Resprouting in the Mediterranean-type shrub Erica australis afffected by soil resource availability

- Resprouting in the Mediterranean-type shrub Erica australis afffected by soil resource availability - 641

Journal of Vegetation Science 13: 641-650, 2002© IAVS; Opulus Press Uppsala. Printed in Sweden

Abstract. Soil resource availability may affect plant regenera-tion by resprouting in disturbed environments directly, by af-fecting plant growth rates, or indirectly by determining alloca-tion to storage in the resprouting organs. Allocation to storagemay be higher in stressful, low resource-supply soils, but undersuch conditions plant growth rates may be lower. These factorscould act in opposite directions leading to poorly known effectson resprouting. This paper analyses the role played by soilresources in the production and growth of resprouts after re-moval of above-ground plant tissues in the Mediterranean shrubErica australis. At 13 sites, differing in substrate, we cut thebase of the stems of six plants of E. australis and allowed themto resprout and grow for two years. Soils were chemicallyanalysed and plant water potential measured during the summerat all sites to characterize soil resource availability. We usedstepwise regression analysis to determine the relationships be-tween the resprouting response [mean site values of the numberof resprouts (RN), maximum length (RML) and biomass (RB)]and soil nutrient content and plant water potential at each site.During the first two years of resprouting there were statisticallysignificant differences among sites in the variables characteriz-ing the resprouting response. RML was always different amongsites and had little relationship with lignotuber area. RN wasless different among sites and was always positively correlatedwith lignotuber area. RB was different among sites after the twoyears of growth. During the first months of resprouting, RN andRML were highly and positively related to the water status ofthe plant during summer. At later dates soil fertility variablescame into play, explaining significant amounts of variance ofthe resprouting variables. Soil extractable cations content wasthe main variable accounting for RML and RB. Our resultