 3ndash14 httpsdoiorg101034j1600-0706200312181x

AugustineDJ(2003)Long‐termlivestock‐mediatedredistributionofnitro-genandphosphorusinanEastAfricansavannaJournal of Applied Ecology 40(1)137ndash149httpsdoiorg101046j1365‐2664200300778x

AugustineDJ(2010)Responseofnativeungulatestodroughtinsemi‐arid Kenyan rangeland African Journal of Ecology 48(4)1009ndash1020httpsdoiorg101111j1365‐2028201001207x

AugustineDJampMcNaughtonSJ(2004)RegulationofshrubdynamicsbynativebrowsingungulatesonEastAfricanrangelandJournal of Applied Ecology 41(1)45ndash58httpsdoiorg101111j1365‐2664200400864x

AugustineDJampMcNaughtonSJ(2006)Interactiveeffectsofungu-lateherbivoressoilfertilityandvariablerainfallonecosystempro-cesses in a semi-arid savanna Ecosystems 9(8)1242ndash1256httpsdoiorg101007s10021-005-0020-y

BartonKE (2016)TougherandthornierGeneralpatterns in the in-ductionofphysicaldefencetraitsFunctional Ecology 30(2)181ndash187httpsdoiorg1011111365‐243512495

BryantJPChapinFSIIIampKleinDR(1983)Carbonnutrientbal-anceofborealplantsinrelationtovertebrateherbivoryOikos 40(3)357ndash368httpsdoiorg1023073544308

emspensp emsp | emsp5Journal of EcologyWIGLEY Et aL

3emsp |emspRESULTS

31emsp|emspDefence strategies

Results fromHornsParallelAnalysis for factor retentionbasedon 5000 iterations using the mean estimate showed that the first six components of the PCA should be retained The PCAbased on the measured plant traits showed that the six domi-nantspeciesatthestudysiteseparatedintothreebroadgroupsalong the first two PC axes which together accounted for al-most 60 of the total variance (Figure 1) This separationwasmost strongly driven by differences in spine length and fibre

onPC1 separating the twospecieswith long spinesand lowerfibre(A etbaica and B pedicellaris)fromthespecieswithhigherfibreandshortspines(A brevispica and A mellifera)ornospines(G tenax andC dichogamus (Table1)OnPC2 the groupsweremoststronglyseparatedbydifferencesinN‐freesecondaryme-tabolitesandspinedensityTwoofthebroad‐leavedspeciesG tenax and C dichogamus weresimilarintermsofhavingnospinesand lower concentrations of N-free secondary metabolites and weremoststronglyseparatedfromA brevispica and A mellifera whichhadhighN‐freesecondarymetabolitesandspinedensities(Figure1Table1)

F I G U R E 1 emsp (a)Plotofalineardiscriminant analysis of the measured defencetraitsshowingthegroupingofthesixMpalawoodyspeciesrelativetoWigleyetals(2018)dominantspeciesfromsouthernAfricansavannas(b)principalcomponentanalysis(PCA)basedonninemeasuredplantdefencetraitsofthesixwoodyplantspeciesgrowinginthepresence(black)andlong‐termabsence(grey)oflargemammalianherbivoresincentralLaikipiaCountyKenyaAxis1explained33ofthevarianceandwasprimarilyassociatedwithvariationinspinelengthandaciddetergentfibre(ADF)Axis2explained26ofthevarianceandwasprimarilyassociatedwithvariationincondensedtannins(CT)totalpolyphenols(TP)andspinedensity(seeTable1foreigenvectorscoresofthePCA)PointsrepresentthemeanlocationofeachspeciesonPCaxes1and2ateachofthethreepairedexclosureandcontrolsitesOvalsshowtheoverallmeans(centre)foreachspeciesandtreatmentanddispersion(ellipses)

TA B L E 1 emspEigenvectorscoresfortheprincipalcomponentanalysis(PCA)basedonninedefencetraits

PC1 (33) PC2 (26) PC3 (15) PC4 (10) PC5 (8) PC6 (4)

Spinelength(mm) minus053 minus005 019 minus007 010 025

Spinedensity(spinescmminus1) minus030 minus046 013 004 043 016

Total available N(mggminus1) 008 038 minus029 026 078 minus026

SLA(cm2 gminus1) 032 minus004 048 minus056 015 minus039

TP() 028 minus049 minus028 009 minus011 minus040

CT() 033 minus042 minus036 009 004 037

ADF() 048 013 007 minus030 024 061

BI(branchescmminus1) minus014 037 minus053 minus040 minus024 005

BSI(g) 028 028 037 059 minus023 012

NoteSLAspecificleafareaTPtotalpolyphenolsCTcondensedtanninsADFaciddetergentfibreBIbranchingindexBSIbitesizeindex

6emsp |emsp emspenspJournal of Ecology WIGLEY Et aL

32emsp|emspEffects of herbivore removal on plant defence traits and strategies

ThePCA(Figure1)illustratestheeffectsofherbivoreremovalandcontroltreatmentsontheplantdefencetraits(andstrategies)withherbivore removal treatments diverging from control treatments alongbothPCaxestovaryingdegrees(Figure1)Howevernoneofthespecieschangeddefencestrategiesasaresultofherbivorere-movalbothstructuralandchemicaldefencesrespondedtovariabledegrees both within and between strategies The long-term removal of large mammalian browsers from this savanna did not affect leaf totalNandaciddetergentfibreforanyofthesixofthewoodyspe-cies(Table2)Specificleafareadeclined(ieleavesbecamesmallerandthicker)withherbivoreexclusionforonefine‐leavedspecies(A mellifera p=005)andincreased(leavesbecamelargerandthinner)withherbivoreexclusionfortwobroad‐leavedspeciesB pedicellaris (p=003)andG tenax(p=009Figure2)HerbivoreremovalresultedinhighertotalpolyphenolsinA etbaica and A mellifera(p = 007 and p=001respectively)andsignificantlyhighercondensedtanninsinA brevispica(p=0003Figure2)andloweravailableNinA mellifera (plt01) andC dichogamus (plt001) Plant architectural defences(spinesandbranchingdensities)werefoundtorelaxwithherbivoreremoval ie became less structurally defended Herbivore removal decreasedthebranchingindexandbranchespervolume(plt0001)and increased the bite size index (iemore leaf biomass per bite

plt00001) for all speciesexceptC dichogamus (Figures3 and4) Herbivore removal resulted in significantly shorter spines inA et-baica(plt0001)andA mellifera (p=002)andasignificantlylowerdensityofspinesinA etbaica (p=001)(Figure3)

33emsp|emspGrowth defence strategies and responses to herbivore removal

Overthe10‐yearperiodbetween1999and2009wedocumentedsubstantialvariationingrowthwhichwepresent inrelationtothethreegroupsWhengrowing in thepresenceofherbivoresgroup1 (A brevispica and A mellifera) decreased in height (minus042plusmn007and minus010plusmn009m respectively) but increased in basal area(269plusmn116and 110plusmn656cm2) In this group herbivore removalresultedinsignificantlytallerplantswithhigherbasalareathanwithherbivorespresent(plt0001Table3Figure5)Group2(A etbaica and B pedicellaris)increasedinheight(031plusmn006and024plusmn044mrespectively)andbasalarea(209plusmn314and346plusmn124cm2)inthepresenceofherbivores Inthisgroupherbivoreremoval increasedplantheight(plt001)butnotbasalarea(Table3Figure5)Plantsingroup3(G tenax and C dichogamus)generallydecreasedinheightbut no consistent trend was evident for basal area when herbivores were present ForG tenax herbivore removal resulted in signifi-cantlytallerplantswithhigherbasalareas(plt0001)C dichogamus plantsshowedaslightdecreaseinheightandincreaseinbasalarea

TA B L E 2 emsp Mean plusmn SEforleaftotalavailablenitrogen(mggminus1)leaftotalnitrogen(mggminus1)specificleafarea(SLAcm2 gminus1)leaftotalpolyphenolics(TP)leafcondensedtannins(CT)leafaciddetergentfibre(ADF)branchingindex(BIbranchescm-1)bitesizeindex(BSIg)andaveragespinelength(ASLmm)

Plant trait treatment Aca bre (1) Aca mel (1) Aca etb (2) Bal ped (2) Gre ten (3) Cro dic (3)

Total available N

In(h‐) 234 plusmn 142 156 plusmn 106 156 plusmn 086 215 plusmn 24 226 plusmn 099 126 plusmn 024

Out(h+) 248 plusmn 111 185 plusmn 105 152 plusmn 114 235 plusmn 171 220 plusmn 093 167 plusmn 082

Leaf total N In(h‐) 361 plusmn 220 344 plusmn 088 329 plusmn 090 283 plusmn 190 340 plusmn 090 214 plusmn 030

Out(h+) 394 plusmn 130 352 plusmn 060 320 plusmn 080 312 plusmn 150 322 plusmn 090 234 plusmn 070

SLA In(h‐) 102 plusmn 734 107 plusmn 544 141 plusmn 786 546 plusmn 507 150 plusmn 514 147 plusmn 600

Out(h+) 119 plusmn 809 118 plusmn 302 139 plusmn 120 407 plusmn 252 136 plusmn 712 146 plusmn 600

TP In(h‐) 331 plusmn 046 626 plusmn 025 155 plusmn 009 055 plusmn 003 109 plusmn 014 126 plusmn 006

Out(h+) 278 plusmn 030 535 plusmn 022 136 plusmn 007 055 plusmn 005 129 plusmn 013 142 plusmn 005

CT In(h‐) 472 plusmn 057 658 plusmn 104 038 plusmn 004 008 plusmn 0007 142 plusmn 022 033 plusmn 003

Out(h+) 248 plusmn 037 715 plusmn 051 041 plusmn 004 009 plusmn 001 154 plusmn 022 037 plusmn 001

ADF In(h‐) 329 plusmn 239 371 plusmn 198 304 plusmn 129 246 plusmn 089 430 plusmn 084 338 plusmn 045

Out(h+) 350 plusmn 216 336 plusmn 183 288 plusmn 158 241 plusmn 060 420 plusmn 105 324 plusmn 116

BI In(h‐) 007 plusmn 003 009 plusmn 003 004 plusmn 0007 025 plusmn 003 016 plusmn 002 067 plusmn 016

Out(h+) 045 plusmn 005 038 plusmn 004 032 plusmn 004 125 plusmn 008 119 plusmn 018 056 plusmn 007

BSI In(h‐) 300 plusmn 023 201 plusmn 015 081 plusmn 006 249 plusmn 023 557 plusmn 048 534 plusmn 05

Out(h+) 087 plusmn 008 037 plusmn 003 011 plusmn 001 044 plusmn 004 108 plusmn 008 427 plusmn 049

ASL In(h‐) 346 plusmn 006 373 plusmn 004 529 plusmn 101 547 plusmn 125 mdash mdash

Out(h+) 347 plusmn 007 385 plusmn 003 586 plusmn 090 550 plusmn 086 mdash mdash

NoteTreatmentsareherbivoresexcluded(h‐)andherbivorespresent(h+)GroupmembershipisindicatedinparenthesesnexttoeachspeciesSignificance levels are indicated byp lt 01 p lt 005 p lt 001 p lt 0001 and are highlighted in bold

emspensp emsp | emsp7Journal of EcologyWIGLEY Et aL

duringthestudyperiodwithherbivorespresenthoweverneitherofthe measures of growth differed between the herbivore removal and herbivores present treatments (pgt005) Based on themeasuredchanges inplantheight andbasal area in thepresenceversus ab-sence of browsers over the course of a decade we ranked the three groupsfromlowtohighintermsoftheircapacitytopersistinthefaceofintensebrowsingpressurewheregroup1ltgroup3ltgroup2(Table3Figure5)

4emsp |emspDISCUSSION

OuroverarchinggoalwastoexaminehowwoodyspeciesinthisEastAfricansavannausesuitesoftraitstocontendwitharelativelyintensebrowsing regime imposedbyadiverseandabundantassemblageoflargemammalianherbivoresThehypotheseswhichunderpinclassicplant defence theory rely heavily on resource availability to predictwhetherplantsinvestingrowthordefence(HermsampMattson1992)and hence do not predict relative investments in structural versuschemicaldefences(Hanleyetal2007)orwhichdefenceswillbemosteffectiveunderfixedresourcelevelsbutvaryingherbivorypressureOurresultsindicatethat(a)woodyspeciesinthissystemcanbeclas-sifiedintothreedifferentgroupsorstrategiesbasedontheirdifferen-tialinvestmentinstructuralandchemicaldefences(b)structuralandchemical defences responded to varying degrees to the removal ofherbivoresbothwithinandacrossgroupsbutplantdefencestrategies

remainedqualitativelyunchangedeven after nearly twodecadesofherbivoreexclusion (c) structuralandchemicaldefencesdidnot re-spondinthesamewaytoherbivoreremovalwithstructuraltraits(es-peciallybranching)typicallyrespondingmorepositively(ieincreased)and (d) thespecies thatweremost resilient to intensebrowsing (ieachievedthehighestgrowth)werethosethatmaintainedhighspinelengthspinedensityandbranchinginthepresenceofherbivores

Noneofthespeciesinourstudysiteappeartobeadoptingtheldquolow nutrienthigh chemical defencerdquo strategy with most specieshavingmoderatetohighleafNcomparedtospeciespreviouslycat-egorised as low nutrienthigh chemical defence strategists in other savannasystems(Wigleyetal2018Figure1a)Thethreestrate-giesweidentifiedthusallfallwithinthebroadremitofldquonutritionanddefencerdquobutspeciesinthesedifferentgroupsappeartoinvestvari-ablyinstructuralandN‐freechemicaldefencesFine‐leavedspeciessuch as A brevispica and A mellifera (group1)havemoderatetohighleafNlowinvestmentinstructuraldefences(shortspines)andhighinvestmentinN‐freechemicaldefences(highCTandTP)A etbaica and B pedicellaris(group2)similarlyhadmoderatetohighleafNandbothspeciesinvestedmoreinphysicaldefences(denselongspinesthorns)thanchemicaldefences(lowN‐freesecondarymetabolites)Finally species suchasG tenax and C dichogamus (group3 bothbroad‐leaved)hadmoderatetohighleafNnospinesandlowN‐freesecondary metabolites

Basedonourmeasuredchanges inplantheightandbasalareainthepresencecomparedtoabsenceofbrowsersoverthecourse

F I G U R E 2 emsp Mean plusmn SE total available leafnitrogen(totalNxavailableNproportion(a)specificleafarea(b)totalpolyphenols(c)andcondensedtannins(d)insideandoutsideoftheexclosuresAcabreAcacia brevispicaAcamelAcacia melliferaAcaetbAcacia etbaica BalpedBalanites pedicellaris Gre ten Grewia tenax Cro dic Croton dichogamus Significance levels are p lt 01 p lt 005 p lt 001

8emsp |emsp emspenspJournal of Ecology WIGLEY Et aL

ofonedecadewerankedthesixwoodyspeciesfromlowtohighintermsoftheircapacitytocoexistwithintensebrowsingpressurewhere G tenaxltA brevispicaltA melliferaltC dichogamusltB pedi-cellarisltA etbaica (Figure5)Our results for the twobroad‐leavedspeciesthat lack investment inspines(group3G tenax and C di-chogamus)provideseveralkeyinsightsregardingstrategiesforldquolivingwithbrowsersrdquoBasedontheframeworkdevelopedfromsouthernAfrican savannas (Wigley et al 2018) we expected both speciesto invest heavily in N-free secondary metabolites as a trade-off to the lackof armamentContrary to thispredictionneither speciesinvestedinN‐freesecondarymetaboliteseitherinthepresenceorabsence of large herbivores

LowerleafN(bothtotalandavailable)wasoneconspicuoustraitdistinguishing Crotonfromtheotherspeciesinthisstudyandmaycontributetothefactthatthisspeciesisgenerallyunpalatableanduneatenbylargebrowsers(Kartzineletal2014)HoweverleafN(~22)wasnot so lowas tocompromise ruminantdigestioneffi-ciency(whichtypicallyoccurscloserto1NVanSoest1994)sug-gesting thatsomeotherpotentiallycostlyandasyetunmeasuredaspect ofCroton leaf chemistry is a key trait allowing this broad‐leavedspeciestocoexistwithlargebrowsersCroton sppingeneralarehighlyaromaticandknowntocontainmultipleuniquesecondarychemicals including cembranoids halimanes crotofolanes sesqui-terpenoids flavonoids and cyclohexanol derivatives (Langat et al2016)someofwhichareverysuccessfulinlimitingvertebrateand

invertebrate herbivory (Kaplan Halitschke Kessler Sardanelli ampDenno 2008 Levin 1976) Including these types of chemical de-fences in a generalised trait framework will be challenging as they arenotubiquitousacrossspeciesandtheirexpressioncandependonspecificherbivoreelicitors(Moreiraetal2013)

The lack of investment in N-free secondary metabolites by the secondbroad‐leavedspeciesG tenaxwasalsosurprisingbutcon-sistentwithourfindingthatgrowthofthisspeciesisseverelyneg-ativelyaffectedbylargebrowsersandwithpreviousworkshowingsubstantial declines in all size classes of G tenaxinthepresenceofbrowsersinthissavanna(AugustineampMcNaughton2004Sankaranetal2013)Ratherthanemployinganyformofcostlychemicalde-fence this species appears to coexist (uneasily)with browsers byincreasing the complexity of its branching architecture (BI) andgrowing in close association with other thorny species that cre-ate structural refugia where G tenax saplings are protected frombrowsers (personalobservationbyallauthorsseealsoCoverdaleGoheenPalmerampPringle2018)OnevalueofthisstrategyisthatwhenbrowsingpressureisremovedorlowthelackofinvestmentincostlydefencescombinedwithhighleafNallowsforrapidgrowth

TheremainingfourspeciesbelongingtotheothertwogroupsallhavesomeformofinvestmentinspinesTheycanbearrayedalongagradientofincreasingspineinvestmentfromgroup1togroup2withspeciesingroup1consistingofthosewithshortstraightspines(A brevispica) or short recurved spines (A mellifera) and those in

F I G U R E 3 emsp Mean plusmn SEspinelength(a)spinediameter(b)bitesizeindex(c)andbranchdensity(d)insideandoutsidetheexclosuresAcabreAcacia brevispica AcamelAcacia mellifera AcaetbAcacia etbaicaBalpedBalanites pedicellaris Gre ten Grewia tenax Cro dic Croton dichogamus Significance levels are p lt 005 p lt 001 p lt 0001

(a) (b)

(c) (d)

emspensp emsp | emsp9Journal of EcologyWIGLEY Et aL

F I G U R E 4 emspExamplesofAcacia brevispica and Balanites pedicellaris growing in the absence of large mammalianherbivores(ac)andthesamespeciesgrowinginthepresenceoflargemammalianherbivores(bd)incentralLaikipiaCountyKenyaNotethecleardifferences in architecture branching densities and browser damage between the treatments

(a) (b)

(c) (d)

TA B L E 3 emsp Mean plusmn SEforplantheightandbasalareain1999and2009forthesixspeciesgrowingwithandwithoutherbivorespresentnetchangesinplantheightandbasalareabetweenthe2yearsarealsoshown

Species Treatment Height 1999 (m) Height 2009 (m)Basal area 1999 (cm2)

Basal area 2009 (cm2) Δ height Δ basal area

Acacia brevispica Out(h+) 195 plusmn 007 152 plusmn 005 1379 plusmn 174 1649 plusmn 209 minus043plusmn007 269 plusmn 117

Acacia brevispica In(h‐) 201 plusmn 006 297 plusmn 008 991 plusmn 096 164 plusmn 147 095 plusmn 006 649 plusmn 088

Acacia mellifera Out(h+) 228 plusmn 011 218 plusmn 01 8368 plusmn 1598 9465 plusmn 134 minus010plusmn009 1097 plusmn 657

Acacia mellifera In(h‐) 219 plusmn 009 349 plusmn 01 8048 plusmn 1331 12236 plusmn 1492 13 plusmn 005 4188 plusmn 611

Acacia etbaica Out(h+) 168 plusmn 008 199 plusmn 01 3699 plusmn 616 5786 plusmn 715 031 plusmn 006 2087 plusmn 314

Acacia etbaica In(h‐) 157 plusmn 007 305 plusmn 007 3039 plusmn 754 5183 plusmn 77 148 plusmn 006 2117 plusmn 323

Balanaites pedicellaris Out(h+) 25 plusmn 07 286 plusmn 05 8823 plusmn 4682 12282 plusmn 5008 036 plusmn 047 3459 plusmn 1242

Balanaites pedicellaris In(h‐) 196 plusmn 028 401 plusmn 026 2542 plusmn 829 7193 plusmn 1645 205 plusmn 023 4650 plusmn 1283

Grewia tenax Out(h+) 131 plusmn 009 095 plusmn 024 336 plusmn 044 296 plusmn 048 minus036plusmn024 minus041plusmn035

Grewia tenax In(h‐) 116 plusmn 005 24 plusmn 008 194 plusmn 023 578 plusmn 05 124 plusmn 007 384 plusmn 044

Croton dichogamus Out(h+) 216 plusmn 022 21 plusmn 024 2393 plusmn 831 3323 plusmn 829 minus007plusmn014 929 plusmn 217

Croton dichogamus In(h‐) 159 plusmn 008 182 plusmn 012 932 plusmn 267 1346 plusmn 317 023 plusmn 011 414 plusmn 116

NoteAllmeasurementsarebasedonthesameindividualplantsofeachspeciesthatwerepresentintheplotsin1999andthatwerestillalivein2009

10emsp |emsp emspenspJournal of Ecology WIGLEY Et aL

group2havingeither long straight spines (B pedicellaris)orbothshort recurvedand long straight spines (A etbaica) Specieswiththelongestandmostdensespines(group2)showedlittletonoin-vestmentinCTorTPwhilethespecieswithlowspineinvestment(group 1) showed the greatest concentrations of bothCT and TP(Figure2)The latterstrategyexperiencesgreaterconsumptionbybrowsersinthislandscaperelativetotheformer(Fordetal2014)consistentwithpreviousstudieswhichhavereportedthatspecieswithhigherinvestmentinstructuraldefences(group2)aremostsuc-cessfulatcoexistingwithbrowsersinthissavannasystem(GoheenKeesingAllanOgadaampOstfeld2004Sankaranetal2013)Inthepresenceofbrowsersspeciesinbothgroups1and2maintainedanincreasedcomplexityoftheirbranchingarchitecturecombinedwithsmallerleaves(reflectedinsubstantialreductionsinbitesizeindex)Increased branching even in the absence of changes in thorn length anddensitywhichoccurredinsomespeciesbutnototherscanre-sultindramaticincreasesinthenumberofspinesperunitvolumeofcanopy(ArchibaldampBond2003StaverBondCramerampWakeling2012)servingasaneffectivedeterrentforbrowsers

To summarise the strategies expressed by spinescent speciesin termsof ldquolivingwithbrowsersrdquo canbecharacterisedas (a)veryhigh and inducible investment in spinesbutnotN‐free secondarymetabolites and low available N leading to greatest success under intensebrowsing(A etbaica)(b)moderateandinducibleinvestmentinstraightspinescombinedwith thickermore fibrous leavesbuthigh available N leading to sufficient success under intense brows-ing(B pedicellaris)(c)lowandnon‐inducibleinvestmentinrecurvedspines combinedwithhighN‐free secondarymetabolites and lowavailableNleadingtoreducedsuccessunderbrowsing(A mellifera)

and(d)lowandnon‐inducibleinvestmentinshortspinescombinedwith moderate investment in N-free secondary metabolites and high availableN leading to low success under browsing (A brevispica)Fornara and Du Toit (2008) suggested that physical defences to-getherwithmasscompensatorygrowthabilitiesarekeyadaptationstolivingwithhighbrowsingpressureOurfindingslendstrongsup-porttothisnotionasthemostheavilystructurallydefendedspecies(A etbaica and B pedicellaris)performedthebestinthepresenceofherbivoresdespitelowinvestmentsinN‐freesecondarymetabolitesHowever Scogingset al (2011)predicted that the concentrationsofN‐freesecondarycompoundswoulddependonbrowsinginten-sity with the highest concentrations occurring at intermediate levels ofbrowsingforcertainfunctionalgroupsForexampleScogingsetal(2011)predictalinearincreaseinN‐freesecondarycompoundswithbrowsingintensitywithapotentiallysuddendecreaseatveryhigh levelsofbrowsingpressure indeciduoussavannaspeciesAsourexperimentaldesign lackedan intermediatebrowsingpressuretreatmentwewereunabletotestthesepredictions

Our analyses of available Nprovidedsomeinterestingandvalu-ableinsightsFirstlyinterpretationsofbrowsequalitydifferdepend-ing on whether total N (also commonly converted to crudeproteinby multiplying by 625 eg Cooper et al 1988) or available N isevaluatedForexamplethethreeAcaciaspeciesinthisstudyA bre-vispica A mellifera and A etbaica all had similar and relatively high con-centrations of leaf total N (~3835and33respectively)buttheproportionofthetotalN available to herbivores in A brevispica was ca 20 higher than in A mellifera and A etbaica(~2417and15respectively seeTable2) Ifweonly lookedat totalN they would all appear tobeofhighqualityhowever ifwe lookat availableN

F I G U R E 5 emsp (a)MeanplusmnSE change in plantheightbetween1999and2009forthesixdominantspeciesgrowinginsideandoutsidetheexclosures(b)Mean plusmn SE change in stem basal area between 1999 and 2009 for the same six speciesSignificancelevelsarep lt 001 p lt 0001

emspensp emsp | emsp11Journal of EcologyWIGLEY Et aL

itappearsasifA brevispicaisofmuchhigherqualitythantheothertwospeciesSecondlyavailableNappearstobemoreresponsivetoherbivore removal than total leaf N No differences in total N were evident between herbivory treatments while available N was signifi-cantly higher in A mellifera and C dichogamus when herbivores were presentThirdlydifferencesinavailableNdonotseemtobelinkedtophylogeny(eghighinsomeAcaciaspecieslowinothers)functionaltype(egfine‐leavedvsbroad‐leaved)norconcentrationsofN‐freesecondarymetabolites (availableNfoundtobe low inspecieswithbothhighand lowN‐freesecondarymetabolites)Finally these re-sults raise some importantquestions regarding the commonlyheldview (especially in African savannas) that fine‐leaved species (egAcacia)havehigherqualityleavesandarestructurallydefendedwhilebroad‐leavedspeciesarelessnutritiousandchemicallydefendedForexampleG tenax and B pedicellarisbothbroad‐leavedspecieshadhigher available Nthanthetwofine‐leavedspeciesA mellifera and A etbaica These findings highlight the need for further studies to eluci-date the determinants of leaf available N and that caution needs to be taken when using total NasameasureofbrowsequalityFurthermoreour findings suggest that neither total N nor available N are sufficient bythemselvestopredictbrowserutilisationhighlightingtheneedtoconsideralltraitscollectively(iedefencestrategies)

5emsp |emspCONCLUSIONS

Allofthedominantspeciesatourstudysitehadmoderatetohighleaf N concentrations but differed in available N and in the way they combined chemical and structural defences thus fitting under thebroaderldquonutritionanddefencerdquosyndromepreviouslydescribedforsavannasStructuraldefenceshadmorepronouncedresponsesto herbivory through increased spine length and density andorincreased branching while N-free secondary metabolites did not increaseinresponsetoherbivoryHighinvestmentinstructuralde-fences was the most successful defence strategy at our study sites withmoderateresourcelevels(relativetootherAfricansavannas)andhighherbivorypressureresultinginthehighestgrowthinthissavanna system This work has shown that within one overarching defence syndrome (nutritionanddefence) specieswithina com-munity can widely diverge in their adopted defence strategiesEndaraetal(2015)reportedhighlevelsofdissimilarityinthede-fencesof closely relatedco‐occurring forest speciesand suggestthat this would be necessary for the coexistence of closely related neighboursandcouldaccountforthehighlocaldiversityoftropicalforests Similarly we suggest that diverse defence strategies en-able savanna species (which are often closely related) to occupydifferentnichesanddefendagainstdifferent typesofherbivoresresultinginmoreresilientandspeciesrichwoodycommunities

ACKNOWLEDG EMENTS

WethankJParsareforassistanceinthefieldandChengappaSKand Balavignesh B for their assistance with laboratory analyses

This work was funded by the National Centre for Biological Sciences TIFR India andbyNationalGeographic (GrantNumber982815)andNERC (GrantNE‐E017436‐1)Wearehugely grateful to theOfficeofthePresidentoftheRepublicofKenyaforpermissiontoconducttheresearchandtheMpalaResearchCentreforlogisti-calsupport

AUTHORSrsquo CONTRIBUTIONS

BJWCCDJAandMSco‐wrotethemanuscriptBJWandMS collected the data DJA MS and JR implemented theexclosure experiment DH conceptualised and performed theanalyticalapproacheswithrespecttofibreandavailableNdeter-mination andperformed condensed tannin and total polyphenolanalysesAllauthorscommentedandaddedtoearlierversionsofthemanuscript

DATA ACCE SSIBILIT Y

Data available from the Dryad Digital Repository httpsdoiorg105061dryadp2d00sf(Wigleyetal2019)

ORCID

Benjamin J Wigley httpsorcidorg0000‐0002‐6964‐3624

Mahesh Sankaran httpsorcidorg0000‐0002‐1661‐6542

R E FE R E N C E S

Agrawal A A (2011) Current trends in the evolutionary ecology ofplant defence Functional Ecology 25(2) 420ndash432 httpsdoiorg101111j1365-2435201001796x

Agrawal A A amp Fishbein M (2006) Plant defense syndromesEcology 87(sp7) 132ndash149 httpsdoiorg1018900012‐9658 (2006)87[132PDS]20CO2

Archibald S ampBond W J (2003) Growing tall vs growing wideTree architecture and allometry of Acacia karroo in forest sa-vanna and arid environments Oikos 102(1) 3ndash14 httpsdoiorg101034j1600-0706200312181x

AugustineDJ(2003)Long‐termlivestock‐mediatedredistributionofnitro-genandphosphorusinanEastAfricansavannaJournal of Applied Ecology 40(1)137ndash149httpsdoiorg101046j1365‐2664200300778x

AugustineDJ(2010)Responseofnativeungulatestodroughtinsemi‐arid Kenyan rangeland African Journal of Ecology 48(4)1009ndash1020httpsdoiorg101111j1365‐2028201001207x

AugustineDJampMcNaughtonSJ(2004)RegulationofshrubdynamicsbynativebrowsingungulatesonEastAfricanrangelandJournal of Applied Ecology 41(1)45ndash58httpsdoiorg101111j1365‐2664200400864x

AugustineDJampMcNaughtonSJ(2006)Interactiveeffectsofungu-lateherbivoressoilfertilityandvariablerainfallonecosystempro-cesses in a semi-arid savanna Ecosystems 9(8)1242ndash1256httpsdoiorg101007s10021-005-0020-y

BartonKE (2016)TougherandthornierGeneralpatterns in the in-ductionofphysicaldefencetraitsFunctional Ecology 30(2)181ndash187httpsdoiorg1011111365‐243512495

BryantJPChapinFSIIIampKleinDR(1983)Carbonnutrientbal-anceofborealplantsinrelationtovertebrateherbivoryOikos 40(3)357ndash368httpsdoiorg1023073544308

6emsp |emsp emspenspJournal of Ecology WIGLEY Et aL

32emsp|emspEffects of herbivore removal on plant defence traits and strategies

ThePCA(Figure1)illustratestheeffectsofherbivoreremovalandcontroltreatmentsontheplantdefencetraits(andstrategies)withherbivore removal treatments diverging from control treatments alongbothPCaxestovaryingdegrees(Figure1)Howevernoneofthespecieschangeddefencestrategiesasaresultofherbivorere-movalbothstructuralandchemicaldefencesrespondedtovariabledegrees both within and between strategies The long-term removal of large mammalian browsers from this savanna did not affect leaf totalNandaciddetergentfibreforanyofthesixofthewoodyspe-cies(Table2)Specificleafareadeclined(ieleavesbecamesmallerandthicker)withherbivoreexclusionforonefine‐leavedspecies(A mellifera p=005)andincreased(leavesbecamelargerandthinner)withherbivoreexclusionfortwobroad‐leavedspeciesB pedicellaris (p=003)andG tenax(p=009Figure2)HerbivoreremovalresultedinhighertotalpolyphenolsinA etbaica and A mellifera(p = 007 and p=001respectively)andsignificantlyhighercondensedtanninsinA brevispica(p=0003Figure2)andloweravailableNinA mellifera (plt01) andC dichogamus (plt001) Plant architectural defences(spinesandbranchingdensities)werefoundtorelaxwithherbivoreremoval ie became less structurally defended Herbivore removal decreasedthebranchingindexandbranchespervolume(plt0001)and increased the bite size index (iemore leaf biomass per bite

plt00001) for all speciesexceptC dichogamus (Figures3 and4) Herbivore removal resulted in significantly shorter spines inA et-baica(plt0001)andA mellifera (p=002)andasignificantlylowerdensityofspinesinA etbaica (p=001)(Figure3)

33emsp|emspGrowth defence strategies and responses to herbivore removal

Overthe10‐yearperiodbetween1999and2009wedocumentedsubstantialvariationingrowthwhichwepresent inrelationtothethreegroupsWhengrowing in thepresenceofherbivoresgroup1 (A brevispica and A mellifera) decreased in height (minus042plusmn007and minus010plusmn009m respectively) but increased in basal area(269plusmn116and 110plusmn656cm2) In this group herbivore removalresultedinsignificantlytallerplantswithhigherbasalareathanwithherbivorespresent(plt0001Table3Figure5)Group2(A etbaica and B pedicellaris)increasedinheight(031plusmn006and024plusmn044mrespectively)andbasalarea(209plusmn314and346plusmn124cm2)inthepresenceofherbivores Inthisgroupherbivoreremoval increasedplantheight(plt001)butnotbasalarea(Table3Figure5)Plantsingroup3(G tenax and C dichogamus)generallydecreasedinheightbut no consistent trend was evident for basal area when herbivores were present ForG tenax herbivore removal resulted in signifi-cantlytallerplantswithhigherbasalareas(plt0001)C dichogamus plantsshowedaslightdecreaseinheightandincreaseinbasalarea

TA B L E 2 emsp Mean plusmn SEforleaftotalavailablenitrogen(mggminus1)leaftotalnitrogen(mggminus1)specificleafarea(SLAcm2 gminus1)leaftotalpolyphenolics(TP)leafcondensedtannins(CT)leafaciddetergentfibre(ADF)branchingindex(BIbranchescm-1)bitesizeindex(BSIg)andaveragespinelength(ASLmm)

Plant trait treatment Aca bre (1) Aca mel (1) Aca etb (2) Bal ped (2) Gre ten (3) Cro dic (3)

Total available N

In(h‐) 234 plusmn 142 156 plusmn 106 156 plusmn 086 215 plusmn 24 226 plusmn 099 126 plusmn 024

Out(h+) 248 plusmn 111 185 plusmn 105 152 plusmn 114 235 plusmn 171 220 plusmn 093 167 plusmn 082

Leaf total N In(h‐) 361 plusmn 220 344 plusmn 088 329 plusmn 090 283 plusmn 190 340 plusmn 090 214 plusmn 030

Out(h+) 394 plusmn 130 352 plusmn 060 320 plusmn 080 312 plusmn 150 322 plusmn 090 234 plusmn 070

SLA In(h‐) 102 plusmn 734 107 plusmn 544 141 plusmn 786 546 plusmn 507 150 plusmn 514 147 plusmn 600

Out(h+) 119 plusmn 809 118 plusmn 302 139 plusmn 120 407 plusmn 252 136 plusmn 712 146 plusmn 600

TP In(h‐) 331 plusmn 046 626 plusmn 025 155 plusmn 009 055 plusmn 003 109 plusmn 014 126 plusmn 006

Out(h+) 278 plusmn 030 535 plusmn 022 136 plusmn 007 055 plusmn 005 129 plusmn 013 142 plusmn 005

CT In(h‐) 472 plusmn 057 658 plusmn 104 038 plusmn 004 008 plusmn 0007 142 plusmn 022 033 plusmn 003

Out(h+) 248 plusmn 037 715 plusmn 051 041 plusmn 004 009 plusmn 001 154 plusmn 022 037 plusmn 001

ADF In(h‐) 329 plusmn 239 371 plusmn 198 304 plusmn 129 246 plusmn 089 430 plusmn 084 338 plusmn 045

Out(h+) 350 plusmn 216 336 plusmn 183 288 plusmn 158 241 plusmn 060 420 plusmn 105 324 plusmn 116

BI In(h‐) 007 plusmn 003 009 plusmn 003 004 plusmn 0007 025 plusmn 003 016 plusmn 002 067 plusmn 016

Out(h+) 045 plusmn 005 038 plusmn 004 032 plusmn 004 125 plusmn 008 119 plusmn 018 056 plusmn 007

BSI In(h‐) 300 plusmn 023 201 plusmn 015 081 plusmn 006 249 plusmn 023 557 plusmn 048 534 plusmn 05

Out(h+) 087 plusmn 008 037 plusmn 003 011 plusmn 001 044 plusmn 004 108 plusmn 008 427 plusmn 049

ASL In(h‐) 346 plusmn 006 373 plusmn 004 529 plusmn 101 547 plusmn 125 mdash mdash

Out(h+) 347 plusmn 007 385 plusmn 003 586 plusmn 090 550 plusmn 086 mdash mdash

NoteTreatmentsareherbivoresexcluded(h‐)andherbivorespresent(h+)GroupmembershipisindicatedinparenthesesnexttoeachspeciesSignificance levels are indicated byp lt 01 p lt 005 p lt 001 p lt 0001 and are highlighted in bold

emspensp emsp | emsp7Journal of EcologyWIGLEY Et aL

duringthestudyperiodwithherbivorespresenthoweverneitherofthe measures of growth differed between the herbivore removal and herbivores present treatments (pgt005) Based on themeasuredchanges inplantheight andbasal area in thepresenceversus ab-sence of browsers over the course of a decade we ranked the three groupsfromlowtohighintermsoftheircapacitytopersistinthefaceofintensebrowsingpressurewheregroup1ltgroup3ltgroup2(Table3Figure5)

4emsp |emspDISCUSSION

OuroverarchinggoalwastoexaminehowwoodyspeciesinthisEastAfricansavannausesuitesoftraitstocontendwitharelativelyintensebrowsing regime imposedbyadiverseandabundantassemblageoflargemammalianherbivoresThehypotheseswhichunderpinclassicplant defence theory rely heavily on resource availability to predictwhetherplantsinvestingrowthordefence(HermsampMattson1992)and hence do not predict relative investments in structural versuschemicaldefences(Hanleyetal2007)orwhichdefenceswillbemosteffectiveunderfixedresourcelevelsbutvaryingherbivorypressureOurresultsindicatethat(a)woodyspeciesinthissystemcanbeclas-sifiedintothreedifferentgroupsorstrategiesbasedontheirdifferen-tialinvestmentinstructuralandchemicaldefences(b)structuralandchemical defences responded to varying degrees to the removal ofherbivoresbothwithinandacrossgroupsbutplantdefencestrategies

remainedqualitativelyunchangedeven after nearly twodecadesofherbivoreexclusion (c) structuralandchemicaldefencesdidnot re-spondinthesamewaytoherbivoreremovalwithstructuraltraits(es-peciallybranching)typicallyrespondingmorepositively(ieincreased)and (d) thespecies thatweremost resilient to intensebrowsing (ieachievedthehighestgrowth)werethosethatmaintainedhighspinelengthspinedensityandbranchinginthepresenceofherbivores

Noneofthespeciesinourstudysiteappeartobeadoptingtheldquolow nutrienthigh chemical defencerdquo strategy with most specieshavingmoderatetohighleafNcomparedtospeciespreviouslycat-egorised as low nutrienthigh chemical defence strategists in other savannasystems(Wigleyetal2018Figure1a)Thethreestrate-giesweidentifiedthusallfallwithinthebroadremitofldquonutritionanddefencerdquobutspeciesinthesedifferentgroupsappeartoinvestvari-ablyinstructuralandN‐freechemicaldefencesFine‐leavedspeciessuch as A brevispica and A mellifera (group1)havemoderatetohighleafNlowinvestmentinstructuraldefences(shortspines)andhighinvestmentinN‐freechemicaldefences(highCTandTP)A etbaica and B pedicellaris(group2)similarlyhadmoderatetohighleafNandbothspeciesinvestedmoreinphysicaldefences(denselongspinesthorns)thanchemicaldefences(lowN‐freesecondarymetabolites)Finally species suchasG tenax and C dichogamus (group3 bothbroad‐leaved)hadmoderatetohighleafNnospinesandlowN‐freesecondary metabolites

Basedonourmeasuredchanges inplantheightandbasalareainthepresencecomparedtoabsenceofbrowsersoverthecourse

F I G U R E 2 emsp Mean plusmn SE total available leafnitrogen(totalNxavailableNproportion(a)specificleafarea(b)totalpolyphenols(c)andcondensedtannins(d)insideandoutsideoftheexclosuresAcabreAcacia brevispicaAcamelAcacia melliferaAcaetbAcacia etbaica BalpedBalanites pedicellaris Gre ten Grewia tenax Cro dic Croton dichogamus Significance levels are p lt 01 p lt 005 p lt 001

8emsp |emsp emspenspJournal of Ecology WIGLEY Et aL

ofonedecadewerankedthesixwoodyspeciesfromlowtohighintermsoftheircapacitytocoexistwithintensebrowsingpressurewhere G tenaxltA brevispicaltA melliferaltC dichogamusltB pedi-cellarisltA etbaica (Figure5)Our results for the twobroad‐leavedspeciesthat lack investment inspines(group3G tenax and C di-chogamus)provideseveralkeyinsightsregardingstrategiesforldquolivingwithbrowsersrdquoBasedontheframeworkdevelopedfromsouthernAfrican savannas (Wigley et al 2018) we expected both speciesto invest heavily in N-free secondary metabolites as a trade-off to the lackof armamentContrary to thispredictionneither speciesinvestedinN‐freesecondarymetaboliteseitherinthepresenceorabsence of large herbivores

LowerleafN(bothtotalandavailable)wasoneconspicuoustraitdistinguishing Crotonfromtheotherspeciesinthisstudyandmaycontributetothefactthatthisspeciesisgenerallyunpalatableanduneatenbylargebrowsers(Kartzineletal2014)HoweverleafN(~22)wasnot so lowas tocompromise ruminantdigestioneffi-ciency(whichtypicallyoccurscloserto1NVanSoest1994)sug-gesting thatsomeotherpotentiallycostlyandasyetunmeasuredaspect ofCroton leaf chemistry is a key trait allowing this broad‐leavedspeciestocoexistwithlargebrowsersCroton sppingeneralarehighlyaromaticandknowntocontainmultipleuniquesecondarychemicals including cembranoids halimanes crotofolanes sesqui-terpenoids flavonoids and cyclohexanol derivatives (Langat et al2016)someofwhichareverysuccessfulinlimitingvertebrateand

invertebrate herbivory (Kaplan Halitschke Kessler Sardanelli ampDenno 2008 Levin 1976) Including these types of chemical de-fences in a generalised trait framework will be challenging as they arenotubiquitousacrossspeciesandtheirexpressioncandependonspecificherbivoreelicitors(Moreiraetal2013)

The lack of investment in N-free secondary metabolites by the secondbroad‐leavedspeciesG tenaxwasalsosurprisingbutcon-sistentwithourfindingthatgrowthofthisspeciesisseverelyneg-ativelyaffectedbylargebrowsersandwithpreviousworkshowingsubstantial declines in all size classes of G tenaxinthepresenceofbrowsersinthissavanna(AugustineampMcNaughton2004Sankaranetal2013)Ratherthanemployinganyformofcostlychemicalde-fence this species appears to coexist (uneasily)with browsers byincreasing the complexity of its branching architecture (BI) andgrowing in close association with other thorny species that cre-ate structural refugia where G tenax saplings are protected frombrowsers (personalobservationbyallauthorsseealsoCoverdaleGoheenPalmerampPringle2018)OnevalueofthisstrategyisthatwhenbrowsingpressureisremovedorlowthelackofinvestmentincostlydefencescombinedwithhighleafNallowsforrapidgrowth

TheremainingfourspeciesbelongingtotheothertwogroupsallhavesomeformofinvestmentinspinesTheycanbearrayedalongagradientofincreasingspineinvestmentfromgroup1togroup2withspeciesingroup1consistingofthosewithshortstraightspines(A brevispica) or short recurved spines (A mellifera) and those in

F I G U R E 3 emsp Mean plusmn SEspinelength(a)spinediameter(b)bitesizeindex(c)andbranchdensity(d)insideandoutsidetheexclosuresAcabreAcacia brevispica AcamelAcacia mellifera AcaetbAcacia etbaicaBalpedBalanites pedicellaris Gre ten Grewia tenax Cro dic Croton dichogamus Significance levels are p lt 005 p lt 001 p lt 0001

(a) (b)

(c) (d)

emspensp emsp | emsp9Journal of EcologyWIGLEY Et aL

F I G U R E 4 emspExamplesofAcacia brevispica and Balanites pedicellaris growing in the absence of large mammalianherbivores(ac)andthesamespeciesgrowinginthepresenceoflargemammalianherbivores(bd)incentralLaikipiaCountyKenyaNotethecleardifferences in architecture branching densities and browser damage between the treatments

(a) (b)

(c) (d)

TA B L E 3 emsp Mean plusmn SEforplantheightandbasalareain1999and2009forthesixspeciesgrowingwithandwithoutherbivorespresentnetchangesinplantheightandbasalareabetweenthe2yearsarealsoshown

Species Treatment Height 1999 (m) Height 2009 (m)Basal area 1999 (cm2)

Basal area 2009 (cm2) Δ height Δ basal area

Acacia brevispica Out(h+) 195 plusmn 007 152 plusmn 005 1379 plusmn 174 1649 plusmn 209 minus043plusmn007 269 plusmn 117

Acacia brevispica In(h‐) 201 plusmn 006 297 plusmn 008 991 plusmn 096 164 plusmn 147 095 plusmn 006 649 plusmn 088

Acacia mellifera Out(h+) 228 plusmn 011 218 plusmn 01 8368 plusmn 1598 9465 plusmn 134 minus010plusmn009 1097 plusmn 657

Acacia mellifera In(h‐) 219 plusmn 009 349 plusmn 01 8048 plusmn 1331 12236 plusmn 1492 13 plusmn 005 4188 plusmn 611

Acacia etbaica Out(h+) 168 plusmn 008 199 plusmn 01 3699 plusmn 616 5786 plusmn 715 031 plusmn 006 2087 plusmn 314

Acacia etbaica In(h‐) 157 plusmn 007 305 plusmn 007 3039 plusmn 754 5183 plusmn 77 148 plusmn 006 2117 plusmn 323

Balanaites pedicellaris Out(h+) 25 plusmn 07 286 plusmn 05 8823 plusmn 4682 12282 plusmn 5008 036 plusmn 047 3459 plusmn 1242

Balanaites pedicellaris In(h‐) 196 plusmn 028 401 plusmn 026 2542 plusmn 829 7193 plusmn 1645 205 plusmn 023 4650 plusmn 1283

Grewia tenax Out(h+) 131 plusmn 009 095 plusmn 024 336 plusmn 044 296 plusmn 048 minus036plusmn024 minus041plusmn035

Grewia tenax In(h‐) 116 plusmn 005 24 plusmn 008 194 plusmn 023 578 plusmn 05 124 plusmn 007 384 plusmn 044

Croton dichogamus Out(h+) 216 plusmn 022 21 plusmn 024 2393 plusmn 831 3323 plusmn 829 minus007plusmn014 929 plusmn 217

Croton dichogamus In(h‐) 159 plusmn 008 182 plusmn 012 932 plusmn 267 1346 plusmn 317 023 plusmn 011 414 plusmn 116

NoteAllmeasurementsarebasedonthesameindividualplantsofeachspeciesthatwerepresentintheplotsin1999andthatwerestillalivein2009

10emsp |emsp emspenspJournal of Ecology WIGLEY Et aL

group2havingeither long straight spines (B pedicellaris)orbothshort recurvedand long straight spines (A etbaica) Specieswiththelongestandmostdensespines(group2)showedlittletonoin-vestmentinCTorTPwhilethespecieswithlowspineinvestment(group 1) showed the greatest concentrations of bothCT and TP(Figure2)The latterstrategyexperiencesgreaterconsumptionbybrowsersinthislandscaperelativetotheformer(Fordetal2014)consistentwithpreviousstudieswhichhavereportedthatspecieswithhigherinvestmentinstructuraldefences(group2)aremostsuc-cessfulatcoexistingwithbrowsersinthissavannasystem(GoheenKeesingAllanOgadaampOstfeld2004Sankaranetal2013)Inthepresenceofbrowsersspeciesinbothgroups1and2maintainedanincreasedcomplexityoftheirbranchingarchitecturecombinedwithsmallerleaves(reflectedinsubstantialreductionsinbitesizeindex)Increased branching even in the absence of changes in thorn length anddensitywhichoccurredinsomespeciesbutnototherscanre-sultindramaticincreasesinthenumberofspinesperunitvolumeofcanopy(ArchibaldampBond2003StaverBondCramerampWakeling2012)servingasaneffectivedeterrentforbrowsers

To summarise the strategies expressed by spinescent speciesin termsof ldquolivingwithbrowsersrdquo canbecharacterisedas (a)veryhigh and inducible investment in spinesbutnotN‐free secondarymetabolites and low available N leading to greatest success under intensebrowsing(A etbaica)(b)moderateandinducibleinvestmentinstraightspinescombinedwith thickermore fibrous leavesbuthigh available N leading to sufficient success under intense brows-ing(B pedicellaris)(c)lowandnon‐inducibleinvestmentinrecurvedspines combinedwithhighN‐free secondarymetabolites and lowavailableNleadingtoreducedsuccessunderbrowsing(A mellifera)

and(d)lowandnon‐inducibleinvestmentinshortspinescombinedwith moderate investment in N-free secondary metabolites and high availableN leading to low success under browsing (A brevispica)Fornara and Du Toit (2008) suggested that physical defences to-getherwithmasscompensatorygrowthabilitiesarekeyadaptationstolivingwithhighbrowsingpressureOurfindingslendstrongsup-porttothisnotionasthemostheavilystructurallydefendedspecies(A etbaica and B pedicellaris)performedthebestinthepresenceofherbivoresdespitelowinvestmentsinN‐freesecondarymetabolitesHowever Scogingset al (2011)predicted that the concentrationsofN‐freesecondarycompoundswoulddependonbrowsinginten-sity with the highest concentrations occurring at intermediate levels ofbrowsingforcertainfunctionalgroupsForexampleScogingsetal(2011)predictalinearincreaseinN‐freesecondarycompoundswithbrowsingintensitywithapotentiallysuddendecreaseatveryhigh levelsofbrowsingpressure indeciduoussavannaspeciesAsourexperimentaldesign lackedan intermediatebrowsingpressuretreatmentwewereunabletotestthesepredictions

Our analyses of available Nprovidedsomeinterestingandvalu-ableinsightsFirstlyinterpretationsofbrowsequalitydifferdepend-ing on whether total N (also commonly converted to crudeproteinby multiplying by 625 eg Cooper et al 1988) or available N isevaluatedForexamplethethreeAcaciaspeciesinthisstudyA bre-vispica A mellifera and A etbaica all had similar and relatively high con-centrations of leaf total N (~3835and33respectively)buttheproportionofthetotalN available to herbivores in A brevispica was ca 20 higher than in A mellifera and A etbaica(~2417and15respectively seeTable2) Ifweonly lookedat totalN they would all appear tobeofhighqualityhowever ifwe lookat availableN

F I G U R E 5 emsp (a)MeanplusmnSE change in plantheightbetween1999and2009forthesixdominantspeciesgrowinginsideandoutsidetheexclosures(b)Mean plusmn SE change in stem basal area between 1999 and 2009 for the same six speciesSignificancelevelsarep lt 001 p lt 0001

emspensp emsp | emsp11Journal of EcologyWIGLEY Et aL

itappearsasifA brevispicaisofmuchhigherqualitythantheothertwospeciesSecondlyavailableNappearstobemoreresponsivetoherbivore removal than total leaf N No differences in total N were evident between herbivory treatments while available N was signifi-cantly higher in A mellifera and C dichogamus when herbivores were presentThirdlydifferencesinavailableNdonotseemtobelinkedtophylogeny(eghighinsomeAcaciaspecieslowinothers)functionaltype(egfine‐leavedvsbroad‐leaved)norconcentrationsofN‐freesecondarymetabolites (availableNfoundtobe low inspecieswithbothhighand lowN‐freesecondarymetabolites)Finally these re-sults raise some importantquestions regarding the commonlyheldview (especially in African savannas) that fine‐leaved species (egAcacia)havehigherqualityleavesandarestructurallydefendedwhilebroad‐leavedspeciesarelessnutritiousandchemicallydefendedForexampleG tenax and B pedicellarisbothbroad‐leavedspecieshadhigher available Nthanthetwofine‐leavedspeciesA mellifera and A etbaica These findings highlight the need for further studies to eluci-date the determinants of leaf available N and that caution needs to be taken when using total NasameasureofbrowsequalityFurthermoreour findings suggest that neither total N nor available N are sufficient bythemselvestopredictbrowserutilisationhighlightingtheneedtoconsideralltraitscollectively(iedefencestrategies)

5emsp |emspCONCLUSIONS

Allofthedominantspeciesatourstudysitehadmoderatetohighleaf N concentrations but differed in available N and in the way they combined chemical and structural defences thus fitting under thebroaderldquonutritionanddefencerdquosyndromepreviouslydescribedforsavannasStructuraldefenceshadmorepronouncedresponsesto herbivory through increased spine length and density andorincreased branching while N-free secondary metabolites did not increaseinresponsetoherbivoryHighinvestmentinstructuralde-fences was the most successful defence strategy at our study sites withmoderateresourcelevels(relativetootherAfricansavannas)andhighherbivorypressureresultinginthehighestgrowthinthissavanna system This work has shown that within one overarching defence syndrome (nutritionanddefence) specieswithina com-munity can widely diverge in their adopted defence strategiesEndaraetal(2015)reportedhighlevelsofdissimilarityinthede-fencesof closely relatedco‐occurring forest speciesand suggestthat this would be necessary for the coexistence of closely related neighboursandcouldaccountforthehighlocaldiversityoftropicalforests Similarly we suggest that diverse defence strategies en-able savanna species (which are often closely related) to occupydifferentnichesanddefendagainstdifferent typesofherbivoresresultinginmoreresilientandspeciesrichwoodycommunities

ACKNOWLEDG EMENTS

WethankJParsareforassistanceinthefieldandChengappaSKand Balavignesh B for their assistance with laboratory analyses

This work was funded by the National Centre for Biological Sciences TIFR India andbyNationalGeographic (GrantNumber982815)andNERC (GrantNE‐E017436‐1)Wearehugely grateful to theOfficeofthePresidentoftheRepublicofKenyaforpermissiontoconducttheresearchandtheMpalaResearchCentreforlogisti-calsupport

AUTHORSrsquo CONTRIBUTIONS

BJWCCDJAandMSco‐wrotethemanuscriptBJWandMS collected the data DJA MS and JR implemented theexclosure experiment DH conceptualised and performed theanalyticalapproacheswithrespecttofibreandavailableNdeter-mination andperformed condensed tannin and total polyphenolanalysesAllauthorscommentedandaddedtoearlierversionsofthemanuscript

DATA ACCE SSIBILIT Y

Data available from the Dryad Digital Repository httpsdoiorg105061dryadp2d00sf(Wigleyetal2019)

ORCID

Benjamin J Wigley httpsorcidorg0000‐0002‐6964‐3624

Mahesh Sankaran httpsorcidorg0000‐0002‐1661‐6542

R E FE R E N C E S

Agrawal A A (2011) Current trends in the evolutionary ecology ofplant defence Functional Ecology 25(2) 420ndash432 httpsdoiorg101111j1365-2435201001796x

Agrawal A A amp Fishbein M (2006) Plant defense syndromesEcology 87(sp7) 132ndash149 httpsdoiorg1018900012‐9658 (2006)87[132PDS]20CO2

Archibald S ampBond W J (2003) Growing tall vs growing wideTree architecture and allometry of Acacia karroo in forest sa-vanna and arid environments Oikos 102(1) 3ndash14 httpsdoiorg101034j1600-0706200312181x

AugustineDJ(2003)Long‐termlivestock‐mediatedredistributionofnitro-genandphosphorusinanEastAfricansavannaJournal of Applied Ecology 40(1)137ndash149httpsdoiorg101046j1365‐2664200300778x

AugustineDJ(2010)Responseofnativeungulatestodroughtinsemi‐arid Kenyan rangeland African Journal of Ecology 48(4)1009ndash1020httpsdoiorg101111j1365‐2028201001207x

AugustineDJampMcNaughtonSJ(2004)RegulationofshrubdynamicsbynativebrowsingungulatesonEastAfricanrangelandJournal of Applied Ecology 41(1)45ndash58httpsdoiorg101111j1365‐2664200400864x

AugustineDJampMcNaughtonSJ(2006)Interactiveeffectsofungu-lateherbivoressoilfertilityandvariablerainfallonecosystempro-cesses in a semi-arid savanna Ecosystems 9(8)1242ndash1256httpsdoiorg101007s10021-005-0020-y

BartonKE (2016)TougherandthornierGeneralpatterns in the in-ductionofphysicaldefencetraitsFunctional Ecology 30(2)181ndash187httpsdoiorg1011111365‐243512495

BryantJPChapinFSIIIampKleinDR(1983)Carbonnutrientbal-anceofborealplantsinrelationtovertebrateherbivoryOikos 40(3)357ndash368httpsdoiorg1023073544308

emspensp emsp | emsp7Journal of EcologyWIGLEY Et aL

duringthestudyperiodwithherbivorespresenthoweverneitherofthe measures of growth differed between the herbivore removal and herbivores present treatments (pgt005) Based on themeasuredchanges inplantheight andbasal area in thepresenceversus ab-sence of browsers over the course of a decade we ranked the three groupsfromlowtohighintermsoftheircapacitytopersistinthefaceofintensebrowsingpressurewheregroup1ltgroup3ltgroup2(Table3Figure5)

4emsp |emspDISCUSSION

OuroverarchinggoalwastoexaminehowwoodyspeciesinthisEastAfricansavannausesuitesoftraitstocontendwitharelativelyintensebrowsing regime imposedbyadiverseandabundantassemblageoflargemammalianherbivoresThehypotheseswhichunderpinclassicplant defence theory rely heavily on resource availability to predictwhetherplantsinvestingrowthordefence(HermsampMattson1992)and hence do not predict relative investments in structural versuschemicaldefences(Hanleyetal2007)orwhichdefenceswillbemosteffectiveunderfixedresourcelevelsbutvaryingherbivorypressureOurresultsindicatethat(a)woodyspeciesinthissystemcanbeclas-sifiedintothreedifferentgroupsorstrategiesbasedontheirdifferen-tialinvestmentinstructuralandchemicaldefences(b)structuralandchemical defences responded to varying degrees to the removal ofherbivoresbothwithinandacrossgroupsbutplantdefencestrategies

remainedqualitativelyunchangedeven after nearly twodecadesofherbivoreexclusion (c) structuralandchemicaldefencesdidnot re-spondinthesamewaytoherbivoreremovalwithstructuraltraits(es-peciallybranching)typicallyrespondingmorepositively(ieincreased)and (d) thespecies thatweremost resilient to intensebrowsing (ieachievedthehighestgrowth)werethosethatmaintainedhighspinelengthspinedensityandbranchinginthepresenceofherbivores

Noneofthespeciesinourstudysiteappeartobeadoptingtheldquolow nutrienthigh chemical defencerdquo strategy with most specieshavingmoderatetohighleafNcomparedtospeciespreviouslycat-egorised as low nutrienthigh chemical defence strategists in other savannasystems(Wigleyetal2018Figure1a)Thethreestrate-giesweidentifiedthusallfallwithinthebroadremitofldquonutritionanddefencerdquobutspeciesinthesedifferentgroupsappeartoinvestvari-ablyinstructuralandN‐freechemicaldefencesFine‐leavedspeciessuch as A brevispica and A mellifera (group1)havemoderatetohighleafNlowinvestmentinstructuraldefences(shortspines)andhighinvestmentinN‐freechemicaldefences(highCTandTP)A etbaica and B pedicellaris(group2)similarlyhadmoderatetohighleafNandbothspeciesinvestedmoreinphysicaldefences(denselongspinesthorns)thanchemicaldefences(lowN‐freesecondarymetabolites)Finally species suchasG tenax and C dichogamus (group3 bothbroad‐leaved)hadmoderatetohighleafNnospinesandlowN‐freesecondary metabolites

Basedonourmeasuredchanges inplantheightandbasalareainthepresencecomparedtoabsenceofbrowsersoverthecourse

F I G U R E 2 emsp Mean plusmn SE total available leafnitrogen(totalNxavailableNproportion(a)specificleafarea(b)totalpolyphenols(c)andcondensedtannins(d)insideandoutsideoftheexclosuresAcabreAcacia brevispicaAcamelAcacia melliferaAcaetbAcacia etbaica BalpedBalanites pedicellaris Gre ten Grewia tenax Cro dic Croton dichogamus Significance levels are p lt 01 p lt 005 p lt 001

8emsp |emsp emspenspJournal of Ecology WIGLEY Et aL

ofonedecadewerankedthesixwoodyspeciesfromlowtohighintermsoftheircapacitytocoexistwithintensebrowsingpressurewhere G tenaxltA brevispicaltA melliferaltC dichogamusltB pedi-cellarisltA etbaica (Figure5)Our results for the twobroad‐leavedspeciesthat lack investment inspines(group3G tenax and C di-chogamus)provideseveralkeyinsightsregardingstrategiesforldquolivingwithbrowsersrdquoBasedontheframeworkdevelopedfromsouthernAfrican savannas (Wigley et al 2018) we expected both speciesto invest heavily in N-free secondary metabolites as a trade-off to the lackof armamentContrary to thispredictionneither speciesinvestedinN‐freesecondarymetaboliteseitherinthepresenceorabsence of large herbivores

LowerleafN(bothtotalandavailable)wasoneconspicuoustraitdistinguishing Crotonfromtheotherspeciesinthisstudyandmaycontributetothefactthatthisspeciesisgenerallyunpalatableanduneatenbylargebrowsers(Kartzineletal2014)HoweverleafN(~22)wasnot so lowas tocompromise ruminantdigestioneffi-ciency(whichtypicallyoccurscloserto1NVanSoest1994)sug-gesting thatsomeotherpotentiallycostlyandasyetunmeasuredaspect ofCroton leaf chemistry is a key trait allowing this broad‐leavedspeciestocoexistwithlargebrowsersCroton sppingeneralarehighlyaromaticandknowntocontainmultipleuniquesecondarychemicals including cembranoids halimanes crotofolanes sesqui-terpenoids flavonoids and cyclohexanol derivatives (Langat et al2016)someofwhichareverysuccessfulinlimitingvertebrateand

invertebrate herbivory (Kaplan Halitschke Kessler Sardanelli ampDenno 2008 Levin 1976) Including these types of chemical de-fences in a generalised trait framework will be challenging as they arenotubiquitousacrossspeciesandtheirexpressioncandependonspecificherbivoreelicitors(Moreiraetal2013)

The lack of investment in N-free secondary metabolites by the secondbroad‐leavedspeciesG tenaxwasalsosurprisingbutcon-sistentwithourfindingthatgrowthofthisspeciesisseverelyneg-ativelyaffectedbylargebrowsersandwithpreviousworkshowingsubstantial declines in all size classes of G tenaxinthepresenceofbrowsersinthissavanna(AugustineampMcNaughton2004Sankaranetal2013)Ratherthanemployinganyformofcostlychemicalde-fence this species appears to coexist (uneasily)with browsers byincreasing the complexity of its branching architecture (BI) andgrowing in close association with other thorny species that cre-ate structural refugia where G tenax saplings are protected frombrowsers (personalobservationbyallauthorsseealsoCoverdaleGoheenPalmerampPringle2018)OnevalueofthisstrategyisthatwhenbrowsingpressureisremovedorlowthelackofinvestmentincostlydefencescombinedwithhighleafNallowsforrapidgrowth

TheremainingfourspeciesbelongingtotheothertwogroupsallhavesomeformofinvestmentinspinesTheycanbearrayedalongagradientofincreasingspineinvestmentfromgroup1togroup2withspeciesingroup1consistingofthosewithshortstraightspines(A brevispica) or short recurved spines (A mellifera) and those in

F I G U R E 3 emsp Mean plusmn SEspinelength(a)spinediameter(b)bitesizeindex(c)andbranchdensity(d)insideandoutsidetheexclosuresAcabreAcacia brevispica AcamelAcacia mellifera AcaetbAcacia etbaicaBalpedBalanites pedicellaris Gre ten Grewia tenax Cro dic Croton dichogamus Significance levels are p lt 005 p lt 001 p lt 0001

(a) (b)

(c) (d)

emspensp emsp | emsp9Journal of EcologyWIGLEY Et aL

F I G U R E 4 emspExamplesofAcacia brevispica and Balanites pedicellaris growing in the absence of large mammalianherbivores(ac)andthesamespeciesgrowinginthepresenceoflargemammalianherbivores(bd)incentralLaikipiaCountyKenyaNotethecleardifferences in architecture branching densities and browser damage between the treatments

(a) (b)

(c) (d)

TA B L E 3 emsp Mean plusmn SEforplantheightandbasalareain1999and2009forthesixspeciesgrowingwithandwithoutherbivorespresentnetchangesinplantheightandbasalareabetweenthe2yearsarealsoshown

Species Treatment Height 1999 (m) Height 2009 (m)Basal area 1999 (cm2)

Basal area 2009 (cm2) Δ height Δ basal area

Acacia brevispica Out(h+) 195 plusmn 007 152 plusmn 005 1379 plusmn 174 1649 plusmn 209 minus043plusmn007 269 plusmn 117

Acacia brevispica In(h‐) 201 plusmn 006 297 plusmn 008 991 plusmn 096 164 plusmn 147 095 plusmn 006 649 plusmn 088

Acacia mellifera Out(h+) 228 plusmn 011 218 plusmn 01 8368 plusmn 1598 9465 plusmn 134 minus010plusmn009 1097 plusmn 657

Acacia mellifera In(h‐) 219 plusmn 009 349 plusmn 01 8048 plusmn 1331 12236 plusmn 1492 13 plusmn 005 4188 plusmn 611

Acacia etbaica Out(h+) 168 plusmn 008 199 plusmn 01 3699 plusmn 616 5786 plusmn 715 031 plusmn 006 2087 plusmn 314

Acacia etbaica In(h‐) 157 plusmn 007 305 plusmn 007 3039 plusmn 754 5183 plusmn 77 148 plusmn 006 2117 plusmn 323

Balanaites pedicellaris Out(h+) 25 plusmn 07 286 plusmn 05 8823 plusmn 4682 12282 plusmn 5008 036 plusmn 047 3459 plusmn 1242

Balanaites pedicellaris In(h‐) 196 plusmn 028 401 plusmn 026 2542 plusmn 829 7193 plusmn 1645 205 plusmn 023 4650 plusmn 1283

Grewia tenax Out(h+) 131 plusmn 009 095 plusmn 024 336 plusmn 044 296 plusmn 048 minus036plusmn024 minus041plusmn035

Grewia tenax In(h‐) 116 plusmn 005 24 plusmn 008 194 plusmn 023 578 plusmn 05 124 plusmn 007 384 plusmn 044

Croton dichogamus Out(h+) 216 plusmn 022 21 plusmn 024 2393 plusmn 831 3323 plusmn 829 minus007plusmn014 929 plusmn 217

Croton dichogamus In(h‐) 159 plusmn 008 182 plusmn 012 932 plusmn 267 1346 plusmn 317 023 plusmn 011 414 plusmn 116

NoteAllmeasurementsarebasedonthesameindividualplantsofeachspeciesthatwerepresentintheplotsin1999andthatwerestillalivein2009

10emsp |emsp emspenspJournal of Ecology WIGLEY Et aL

group2havingeither long straight spines (B pedicellaris)orbothshort recurvedand long straight spines (A etbaica) Specieswiththelongestandmostdensespines(group2)showedlittletonoin-vestmentinCTorTPwhilethespecieswithlowspineinvestment(group 1) showed the greatest concentrations of bothCT and TP(Figure2)The latterstrategyexperiencesgreaterconsumptionbybrowsersinthislandscaperelativetotheformer(Fordetal2014)consistentwithpreviousstudieswhichhavereportedthatspecieswithhigherinvestmentinstructuraldefences(group2)aremostsuc-cessfulatcoexistingwithbrowsersinthissavannasystem(GoheenKeesingAllanOgadaampOstfeld2004Sankaranetal2013)Inthepresenceofbrowsersspeciesinbothgroups1and2maintainedanincreasedcomplexityoftheirbranchingarchitecturecombinedwithsmallerleaves(reflectedinsubstantialreductionsinbitesizeindex)Increased branching even in the absence of changes in thorn length anddensitywhichoccurredinsomespeciesbutnototherscanre-sultindramaticincreasesinthenumberofspinesperunitvolumeofcanopy(ArchibaldampBond2003StaverBondCramerampWakeling2012)servingasaneffectivedeterrentforbrowsers

To summarise the strategies expressed by spinescent speciesin termsof ldquolivingwithbrowsersrdquo canbecharacterisedas (a)veryhigh and inducible investment in spinesbutnotN‐free secondarymetabolites and low available N leading to greatest success under intensebrowsing(A etbaica)(b)moderateandinducibleinvestmentinstraightspinescombinedwith thickermore fibrous leavesbuthigh available N leading to sufficient success under intense brows-ing(B pedicellaris)(c)lowandnon‐inducibleinvestmentinrecurvedspines combinedwithhighN‐free secondarymetabolites and lowavailableNleadingtoreducedsuccessunderbrowsing(A mellifera)

and(d)lowandnon‐inducibleinvestmentinshortspinescombinedwith moderate investment in N-free secondary metabolites and high availableN leading to low success under browsing (A brevispica)Fornara and Du Toit (2008) suggested that physical defences to-getherwithmasscompensatorygrowthabilitiesarekeyadaptationstolivingwithhighbrowsingpressureOurfindingslendstrongsup-porttothisnotionasthemostheavilystructurallydefendedspecies(A etbaica and B pedicellaris)performedthebestinthepresenceofherbivoresdespitelowinvestmentsinN‐freesecondarymetabolitesHowever Scogingset al (2011)predicted that the concentrationsofN‐freesecondarycompoundswoulddependonbrowsinginten-sity with the highest concentrations occurring at intermediate levels ofbrowsingforcertainfunctionalgroupsForexampleScogingsetal(2011)predictalinearincreaseinN‐freesecondarycompoundswithbrowsingintensitywithapotentiallysuddendecreaseatveryhigh levelsofbrowsingpressure indeciduoussavannaspeciesAsourexperimentaldesign lackedan intermediatebrowsingpressuretreatmentwewereunabletotestthesepredictions

Our analyses of available Nprovidedsomeinterestingandvalu-ableinsightsFirstlyinterpretationsofbrowsequalitydifferdepend-ing on whether total N (also commonly converted to crudeproteinby multiplying by 625 eg Cooper et al 1988) or available N isevaluatedForexamplethethreeAcaciaspeciesinthisstudyA bre-vispica A mellifera and A etbaica all had similar and relatively high con-centrations of leaf total N (~3835and33respectively)buttheproportionofthetotalN available to herbivores in A brevispica was ca 20 higher than in A mellifera and A etbaica(~2417and15respectively seeTable2) Ifweonly lookedat totalN they would all appear tobeofhighqualityhowever ifwe lookat availableN

F I G U R E 5 emsp (a)MeanplusmnSE change in plantheightbetween1999and2009forthesixdominantspeciesgrowinginsideandoutsidetheexclosures(b)Mean plusmn SE change in stem basal area between 1999 and 2009 for the same six speciesSignificancelevelsarep lt 001 p lt 0001

emspensp emsp | emsp11Journal of EcologyWIGLEY Et aL

itappearsasifA brevispicaisofmuchhigherqualitythantheothertwospeciesSecondlyavailableNappearstobemoreresponsivetoherbivore removal than total leaf N No differences in total N were evident between herbivory treatments while available N was signifi-cantly higher in A mellifera and C dichogamus when herbivores were presentThirdlydifferencesinavailableNdonotseemtobelinkedtophylogeny(eghighinsomeAcaciaspecieslowinothers)functionaltype(egfine‐leavedvsbroad‐leaved)norconcentrationsofN‐freesecondarymetabolites (availableNfoundtobe low inspecieswithbothhighand lowN‐freesecondarymetabolites)Finally these re-sults raise some importantquestions regarding the commonlyheldview (especially in African savannas) that fine‐leaved species (egAcacia)havehigherqualityleavesandarestructurallydefendedwhilebroad‐leavedspeciesarelessnutritiousandchemicallydefendedForexampleG tenax and B pedicellarisbothbroad‐leavedspecieshadhigher available Nthanthetwofine‐leavedspeciesA mellifera and A etbaica These findings highlight the need for further studies to eluci-date the determinants of leaf available N and that caution needs to be taken when using total NasameasureofbrowsequalityFurthermoreour findings suggest that neither total N nor available N are sufficient bythemselvestopredictbrowserutilisationhighlightingtheneedtoconsideralltraitscollectively(iedefencestrategies)

5emsp |emspCONCLUSIONS

Allofthedominantspeciesatourstudysitehadmoderatetohighleaf N concentrations but differed in available N and in the way they combined chemical and structural defences thus fitting under thebroaderldquonutritionanddefencerdquosyndromepreviouslydescribedforsavannasStructuraldefenceshadmorepronouncedresponsesto herbivory through increased spine length and density andorincreased branching while N-free secondary metabolites did not increaseinresponsetoherbivoryHighinvestmentinstructuralde-fences was the most successful defence strategy at our study sites withmoderateresourcelevels(relativetootherAfricansavannas)andhighherbivorypressureresultinginthehighestgrowthinthissavanna system This work has shown that within one overarching defence syndrome (nutritionanddefence) specieswithina com-munity can widely diverge in their adopted defence strategiesEndaraetal(2015)reportedhighlevelsofdissimilarityinthede-fencesof closely relatedco‐occurring forest speciesand suggestthat this would be necessary for the coexistence of closely related neighboursandcouldaccountforthehighlocaldiversityoftropicalforests Similarly we suggest that diverse defence strategies en-able savanna species (which are often closely related) to occupydifferentnichesanddefendagainstdifferent typesofherbivoresresultinginmoreresilientandspeciesrichwoodycommunities

ACKNOWLEDG EMENTS

WethankJParsareforassistanceinthefieldandChengappaSKand Balavignesh B for their assistance with laboratory analyses

This work was funded by the National Centre for Biological Sciences TIFR India andbyNationalGeographic (GrantNumber982815)andNERC (GrantNE‐E017436‐1)Wearehugely grateful to theOfficeofthePresidentoftheRepublicofKenyaforpermissiontoconducttheresearchandtheMpalaResearchCentreforlogisti-calsupport

AUTHORSrsquo CONTRIBUTIONS

BJWCCDJAandMSco‐wrotethemanuscriptBJWandMS collected the data DJA MS and JR implemented theexclosure experiment DH conceptualised and performed theanalyticalapproacheswithrespecttofibreandavailableNdeter-mination andperformed condensed tannin and total polyphenolanalysesAllauthorscommentedandaddedtoearlierversionsofthemanuscript

DATA ACCE SSIBILIT Y

Data available from the Dryad Digital Repository httpsdoiorg105061dryadp2d00sf(Wigleyetal2019)

ORCID

Benjamin J Wigley httpsorcidorg0000‐0002‐6964‐3624

Mahesh Sankaran httpsorcidorg0000‐0002‐1661‐6542

R E FE R E N C E S

Agrawal A A (2011) Current trends in the evolutionary ecology ofplant defence Functional Ecology 25(2) 420ndash432 httpsdoiorg101111j1365-2435201001796x

Agrawal A A amp Fishbein M (2006) Plant defense syndromesEcology 87(sp7) 132ndash149 httpsdoiorg1018900012‐9658 (2006)87[132PDS]20CO2

Archibald S ampBond W J (2003) Growing tall vs growing wideTree architecture and allometry of Acacia karroo in forest sa-vanna and arid environments Oikos 102(1) 3ndash14 httpsdoiorg101034j1600-0706200312181x

AugustineDJ(2003)Long‐termlivestock‐mediatedredistributionofnitro-genandphosphorusinanEastAfricansavannaJournal of Applied Ecology 40(1)137ndash149httpsdoiorg101046j1365‐2664200300778x

AugustineDJ(2010)Responseofnativeungulatestodroughtinsemi‐arid Kenyan rangeland African Journal of Ecology 48(4)1009ndash1020httpsdoiorg101111j1365‐2028201001207x

AugustineDJampMcNaughtonSJ(2004)RegulationofshrubdynamicsbynativebrowsingungulatesonEastAfricanrangelandJournal of Applied Ecology 41(1)45ndash58httpsdoiorg101111j1365‐2664200400864x

AugustineDJampMcNaughtonSJ(2006)Interactiveeffectsofungu-lateherbivoressoilfertilityandvariablerainfallonecosystempro-cesses in a semi-arid savanna Ecosystems 9(8)1242ndash1256httpsdoiorg101007s10021-005-0020-y

BartonKE (2016)TougherandthornierGeneralpatterns in the in-ductionofphysicaldefencetraitsFunctional Ecology 30(2)181ndash187httpsdoiorg1011111365‐243512495

BryantJPChapinFSIIIampKleinDR(1983)Carbonnutrientbal-anceofborealplantsinrelationtovertebrateherbivoryOikos 40(3)357ndash368httpsdoiorg1023073544308

8emsp |emsp emspenspJournal of Ecology WIGLEY Et aL

ofonedecadewerankedthesixwoodyspeciesfromlowtohighintermsoftheircapacitytocoexistwithintensebrowsingpressurewhere G tenaxltA brevispicaltA melliferaltC dichogamusltB pedi-cellarisltA etbaica (Figure5)Our results for the twobroad‐leavedspeciesthat lack investment inspines(group3G tenax and C di-chogamus)provideseveralkeyinsightsregardingstrategiesforldquolivingwithbrowsersrdquoBasedontheframeworkdevelopedfromsouthernAfrican savannas (Wigley et al 2018) we expected both speciesto invest heavily in N-free secondary metabolites as a trade-off to the lackof armamentContrary to thispredictionneither speciesinvestedinN‐freesecondarymetaboliteseitherinthepresenceorabsence of large herbivores

LowerleafN(bothtotalandavailable)wasoneconspicuoustraitdistinguishing Crotonfromtheotherspeciesinthisstudyandmaycontributetothefactthatthisspeciesisgenerallyunpalatableanduneatenbylargebrowsers(Kartzineletal2014)HoweverleafN(~22)wasnot so lowas tocompromise ruminantdigestioneffi-ciency(whichtypicallyoccurscloserto1NVanSoest1994)sug-gesting thatsomeotherpotentiallycostlyandasyetunmeasuredaspect ofCroton leaf chemistry is a key trait allowing this broad‐leavedspeciestocoexistwithlargebrowsersCroton sppingeneralarehighlyaromaticandknowntocontainmultipleuniquesecondarychemicals including cembranoids halimanes crotofolanes sesqui-terpenoids flavonoids and cyclohexanol derivatives (Langat et al2016)someofwhichareverysuccessfulinlimitingvertebrateand

invertebrate herbivory (Kaplan Halitschke Kessler Sardanelli ampDenno 2008 Levin 1976) Including these types of chemical de-fences in a generalised trait framework will be challenging as they arenotubiquitousacrossspeciesandtheirexpressioncandependonspecificherbivoreelicitors(Moreiraetal2013)

The lack of investment in N-free secondary metabolites by the secondbroad‐leavedspeciesG tenaxwasalsosurprisingbutcon-sistentwithourfindingthatgrowthofthisspeciesisseverelyneg-ativelyaffectedbylargebrowsersandwithpreviousworkshowingsubstantial declines in all size classes of G tenaxinthepresenceofbrowsersinthissavanna(AugustineampMcNaughton2004Sankaranetal2013)Ratherthanemployinganyformofcostlychemicalde-fence this species appears to coexist (uneasily)with browsers byincreasing the complexity of its branching architecture (BI) andgrowing in close association with other thorny species that cre-ate structural refugia where G tenax saplings are protected frombrowsers (personalobservationbyallauthorsseealsoCoverdaleGoheenPalmerampPringle2018)OnevalueofthisstrategyisthatwhenbrowsingpressureisremovedorlowthelackofinvestmentincostlydefencescombinedwithhighleafNallowsforrapidgrowth

TheremainingfourspeciesbelongingtotheothertwogroupsallhavesomeformofinvestmentinspinesTheycanbearrayedalongagradientofincreasingspineinvestmentfromgroup1togroup2withspeciesingroup1consistingofthosewithshortstraightspines(A brevispica) or short recurved spines (A mellifera) and those in

F I G U R E 3 emsp Mean plusmn SEspinelength(a)spinediameter(b)bitesizeindex(c)andbranchdensity(d)insideandoutsidetheexclosuresAcabreAcacia brevispica AcamelAcacia mellifera AcaetbAcacia etbaicaBalpedBalanites pedicellaris Gre ten Grewia tenax Cro dic Croton dichogamus Significance levels are p lt 005 p lt 001 p lt 0001

(a) (b)

(c) (d)

emspensp emsp | emsp9Journal of EcologyWIGLEY Et aL

F I G U R E 4 emspExamplesofAcacia brevispica and Balanites pedicellaris growing in the absence of large mammalianherbivores(ac)andthesamespeciesgrowinginthepresenceoflargemammalianherbivores(bd)incentralLaikipiaCountyKenyaNotethecleardifferences in architecture branching densities and browser damage between the treatments

(a) (b)

(c) (d)

TA B L E 3 emsp Mean plusmn SEforplantheightandbasalareain1999and2009forthesixspeciesgrowingwithandwithoutherbivorespresentnetchangesinplantheightandbasalareabetweenthe2yearsarealsoshown

Species Treatment Height 1999 (m) Height 2009 (m)Basal area 1999 (cm2)

Basal area 2009 (cm2) Δ height Δ basal area

Acacia brevispica Out(h+) 195 plusmn 007 152 plusmn 005 1379 plusmn 174 1649 plusmn 209 minus043plusmn007 269 plusmn 117

Acacia brevispica In(h‐) 201 plusmn 006 297 plusmn 008 991 plusmn 096 164 plusmn 147 095 plusmn 006 649 plusmn 088

Acacia mellifera Out(h+) 228 plusmn 011 218 plusmn 01 8368 plusmn 1598 9465 plusmn 134 minus010plusmn009 1097 plusmn 657

Acacia mellifera In(h‐) 219 plusmn 009 349 plusmn 01 8048 plusmn 1331 12236 plusmn 1492 13 plusmn 005 4188 plusmn 611

Acacia etbaica Out(h+) 168 plusmn 008 199 plusmn 01 3699 plusmn 616 5786 plusmn 715 031 plusmn 006 2087 plusmn 314

Acacia etbaica In(h‐) 157 plusmn 007 305 plusmn 007 3039 plusmn 754 5183 plusmn 77 148 plusmn 006 2117 plusmn 323

Balanaites pedicellaris Out(h+) 25 plusmn 07 286 plusmn 05 8823 plusmn 4682 12282 plusmn 5008 036 plusmn 047 3459 plusmn 1242

Balanaites pedicellaris In(h‐) 196 plusmn 028 401 plusmn 026 2542 plusmn 829 7193 plusmn 1645 205 plusmn 023 4650 plusmn 1283

Grewia tenax Out(h+) 131 plusmn 009 095 plusmn 024 336 plusmn 044 296 plusmn 048 minus036plusmn024 minus041plusmn035

Grewia tenax In(h‐) 116 plusmn 005 24 plusmn 008 194 plusmn 023 578 plusmn 05 124 plusmn 007 384 plusmn 044

Croton dichogamus Out(h+) 216 plusmn 022 21 plusmn 024 2393 plusmn 831 3323 plusmn 829 minus007plusmn014 929 plusmn 217

Croton dichogamus In(h‐) 159 plusmn 008 182 plusmn 012 932 plusmn 267 1346 plusmn 317 023 plusmn 011 414 plusmn 116

NoteAllmeasurementsarebasedonthesameindividualplantsofeachspeciesthatwerepresentintheplotsin1999andthatwerestillalivein2009

10emsp |emsp emspenspJournal of Ecology WIGLEY Et aL

group2havingeither long straight spines (B pedicellaris)orbothshort recurvedand long straight spines (A etbaica) Specieswiththelongestandmostdensespines(group2)showedlittletonoin-vestmentinCTorTPwhilethespecieswithlowspineinvestment(group 1) showed the greatest concentrations of bothCT and TP(Figure2)The latterstrategyexperiencesgreaterconsumptionbybrowsersinthislandscaperelativetotheformer(Fordetal2014)consistentwithpreviousstudieswhichhavereportedthatspecieswithhigherinvestmentinstructuraldefences(group2)aremostsuc-cessfulatcoexistingwithbrowsersinthissavannasystem(GoheenKeesingAllanOgadaampOstfeld2004Sankaranetal2013)Inthepresenceofbrowsersspeciesinbothgroups1and2maintainedanincreasedcomplexityoftheirbranchingarchitecturecombinedwithsmallerleaves(reflectedinsubstantialreductionsinbitesizeindex)Increased branching even in the absence of changes in thorn length anddensitywhichoccurredinsomespeciesbutnototherscanre-sultindramaticincreasesinthenumberofspinesperunitvolumeofcanopy(ArchibaldampBond2003StaverBondCramerampWakeling2012)servingasaneffectivedeterrentforbrowsers

To summarise the strategies expressed by spinescent speciesin termsof ldquolivingwithbrowsersrdquo canbecharacterisedas (a)veryhigh and inducible investment in spinesbutnotN‐free secondarymetabolites and low available N leading to greatest success under intensebrowsing(A etbaica)(b)moderateandinducibleinvestmentinstraightspinescombinedwith thickermore fibrous leavesbuthigh available N leading to sufficient success under intense brows-ing(B pedicellaris)(c)lowandnon‐inducibleinvestmentinrecurvedspines combinedwithhighN‐free secondarymetabolites and lowavailableNleadingtoreducedsuccessunderbrowsing(A mellifera)

and(d)lowandnon‐inducibleinvestmentinshortspinescombinedwith moderate investment in N-free secondary metabolites and high availableN leading to low success under browsing (A brevispica)Fornara and Du Toit (2008) suggested that physical defences to-getherwithmasscompensatorygrowthabilitiesarekeyadaptationstolivingwithhighbrowsingpressureOurfindingslendstrongsup-porttothisnotionasthemostheavilystructurallydefendedspecies(A etbaica and B pedicellaris)performedthebestinthepresenceofherbivoresdespitelowinvestmentsinN‐freesecondarymetabolitesHowever Scogingset al (2011)predicted that the concentrationsofN‐freesecondarycompoundswoulddependonbrowsinginten-sity with the highest concentrations occurring at intermediate levels ofbrowsingforcertainfunctionalgroupsForexampleScogingsetal(2011)predictalinearincreaseinN‐freesecondarycompoundswithbrowsingintensitywithapotentiallysuddendecreaseatveryhigh levelsofbrowsingpressure indeciduoussavannaspeciesAsourexperimentaldesign lackedan intermediatebrowsingpressuretreatmentwewereunabletotestthesepredictions

Our analyses of available Nprovidedsomeinterestingandvalu-ableinsightsFirstlyinterpretationsofbrowsequalitydifferdepend-ing on whether total N (also commonly converted to crudeproteinby multiplying by 625 eg Cooper et al 1988) or available N isevaluatedForexamplethethreeAcaciaspeciesinthisstudyA bre-vispica A mellifera and A etbaica all had similar and relatively high con-centrations of leaf total N (~3835and33respectively)buttheproportionofthetotalN available to herbivores in A brevispica was ca 20 higher than in A mellifera and A etbaica(~2417and15respectively seeTable2) Ifweonly lookedat totalN they would all appear tobeofhighqualityhowever ifwe lookat availableN

F I G U R E 5 emsp (a)MeanplusmnSE change in plantheightbetween1999and2009forthesixdominantspeciesgrowinginsideandoutsidetheexclosures(b)Mean plusmn SE change in stem basal area between 1999 and 2009 for the same six speciesSignificancelevelsarep lt 001 p lt 0001

emspensp emsp | emsp11Journal of EcologyWIGLEY Et aL

itappearsasifA brevispicaisofmuchhigherqualitythantheothertwospeciesSecondlyavailableNappearstobemoreresponsivetoherbivore removal than total leaf N No differences in total N were evident between herbivory treatments while available N was signifi-cantly higher in A mellifera and C dichogamus when herbivores were presentThirdlydifferencesinavailableNdonotseemtobelinkedtophylogeny(eghighinsomeAcaciaspecieslowinothers)functionaltype(egfine‐leavedvsbroad‐leaved)norconcentrationsofN‐freesecondarymetabolites (availableNfoundtobe low inspecieswithbothhighand lowN‐freesecondarymetabolites)Finally these re-sults raise some importantquestions regarding the commonlyheldview (especially in African savannas) that fine‐leaved species (egAcacia)havehigherqualityleavesandarestructurallydefendedwhilebroad‐leavedspeciesarelessnutritiousandchemicallydefendedForexampleG tenax and B pedicellarisbothbroad‐leavedspecieshadhigher available Nthanthetwofine‐leavedspeciesA mellifera and A etbaica These findings highlight the need for further studies to eluci-date the determinants of leaf available N and that caution needs to be taken when using total NasameasureofbrowsequalityFurthermoreour findings suggest that neither total N nor available N are sufficient bythemselvestopredictbrowserutilisationhighlightingtheneedtoconsideralltraitscollectively(iedefencestrategies)

5emsp |emspCONCLUSIONS

Allofthedominantspeciesatourstudysitehadmoderatetohighleaf N concentrations but differed in available N and in the way they combined chemical and structural defences thus fitting under thebroaderldquonutritionanddefencerdquosyndromepreviouslydescribedforsavannasStructuraldefenceshadmorepronouncedresponsesto herbivory through increased spine length and density andorincreased branching while N-free secondary metabolites did not increaseinresponsetoherbivoryHighinvestmentinstructuralde-fences was the most successful defence strategy at our study sites withmoderateresourcelevels(relativetootherAfricansavannas)andhighherbivorypressureresultinginthehighestgrowthinthissavanna system This work has shown that within one overarching defence syndrome (nutritionanddefence) specieswithina com-munity can widely diverge in their adopted defence strategiesEndaraetal(2015)reportedhighlevelsofdissimilarityinthede-fencesof closely relatedco‐occurring forest speciesand suggestthat this would be necessary for the coexistence of closely related neighboursandcouldaccountforthehighlocaldiversityoftropicalforests Similarly we suggest that diverse defence strategies en-able savanna species (which are often closely related) to occupydifferentnichesanddefendagainstdifferent typesofherbivoresresultinginmoreresilientandspeciesrichwoodycommunities

ACKNOWLEDG EMENTS

WethankJParsareforassistanceinthefieldandChengappaSKand Balavignesh B for their assistance with laboratory analyses

This work was funded by the National Centre for Biological Sciences TIFR India andbyNationalGeographic (GrantNumber982815)andNERC (GrantNE‐E017436‐1)Wearehugely grateful to theOfficeofthePresidentoftheRepublicofKenyaforpermissiontoconducttheresearchandtheMpalaResearchCentreforlogisti-calsupport

AUTHORSrsquo CONTRIBUTIONS

BJWCCDJAandMSco‐wrotethemanuscriptBJWandMS collected the data DJA MS and JR implemented theexclosure experiment DH conceptualised and performed theanalyticalapproacheswithrespecttofibreandavailableNdeter-mination andperformed condensed tannin and total polyphenolanalysesAllauthorscommentedandaddedtoearlierversionsofthemanuscript

DATA ACCE SSIBILIT Y

Data available from the Dryad Digital Repository httpsdoiorg105061dryadp2d00sf(Wigleyetal2019)

ORCID

Benjamin J Wigley httpsorcidorg0000‐0002‐6964‐3624

Mahesh Sankaran httpsorcidorg0000‐0002‐1661‐6542

R E FE R E N C E S

Agrawal A A (2011) Current trends in the evolutionary ecology ofplant defence Functional Ecology 25(2) 420ndash432 httpsdoiorg101111j1365-2435201001796x

Agrawal A A amp Fishbein M (2006) Plant defense syndromesEcology 87(sp7) 132ndash149 httpsdoiorg1018900012‐9658 (2006)87[132PDS]20CO2

Archibald S ampBond W J (2003) Growing tall vs growing wideTree architecture and allometry of Acacia karroo in forest sa-vanna and arid environments Oikos 102(1) 3ndash14 httpsdoiorg101034j1600-0706200312181x

AugustineDJ(2003)Long‐termlivestock‐mediatedredistributionofnitro-genandphosphorusinanEastAfricansavannaJournal of Applied Ecology 40(1)137ndash149httpsdoiorg101046j1365‐2664200300778x

AugustineDJ(2010)Responseofnativeungulatestodroughtinsemi‐arid Kenyan rangeland African Journal of Ecology 48(4)1009ndash1020httpsdoiorg101111j1365‐2028201001207x

AugustineDJampMcNaughtonSJ(2004)RegulationofshrubdynamicsbynativebrowsingungulatesonEastAfricanrangelandJournal of Applied Ecology 41(1)45ndash58httpsdoiorg101111j1365‐2664200400864x

AugustineDJampMcNaughtonSJ(2006)Interactiveeffectsofungu-lateherbivoressoilfertilityandvariablerainfallonecosystempro-cesses in a semi-arid savanna Ecosystems 9(8)1242ndash1256httpsdoiorg101007s10021-005-0020-y

BartonKE (2016)TougherandthornierGeneralpatterns in the in-ductionofphysicaldefencetraitsFunctional Ecology 30(2)181ndash187httpsdoiorg1011111365‐243512495

BryantJPChapinFSIIIampKleinDR(1983)Carbonnutrientbal-anceofborealplantsinrelationtovertebrateherbivoryOikos 40(3)357ndash368httpsdoiorg1023073544308

emspensp emsp | emsp9Journal of EcologyWIGLEY Et aL

F I G U R E 4 emspExamplesofAcacia brevispica and Balanites pedicellaris growing in the absence of large mammalianherbivores(ac)andthesamespeciesgrowinginthepresenceoflargemammalianherbivores(bd)incentralLaikipiaCountyKenyaNotethecleardifferences in architecture branching densities and browser damage between the treatments

(a) (b)

(c) (d)

TA B L E 3 emsp Mean plusmn SEforplantheightandbasalareain1999and2009forthesixspeciesgrowingwithandwithoutherbivorespresentnetchangesinplantheightandbasalareabetweenthe2yearsarealsoshown

Species Treatment Height 1999 (m) Height 2009 (m)Basal area 1999 (cm2)

Basal area 2009 (cm2) Δ height Δ basal area

Acacia brevispica Out(h+) 195 plusmn 007 152 plusmn 005 1379 plusmn 174 1649 plusmn 209 minus043plusmn007 269 plusmn 117

Acacia brevispica In(h‐) 201 plusmn 006 297 plusmn 008 991 plusmn 096 164 plusmn 147 095 plusmn 006 649 plusmn 088

Acacia mellifera Out(h+) 228 plusmn 011 218 plusmn 01 8368 plusmn 1598 9465 plusmn 134 minus010plusmn009 1097 plusmn 657

Acacia mellifera In(h‐) 219 plusmn 009 349 plusmn 01 8048 plusmn 1331 12236 plusmn 1492 13 plusmn 005 4188 plusmn 611

Acacia etbaica Out(h+) 168 plusmn 008 199 plusmn 01 3699 plusmn 616 5786 plusmn 715 031 plusmn 006 2087 plusmn 314

Acacia etbaica In(h‐) 157 plusmn 007 305 plusmn 007 3039 plusmn 754 5183 plusmn 77 148 plusmn 006 2117 plusmn 323

Balanaites pedicellaris Out(h+) 25 plusmn 07 286 plusmn 05 8823 plusmn 4682 12282 plusmn 5008 036 plusmn 047 3459 plusmn 1242

Balanaites pedicellaris In(h‐) 196 plusmn 028 401 plusmn 026 2542 plusmn 829 7193 plusmn 1645 205 plusmn 023 4650 plusmn 1283

Grewia tenax Out(h+) 131 plusmn 009 095 plusmn 024 336 plusmn 044 296 plusmn 048 minus036plusmn024 minus041plusmn035

Grewia tenax In(h‐) 116 plusmn 005 24 plusmn 008 194 plusmn 023 578 plusmn 05 124 plusmn 007 384 plusmn 044

Croton dichogamus Out(h+) 216 plusmn 022 21 plusmn 024 2393 plusmn 831 3323 plusmn 829 minus007plusmn014 929 plusmn 217

Croton dichogamus In(h‐) 159 plusmn 008 182 plusmn 012 932 plusmn 267 1346 plusmn 317 023 plusmn 011 414 plusmn 116

NoteAllmeasurementsarebasedonthesameindividualplantsofeachspeciesthatwerepresentintheplotsin1999andthatwerestillalivein2009

10emsp |emsp emspenspJournal of Ecology WIGLEY Et aL

group2havingeither long straight spines (B pedicellaris)orbothshort recurvedand long straight spines (A etbaica) Specieswiththelongestandmostdensespines(group2)showedlittletonoin-vestmentinCTorTPwhilethespecieswithlowspineinvestment(group 1) showed the greatest concentrations of bothCT and TP(Figure2)The latterstrategyexperiencesgreaterconsumptionbybrowsersinthislandscaperelativetotheformer(Fordetal2014)consistentwithpreviousstudieswhichhavereportedthatspecieswithhigherinvestmentinstructuraldefences(group2)aremostsuc-cessfulatcoexistingwithbrowsersinthissavannasystem(GoheenKeesingAllanOgadaampOstfeld2004Sankaranetal2013)Inthepresenceofbrowsersspeciesinbothgroups1and2maintainedanincreasedcomplexityoftheirbranchingarchitecturecombinedwithsmallerleaves(reflectedinsubstantialreductionsinbitesizeindex)Increased branching even in the absence of changes in thorn length anddensitywhichoccurredinsomespeciesbutnototherscanre-sultindramaticincreasesinthenumberofspinesperunitvolumeofcanopy(ArchibaldampBond2003StaverBondCramerampWakeling2012)servingasaneffectivedeterrentforbrowsers

To summarise the strategies expressed by spinescent speciesin termsof ldquolivingwithbrowsersrdquo canbecharacterisedas (a)veryhigh and inducible investment in spinesbutnotN‐free secondarymetabolites and low available N leading to greatest success under intensebrowsing(A etbaica)(b)moderateandinducibleinvestmentinstraightspinescombinedwith thickermore fibrous leavesbuthigh available N leading to sufficient success under intense brows-ing(B pedicellaris)(c)lowandnon‐inducibleinvestmentinrecurvedspines combinedwithhighN‐free secondarymetabolites and lowavailableNleadingtoreducedsuccessunderbrowsing(A mellifera)

and(d)lowandnon‐inducibleinvestmentinshortspinescombinedwith moderate investment in N-free secondary metabolites and high availableN leading to low success under browsing (A brevispica)Fornara and Du Toit (2008) suggested that physical defences to-getherwithmasscompensatorygrowthabilitiesarekeyadaptationstolivingwithhighbrowsingpressureOurfindingslendstrongsup-porttothisnotionasthemostheavilystructurallydefendedspecies(A etbaica and B pedicellaris)performedthebestinthepresenceofherbivoresdespitelowinvestmentsinN‐freesecondarymetabolitesHowever Scogingset al (2011)predicted that the concentrationsofN‐freesecondarycompoundswoulddependonbrowsinginten-sity with the highest concentrations occurring at intermediate levels ofbrowsingforcertainfunctionalgroupsForexampleScogingsetal(2011)predictalinearincreaseinN‐freesecondarycompoundswithbrowsingintensitywithapotentiallysuddendecreaseatveryhigh levelsofbrowsingpressure indeciduoussavannaspeciesAsourexperimentaldesign lackedan intermediatebrowsingpressuretreatmentwewereunabletotestthesepredictions

Our analyses of available Nprovidedsomeinterestingandvalu-ableinsightsFirstlyinterpretationsofbrowsequalitydifferdepend-ing on whether total N (also commonly converted to crudeproteinby multiplying by 625 eg Cooper et al 1988) or available N isevaluatedForexamplethethreeAcaciaspeciesinthisstudyA bre-vispica A mellifera and A etbaica all had similar and relatively high con-centrations of leaf total N (~3835and33respectively)buttheproportionofthetotalN available to herbivores in A brevispica was ca 20 higher than in A mellifera and A etbaica(~2417and15respectively seeTable2) Ifweonly lookedat totalN they would all appear tobeofhighqualityhowever ifwe lookat availableN

F I G U R E 5 emsp (a)MeanplusmnSE change in plantheightbetween1999and2009forthesixdominantspeciesgrowinginsideandoutsidetheexclosures(b)Mean plusmn SE change in stem basal area between 1999 and 2009 for the same six speciesSignificancelevelsarep lt 001 p lt 0001

emspensp emsp | emsp11Journal of EcologyWIGLEY Et aL

itappearsasifA brevispicaisofmuchhigherqualitythantheothertwospeciesSecondlyavailableNappearstobemoreresponsivetoherbivore removal than total leaf N No differences in total N were evident between herbivory treatments while available N was signifi-cantly higher in A mellifera and C dichogamus when herbivores were presentThirdlydifferencesinavailableNdonotseemtobelinkedtophylogeny(eghighinsomeAcaciaspecieslowinothers)functionaltype(egfine‐leavedvsbroad‐leaved)norconcentrationsofN‐freesecondarymetabolites (availableNfoundtobe low inspecieswithbothhighand lowN‐freesecondarymetabolites)Finally these re-sults raise some importantquestions regarding the commonlyheldview (especially in African savannas) that fine‐leaved species (egAcacia)havehigherqualityleavesandarestructurallydefendedwhilebroad‐leavedspeciesarelessnutritiousandchemicallydefendedForexampleG tenax and B pedicellarisbothbroad‐leavedspecieshadhigher available Nthanthetwofine‐leavedspeciesA mellifera and A etbaica These findings highlight the need for further studies to eluci-date the determinants of leaf available N and that caution needs to be taken when using total NasameasureofbrowsequalityFurthermoreour findings suggest that neither total N nor available N are sufficient bythemselvestopredictbrowserutilisationhighlightingtheneedtoconsideralltraitscollectively(iedefencestrategies)

5emsp |emspCONCLUSIONS

Allofthedominantspeciesatourstudysitehadmoderatetohighleaf N concentrations but differed in available N and in the way they combined chemical and structural defences thus fitting under thebroaderldquonutritionanddefencerdquosyndromepreviouslydescribedforsavannasStructuraldefenceshadmorepronouncedresponsesto herbivory through increased spine length and density andorincreased branching while N-free secondary metabolites did not increaseinresponsetoherbivoryHighinvestmentinstructuralde-fences was the most successful defence strategy at our study sites withmoderateresourcelevels(relativetootherAfricansavannas)andhighherbivorypressureresultinginthehighestgrowthinthissavanna system This work has shown that within one overarching defence syndrome (nutritionanddefence) specieswithina com-munity can widely diverge in their adopted defence strategiesEndaraetal(2015)reportedhighlevelsofdissimilarityinthede-fencesof closely relatedco‐occurring forest speciesand suggestthat this would be necessary for the coexistence of closely related neighboursandcouldaccountforthehighlocaldiversityoftropicalforests Similarly we suggest that diverse defence strategies en-able savanna species (which are often closely related) to occupydifferentnichesanddefendagainstdifferent typesofherbivoresresultinginmoreresilientandspeciesrichwoodycommunities

ACKNOWLEDG EMENTS

WethankJParsareforassistanceinthefieldandChengappaSKand Balavignesh B for their assistance with laboratory analyses

This work was funded by the National Centre for Biological Sciences TIFR India andbyNationalGeographic (GrantNumber982815)andNERC (GrantNE‐E017436‐1)Wearehugely grateful to theOfficeofthePresidentoftheRepublicofKenyaforpermissiontoconducttheresearchandtheMpalaResearchCentreforlogisti-calsupport

AUTHORSrsquo CONTRIBUTIONS

BJWCCDJAandMSco‐wrotethemanuscriptBJWandMS collected the data DJA MS and JR implemented theexclosure experiment DH conceptualised and performed theanalyticalapproacheswithrespecttofibreandavailableNdeter-mination andperformed condensed tannin and total polyphenolanalysesAllauthorscommentedandaddedtoearlierversionsofthemanuscript

DATA ACCE SSIBILIT Y

Data available from the Dryad Digital Repository httpsdoiorg105061dryadp2d00sf(Wigleyetal2019)

ORCID

Benjamin J Wigley httpsorcidorg0000‐0002‐6964‐3624

Mahesh Sankaran httpsorcidorg0000‐0002‐1661‐6542

R E FE R E N C E S

Agrawal A A (2011) Current trends in the evolutionary ecology ofplant defence Functional Ecology 25(2) 420ndash432 httpsdoiorg101111j1365-2435201001796x

Agrawal A A amp Fishbein M (2006) Plant defense syndromesEcology 87(sp7) 132ndash149 httpsdoiorg1018900012‐9658 (2006)87[132PDS]20CO2

Archibald S ampBond W J (2003) Growing tall vs growing wideTree architecture and allometry of Acacia karroo in forest sa-vanna and arid environments Oikos 102(1) 3ndash14 httpsdoiorg101034j1600-0706200312181x

AugustineDJ(2003)Long‐termlivestock‐mediatedredistributionofnitro-genandphosphorusinanEastAfricansavannaJournal of Applied Ecology 40(1)137ndash149httpsdoiorg101046j1365‐2664200300778x

AugustineDJ(2010)Responseofnativeungulatestodroughtinsemi‐arid Kenyan rangeland African Journal of Ecology 48(4)1009ndash1020httpsdoiorg101111j1365‐2028201001207x

AugustineDJampMcNaughtonSJ(2004)RegulationofshrubdynamicsbynativebrowsingungulatesonEastAfricanrangelandJournal of Applied Ecology 41(1)45ndash58httpsdoiorg101111j1365‐2664200400864x

AugustineDJampMcNaughtonSJ(2006)Interactiveeffectsofungu-lateherbivoressoilfertilityandvariablerainfallonecosystempro-cesses in a semi-arid savanna Ecosystems 9(8)1242ndash1256httpsdoiorg101007s10021-005-0020-y

BartonKE (2016)TougherandthornierGeneralpatterns in the in-ductionofphysicaldefencetraitsFunctional Ecology 30(2)181ndash187httpsdoiorg1011111365‐243512495

BryantJPChapinFSIIIampKleinDR(1983)Carbonnutrientbal-anceofborealplantsinrelationtovertebrateherbivoryOikos 40(3)357ndash368httpsdoiorg1023073544308

10emsp |emsp emspenspJournal of Ecology WIGLEY Et aL

group2havingeither long straight spines (B pedicellaris)orbothshort recurvedand long straight spines (A etbaica) Specieswiththelongestandmostdensespines(group2)showedlittletonoin-vestmentinCTorTPwhilethespecieswithlowspineinvestment(group 1) showed the greatest concentrations of bothCT and TP(Figure2)The latterstrategyexperiencesgreaterconsumptionbybrowsersinthislandscaperelativetotheformer(Fordetal2014)consistentwithpreviousstudieswhichhavereportedthatspecieswithhigherinvestmentinstructuraldefences(group2)aremostsuc-cessfulatcoexistingwithbrowsersinthissavannasystem(GoheenKeesingAllanOgadaampOstfeld2004Sankaranetal2013)Inthepresenceofbrowsersspeciesinbothgroups1and2maintainedanincreasedcomplexityoftheirbranchingarchitecturecombinedwithsmallerleaves(reflectedinsubstantialreductionsinbitesizeindex)Increased branching even in the absence of changes in thorn length anddensitywhichoccurredinsomespeciesbutnototherscanre-sultindramaticincreasesinthenumberofspinesperunitvolumeofcanopy(ArchibaldampBond2003StaverBondCramerampWakeling2012)servingasaneffectivedeterrentforbrowsers

To summarise the strategies expressed by spinescent speciesin termsof ldquolivingwithbrowsersrdquo canbecharacterisedas (a)veryhigh and inducible investment in spinesbutnotN‐free secondarymetabolites and low available N leading to greatest success under intensebrowsing(A etbaica)(b)moderateandinducibleinvestmentinstraightspinescombinedwith thickermore fibrous leavesbuthigh available N leading to sufficient success under intense brows-ing(B pedicellaris)(c)lowandnon‐inducibleinvestmentinrecurvedspines combinedwithhighN‐free secondarymetabolites and lowavailableNleadingtoreducedsuccessunderbrowsing(A mellifera)

and(d)lowandnon‐inducibleinvestmentinshortspinescombinedwith moderate investment in N-free secondary metabolites and high availableN leading to low success under browsing (A brevispica)Fornara and Du Toit (2008) suggested that physical defences to-getherwithmasscompensatorygrowthabilitiesarekeyadaptationstolivingwithhighbrowsingpressureOurfindingslendstrongsup-porttothisnotionasthemostheavilystructurallydefendedspecies(A etbaica and B pedicellaris)performedthebestinthepresenceofherbivoresdespitelowinvestmentsinN‐freesecondarymetabolitesHowever Scogingset al (2011)predicted that the concentrationsofN‐freesecondarycompoundswoulddependonbrowsinginten-sity with the highest concentrations occurring at intermediate levels ofbrowsingforcertainfunctionalgroupsForexampleScogingsetal(2011)predictalinearincreaseinN‐freesecondarycompoundswithbrowsingintensitywithapotentiallysuddendecreaseatveryhigh levelsofbrowsingpressure indeciduoussavannaspeciesAsourexperimentaldesign lackedan intermediatebrowsingpressuretreatmentwewereunabletotestthesepredictions

Our analyses of available Nprovidedsomeinterestingandvalu-ableinsightsFirstlyinterpretationsofbrowsequalitydifferdepend-ing on whether total N (also commonly converted to crudeproteinby multiplying by 625 eg Cooper et al 1988) or available N isevaluatedForexamplethethreeAcaciaspeciesinthisstudyA bre-vispica A mellifera and A etbaica all had similar and relatively high con-centrations of leaf total N (~3835and33respectively)buttheproportionofthetotalN available to herbivores in A brevispica was ca 20 higher than in A mellifera and A etbaica(~2417and15respectively seeTable2) Ifweonly lookedat totalN they would all appear tobeofhighqualityhowever ifwe lookat availableN

F I G U R E 5 emsp (a)MeanplusmnSE change in plantheightbetween1999and2009forthesixdominantspeciesgrowinginsideandoutsidetheexclosures(b)Mean plusmn SE change in stem basal area between 1999 and 2009 for the same six speciesSignificancelevelsarep lt 001 p lt 0001

emspensp emsp | emsp11Journal of EcologyWIGLEY Et aL

itappearsasifA brevispicaisofmuchhigherqualitythantheothertwospeciesSecondlyavailableNappearstobemoreresponsivetoherbivore removal than total leaf N No differences in total N were evident between herbivory treatments while available N was signifi-cantly higher in A mellifera and C dichogamus when herbivores were presentThirdlydifferencesinavailableNdonotseemtobelinkedtophylogeny(eghighinsomeAcaciaspecieslowinothers)functionaltype(egfine‐leavedvsbroad‐leaved)norconcentrationsofN‐freesecondarymetabolites (availableNfoundtobe low inspecieswithbothhighand lowN‐freesecondarymetabolites)Finally these re-sults raise some importantquestions regarding the commonlyheldview (especially in African savannas) that fine‐leaved species (egAcacia)havehigherqualityleavesandarestructurallydefendedwhilebroad‐leavedspeciesarelessnutritiousandchemicallydefendedForexampleG tenax and B pedicellarisbothbroad‐leavedspecieshadhigher available Nthanthetwofine‐leavedspeciesA mellifera and A etbaica These findings highlight the need for further studies to eluci-date the determinants of leaf available N and that caution needs to be taken when using total NasameasureofbrowsequalityFurthermoreour findings suggest that neither total N nor available N are sufficient bythemselvestopredictbrowserutilisationhighlightingtheneedtoconsideralltraitscollectively(iedefencestrategies)

5emsp |emspCONCLUSIONS

Allofthedominantspeciesatourstudysitehadmoderatetohighleaf N concentrations but differed in available N and in the way they combined chemical and structural defences thus fitting under thebroaderldquonutritionanddefencerdquosyndromepreviouslydescribedforsavannasStructuraldefenceshadmorepronouncedresponsesto herbivory through increased spine length and density andorincreased branching while N-free secondary metabolites did not increaseinresponsetoherbivoryHighinvestmentinstructuralde-fences was the most successful defence strategy at our study sites withmoderateresourcelevels(relativetootherAfricansavannas)andhighherbivorypressureresultinginthehighestgrowthinthissavanna system This work has shown that within one overarching defence syndrome (nutritionanddefence) specieswithina com-munity can widely diverge in their adopted defence strategiesEndaraetal(2015)reportedhighlevelsofdissimilarityinthede-fencesof closely relatedco‐occurring forest speciesand suggestthat this would be necessary for the coexistence of closely related neighboursandcouldaccountforthehighlocaldiversityoftropicalforests Similarly we suggest that diverse defence strategies en-able savanna species (which are often closely related) to occupydifferentnichesanddefendagainstdifferent typesofherbivoresresultinginmoreresilientandspeciesrichwoodycommunities

ACKNOWLEDG EMENTS

WethankJParsareforassistanceinthefieldandChengappaSKand Balavignesh B for their assistance with laboratory analyses

This work was funded by the National Centre for Biological Sciences TIFR India andbyNationalGeographic (GrantNumber982815)andNERC (GrantNE‐E017436‐1)Wearehugely grateful to theOfficeofthePresidentoftheRepublicofKenyaforpermissiontoconducttheresearchandtheMpalaResearchCentreforlogisti-calsupport

AUTHORSrsquo CONTRIBUTIONS

BJWCCDJAandMSco‐wrotethemanuscriptBJWandMS collected the data DJA MS and JR implemented theexclosure experiment DH conceptualised and performed theanalyticalapproacheswithrespecttofibreandavailableNdeter-mination andperformed condensed tannin and total polyphenolanalysesAllauthorscommentedandaddedtoearlierversionsofthemanuscript

DATA ACCE SSIBILIT Y

Data available from the Dryad Digital Repository httpsdoiorg105061dryadp2d00sf(Wigleyetal2019)

ORCID

Benjamin J Wigley httpsorcidorg0000‐0002‐6964‐3624

Mahesh Sankaran httpsorcidorg0000‐0002‐1661‐6542

R E FE R E N C E S

Agrawal A A (2011) Current trends in the evolutionary ecology ofplant defence Functional Ecology 25(2) 420ndash432 httpsdoiorg101111j1365-2435201001796x

Agrawal A A amp Fishbein M (2006) Plant defense syndromesEcology 87(sp7) 132ndash149 httpsdoiorg1018900012‐9658 (2006)87[132PDS]20CO2

Archibald S ampBond W J (2003) Growing tall vs growing wideTree architecture and allometry of Acacia karroo in forest sa-vanna and arid environments Oikos 102(1) 3ndash14 httpsdoiorg101034j1600-0706200312181x

AugustineDJ(2003)Long‐termlivestock‐mediatedredistributionofnitro-genandphosphorusinanEastAfricansavannaJournal of Applied Ecology 40(1)137ndash149httpsdoiorg101046j1365‐2664200300778x

AugustineDJ(2010)Responseofnativeungulatestodroughtinsemi‐arid Kenyan rangeland African Journal of Ecology 48(4)1009ndash1020httpsdoiorg101111j1365‐2028201001207x

AugustineDJampMcNaughtonSJ(2004)RegulationofshrubdynamicsbynativebrowsingungulatesonEastAfricanrangelandJournal of Applied Ecology 41(1)45ndash58httpsdoiorg101111j1365‐2664200400864x

AugustineDJampMcNaughtonSJ(2006)Interactiveeffectsofungu-lateherbivoressoilfertilityandvariablerainfallonecosystempro-cesses in a semi-arid savanna Ecosystems 9(8)1242ndash1256httpsdoiorg101007s10021-005-0020-y

BartonKE (2016)TougherandthornierGeneralpatterns in the in-ductionofphysicaldefencetraitsFunctional Ecology 30(2)181ndash187httpsdoiorg1011111365‐243512495

BryantJPChapinFSIIIampKleinDR(1983)Carbonnutrientbal-anceofborealplantsinrelationtovertebrateherbivoryOikos 40(3)357ndash368httpsdoiorg1023073544308

emspensp emsp | emsp11Journal of EcologyWIGLEY Et aL

itappearsasifA brevispicaisofmuchhigherqualitythantheothertwospeciesSecondlyavailableNappearstobemoreresponsivetoherbivore removal than total leaf N No differences in total N were evident between herbivory treatments while available N was signifi-cantly higher in A mellifera and C dichogamus when herbivores were presentThirdlydifferencesinavailableNdonotseemtobelinkedtophylogeny(eghighinsomeAcaciaspecieslowinothers)functionaltype(egfine‐leavedvsbroad‐leaved)norconcentrationsofN‐freesecondarymetabolites (availableNfoundtobe low inspecieswithbothhighand lowN‐freesecondarymetabolites)Finally these re-sults raise some importantquestions regarding the commonlyheldview (especially in African savannas) that fine‐leaved species (egAcacia)havehigherqualityleavesandarestructurallydefendedwhilebroad‐leavedspeciesarelessnutritiousandchemicallydefendedForexampleG tenax and B pedicellarisbothbroad‐leavedspecieshadhigher available Nthanthetwofine‐leavedspeciesA mellifera and A etbaica These findings highlight the need for further studies to eluci-date the determinants of leaf available N and that caution needs to be taken when using total NasameasureofbrowsequalityFurthermoreour findings suggest that neither total N nor available N are sufficient bythemselvestopredictbrowserutilisationhighlightingtheneedtoconsideralltraitscollectively(iedefencestrategies)

5emsp |emspCONCLUSIONS

Allofthedominantspeciesatourstudysitehadmoderatetohighleaf N concentrations but differed in available N and in the way they combined chemical and structural defences thus fitting under thebroaderldquonutritionanddefencerdquosyndromepreviouslydescribedforsavannasStructuraldefenceshadmorepronouncedresponsesto herbivory through increased spine length and density andorincreased branching while N-free secondary metabolites did not increaseinresponsetoherbivoryHighinvestmentinstructuralde-fences was the most successful defence strategy at our study sites withmoderateresourcelevels(relativetootherAfricansavannas)andhighherbivorypressureresultinginthehighestgrowthinthissavanna system This work has shown that within one overarching defence syndrome (nutritionanddefence) specieswithina com-munity can widely diverge in their adopted defence strategiesEndaraetal(2015)reportedhighlevelsofdissimilarityinthede-fencesof closely relatedco‐occurring forest speciesand suggestthat this would be necessary for the coexistence of closely related neighboursandcouldaccountforthehighlocaldiversityoftropicalforests Similarly we suggest that diverse defence strategies en-able savanna species (which are often closely related) to occupydifferentnichesanddefendagainstdifferent typesofherbivoresresultinginmoreresilientandspeciesrichwoodycommunities

ACKNOWLEDG EMENTS

WethankJParsareforassistanceinthefieldandChengappaSKand Balavignesh B for their assistance with laboratory analyses

This work was funded by the National Centre for Biological Sciences TIFR India andbyNationalGeographic (GrantNumber982815)andNERC (GrantNE‐E017436‐1)Wearehugely grateful to theOfficeofthePresidentoftheRepublicofKenyaforpermissiontoconducttheresearchandtheMpalaResearchCentreforlogisti-calsupport

AUTHORSrsquo CONTRIBUTIONS

BJWCCDJAandMSco‐wrotethemanuscriptBJWandMS collected the data DJA MS and JR implemented theexclosure experiment DH conceptualised and performed theanalyticalapproacheswithrespecttofibreandavailableNdeter-mination andperformed condensed tannin and total polyphenolanalysesAllauthorscommentedandaddedtoearlierversionsofthemanuscript

DATA ACCE SSIBILIT Y

Data available from the Dryad Digital Repository httpsdoiorg105061dryadp2d00sf(Wigleyetal2019)

ORCID

Benjamin J Wigley httpsorcidorg0000‐0002‐6964‐3624

Mahesh Sankaran httpsorcidorg0000‐0002‐1661‐6542

R E FE R E N C E S

Agrawal A A (2011) Current trends in the evolutionary ecology ofplant defence Functional Ecology 25(2) 420ndash432 httpsdoiorg101111j1365-2435201001796x

Agrawal A A amp Fishbein M (2006) Plant defense syndromesEcology 87(sp7) 132ndash149 httpsdoiorg1018900012‐9658 (2006)87[132PDS]20CO2

Archibald S ampBond W J (2003) Growing tall vs growing wideTree architecture and allometry of Acacia karroo in forest sa-vanna and arid environments Oikos 102(1) 3ndash14 httpsdoiorg101034j1600-0706200312181x

AugustineDJ(2003)Long‐termlivestock‐mediatedredistributionofnitro-genandphosphorusinanEastAfricansavannaJournal of Applied Ecology 40(1)137ndash149httpsdoiorg101046j1365‐2664200300778x

AugustineDJ(2010)Responseofnativeungulatestodroughtinsemi‐arid Kenyan rangeland African Journal of Ecology 48(4)1009ndash1020httpsdoiorg101111j1365‐2028201001207x

AugustineDJampMcNaughtonSJ(2004)RegulationofshrubdynamicsbynativebrowsingungulatesonEastAfricanrangelandJournal of Applied Ecology 41(1)45ndash58httpsdoiorg101111j1365‐2664200400864x

AugustineDJampMcNaughtonSJ(2006)Interactiveeffectsofungu-lateherbivoressoilfertilityandvariablerainfallonecosystempro-cesses in a semi-arid savanna Ecosystems 9(8)1242ndash1256httpsdoiorg101007s10021-005-0020-y

BartonKE (2016)TougherandthornierGeneralpatterns in the in-ductionofphysicaldefencetraitsFunctional Ecology 30(2)181ndash187httpsdoiorg1011111365‐243512495

BryantJPChapinFSIIIampKleinDR(1983)Carbonnutrientbal-anceofborealplantsinrelationtovertebrateherbivoryOikos 40(3)357ndash368httpsdoiorg1023073544308

12emsp |emsp emspenspJournal of Ecology WIGLEY Et aL

Charles‐Dominique TMidgleyG F ampBondW J (2015) An indexfor assessing effectiveness of plant structural defences againstmammal browsing Plant Ecology 612(10) 1433ndash1440 httpsdoiorg101007s11258-015-0522-4

Charles‐DominiqueTDaviesTJHempsonGPBezengBSDaruBHKabongoRMhellipBondW J (2016) Spinyplantsmammalbrowsers and the origin of African savannas Proceedings of the National Academy of Sciences 113(38) E5572ndashE5579 httpsdoiorg101073pnas1607493113

ClaussMampHummelJ(2005)ThedigestiveperformanceofmammalianherbivoresWhybigmaynotbethatmuchbetterMammal Review 35(2)174ndash187httpsdoiorg101111j1365‐2907200500062x

ColeyPD(1988)Effectsofplantgrowthrateandleaflifetimeontheamount and typeof anti‐herbivoredefenseOecologia 74(4) 531ndash536httpsdoiorg101007BF00380050

ColeyPDBryantJPampChapinFSIII (1985)ResourceavailabilityandplantantiherbivoredefenseScience 230895ndash900httpsdoiorg101126science2304728895

Cooper SMOwen‐SmithNampBryant J P (1988) Foliage accept-ability to browsing ruminants in relation to seasonal changes in the leafchemistryofwoodyplantsinaSouthAfricansavannaOecologia 75(3)336ndash342httpsdoiorg101007BF00376934

CornelissenJHCGwynn‐JonesDvanLogtestijnRSPQuestedHMCallaghanTVampAertsR(2009)Ahypothesisedtriangularmodel combining tradeoffs of foliar defence quality and quantitysupport from subarctic seed plant species InDMing ampM J AWerger(Eds)A spectrum of ecological studies(pp36ndash44)ChongqingChina Southwest China Normal University Press

Coverdale TCGoheen J R Palmer TMampPringleRM (2018)GoodneighborsmakegooddefensesAssociationalrefugesreducedefenseinvestmentinAfricansavannaplantsEcology 99(8)1724ndash1736httpsdoiorg101002ecy2397

CraineJBondWLeeWGReichPBampOllingerS (2003)Theresource economics of chemical and structural defenses across ni-trogen supply gradients Oecologia 137(4) 547ndash556 httpsdoiorg101007s00442-003-1370-9

DaSilvaDMampBatalhaMA(2011)Defensesyndromesagainsther-bivoryinacerradoplantcommunityPlant Ecology 212(2)181ndash193httpsdoiorg101007s11258‐010‐9813‐y

DeGabrielJLWallisIRMooreBDampFoleyWJ(2008)Asim-ple integrativeassaytoquantifynutritionalqualityofbrowses forherbivores Oecologia 156(1) 107ndash116 httpsdoiorg101007s00442-008-0960-y

DraySampDufourAB(2007)Theade4packageImplementingthedu-ality diagram for ecologists Journal of StatisticalSoftware 22(4)1ndash20

Endara M‐J Weinhold A Cox J E Wiggins N L Coley PD amp Kursar T A (2015) Divergent evolution in antiherbi-vore defences within species complexes at a single Amazoniansite Journal of Ecology 103(5) 1107ndash1118 httpsdoiorg1011111365-274512431

FordATGoheenJROtienoTOBidnerL IsbellLAPalmerTMhellipPringleRM (2014)Largecarnivoresmakesavannatreecommunities less thorny Science 346(6207)346ndash349

Fornara D A amp DuToit J T (2008) Community‐level interactionsbetween ungulate browsers and woody plants in an African sa-vanna dominated by palatable‐spinescent Acacia trees Journal of Arid Environments 72(4) 534ndash545 httpsdoiorg101016jjaridenv200707010

Goheen J R Keesing F Allan B F Ogada D amp Ostfeld R S(2004) Net effects of large mammals on Acacia seedling sur-vival in anAfrican savannaEcology 85(6) 1555ndash1561 httpsdoiorg10189003-3060

Grubb P J (1992) Positive distrust in simplicityndashlessons from plantdefences and from competition among plants and among animalsJournal of Ecology 80(4)585ndash610

HanleyMELamontBBFairbanksMMampRaffertyCM(2007)Plant structural traits and their role in anti-herbivore defence Perspectives in Plant Ecology Evolution and Systematics 8(4)157ndash178httpsdoiorg101016jppees200701001

HattasDampJulkunen‐TiittoR(2012)ThequantificationofcondensedtanninsinAfricansavannatreespeciesPhytochemistry Letters 5(2)329ndash334httpsdoiorg101016jphytol201202013

Hattas D Stock W D Mabusela W T amp Green I R (2005)Phytochemical changes in leaves of subtropical grasses and fyn-bos shrubs at elevated atmospheric CO 2 concentrations Global and Planetary Change 47(2) 181ndash192 httpsdoiorg101016jgloplacha200410010

HermsDAampMattsonWJ(1992)ThedilemmaofplantsTogrowordefend The Quarterly Review of Biology 67(3)283ndash335httpsdoiorg101086417659

Kaplan I Halitschke R Kessler A Sardanelli S amp Denno R F(2008) Constitutive and induced defenses to herbivory in above‐andbelowgroundplanttissuesEcology 89(2)392ndash406httpsdoiorg10189007-04711

KartzinelTRGoheenJRCharlesGKDeFrancoEMacleanJEOtieno TOhellipPringle RM (2014) Plant and small‐mammalresponses to large‐herbivoreexclusion in anAfrican savannaFiveyearsoftheUHURUexperimentEcology 95(3)787ndash787httpsdoiorg10189013-1023R1

KorichevaJNykaumlnenHampGianoliE(2004)Meta‐analysisoftrade‐offs among plant antiherbivore fefenses Are plants jacks‐of‐all‐trades masters of all The American Naturalist 163(4)E64ndashE75

Langat M K Crouch N Ndunda B Midiwo J O Aldhaher AAlqahtaniAampMulhollandDA(2016)ThechemistryofAfricanCroton species In L P Christensen amp X Fretteacute (Eds) Planta Medica(Vol82pP384)CopenhagenDenmark

LevinDA (1976)Thechemicaldefensesofplants topathogensandherbivores Annual Review of Ecology and Systematics 7(1)121ndash159httpsdoiorg101146annureves07110176001005

MilewskiAVYoungTPampMaddenD(1991)Thornsasinducedde-fensesExperimentalevidenceOecologia 86(1)70ndash75httpsdoiorg101007BF00317391

MolesATPecoBWallisIRFoleyWJPooreAGSeabloomEWhellipHuiFK(2013)Correlationsbetweenphysicalandchemi-caldefencesinplantsTradeoffssyndromesorjustmanydifferentways to skin a herbivorous cat New Phytologist 198(1)252ndash263

Moreira X Lundborg L Zas R Carrillo‐Gavilaacuten A Borg‐KarlsonA‐KampSampedroL(2013)Inducibilityofchemicaldefencesbytwochewinginsectherbivoresinpinetreesisspecifictotargetedplanttissueparticularherbivore anddefensive traitPhytochemistry 94 113ndash122httpsdoiorg101016jphytochem201305008

NoadTampBirnieA(1990)TreesofKenya(Vol1376914367)NairobiKenyaTCNoadandABirnie308p‐illuscolillusmapISBNRetrievedfromhttpkbdkeworgkbddetailedresultdoxmlid=288220

Nuacutentildeez‐FarfaacutenJFornoniJampValverdePL (2007)Theevolutionofresistance and tolerance to herbivores Annual Review of Ecology Evolution and Systematics 38541ndash566httpsdoiorg101146an-nurevecolsys38091206095822

Perez-Harguindeguy N Diaz S Garnier E Lavorel S Poorter H JaureguiberryPhellipCornelissenJHC(2013)Newhandbookforstandardised measurement of plant functional traits worldwideAustralian Journal of Botany 61(3)167ndash234httpsdoiorg101071BT12225

PringleRMPriorKMPalmerTMYoungTPampGoheenJR(2016)Largeherbivorespromotehabitatspecializationandbetadi-versityofAfricansavannatreesEcology 97(10)2640ndash2657httpsdoiorg101002ecy1522

RDevelopmentCoreTeam(2016)R A language and environment for sta-tistical computing [Computer software]ViennaAustriaRFoundationforStatisticalComputing

emspensp emsp | emsp13Journal of EcologyWIGLEY Et aL

ReadJSansonGDCaldwellEClissoldFJChatainAPeetersPhellipKerrS(2008)Correlationsbetweenleaftoughnessandpheno-licsamongspeciesincontrastingenvironmentsofAustraliaandNewCaledonia Annals of Botany 103(5)757ndash767

SankaranMAugustineDJampRatnamJ(2013)Nativeungulatesofdiversebodysizescollectively regulate long‐termwoodyplantde-mographyandstructureofa semi‐arid savanna Journal of Ecology 101(6)1389ndash1399httpsdoiorg1011111365‐274512147

ScholesRJDowtyPRCaylorKParsonsDABFrostPGHampShugartHH (2002)Trendsinsavannastructureandcomposi-tion along an aridity gradient in the Kalahari Journal of Vegetation Science 13(3)419ndash428httpsdoiorg101111j1654‐11032002tb02066x

ScogingsPFHjaumllteacutenJampSkarpeC(2011)SecondarymetabolitesandnutrientsofwoodyplantsinrelationtobrowsingintensityinAfricansavannas Oecologia 167(4) 1063ndash1073 httpsdoiorg101007s00442-011-2042-9

ScogingsPFHjaumllteacutenJampSkarpeC(2013)Doeslargeherbivorere-moval affect secondary metabolites nutrients and shoot length in woody species in semi‐arid savannasJournal of Arid Environments 884ndash8httpsdoiorg101016jjaridenv201208010

Scogings P F Mamashela T C amp Zobolo A M (2013) Deciduoussapling responses to season and large herbivores in a semi‐aridAfrican savanna Austral Ecology 38(5) 548ndash556 httpsdoiorg101111j1442-9993201202454x

Stamp N (2003) Out of the quagmire of plant defense hypothe-ses The Quarterly Review of Biology 78(1) 23ndash55 httpsdoiorg101086367580

Staver A C Bond W J Cramer M D amp Wakeling J L (2012)Top‐down determinants of niche structure and adaptation amongAfrican Acacias Ecology Letters 15(7) 673ndash679 httpsdoiorg101111j1461-0248201201784x

Steward JLampKeelerKH (1988)Are there trade‐offsamongan-tiherbivore defenses in Ipomoea (Convolvulaceae) Oikos 53(3)79ndash86

StraussSYampAgrawalAA(1999)Theecologyandevolutionofplanttolerance to herbivory Trends in Ecology amp Evolution 14(5)179ndash185httpsdoiorg101016S0169‐5347(98)01576‐6

ThoulessCR(1995)Longdistancemovementsofelephantsinnorth-ern Kenya African Journal of Ecology 33(4) 321ndash334 httpsdoiorg101111j1365-20281995tb01042x

Tomlinson KW vanLangevelde FWardD PrinsHH deBie SVosman B hellipSterck F J (2016)Defence against vertebrate her-bivores trades off into architectural and low nutrient strategies amongstsavannaFabaceaespeciesOikos 125(1)126ndash136httpsdoiorg101111oik02325

Twigg L E amp Socha L V (1996) Physical versus chemical defencemechanisms in toxic Gastrolobium Oecologia 108(1)21ndash28

VanSoestPJ(1994)Nutritional ecology of the ruminant 2nd ed Ithaca NY Cornell University Press

VarmaVampOsuriAM(2013)BlackSpotAplatformforautomatedandrapidestimationofleafareafromscannedimagesPlant Ecology 214(12)1529ndash1534httpsdoiorg101007s11258‐013‐0273‐z

Wigley B J Bond W J Fritz H amp Coetsee C (2015) Mammalbrowsers and rainfall affect Acacia leaf nutrient content defenseand growth in SouthAfrican savannasBiotropica 47(2) 190ndash200httpsdoiorg101111btp12192

WigleyBJFritzHampCoetseeC(2018)DefencestrategiesinAfricansavanna trees Oecologia 187(3)797ndash809httpsdoiorg101007s00442-018-4165-8

Wigley B J Fritz H Coetsee C amp BondW J (2014) HerbivoresshapewoodyplantcommunitiesintheKrugerNationalParkLessonsfrom three long-term exclosures Koedoe 56(1) 1ndash12 httpsdoiorg104102koedoev56i11165

WigleyBCoetseeCAugustineDRatnamJHattasDampSankaranM (2019) Data from A thorny issue Woody plant defence andgrowthinanEastAfricansavannaDryad Digital Repositoryhttpsdoiorg105061dryadp2d00sf

How to cite this articleWigleyBJCoetseeCAugustineDJRatnamJHattasDSankaranMAthornyissueWoodyplantdefenceandgrowthinanEastAfricansavannaJ Ecol 2019001ndash13 httpsdoiorg1011111365‐274513140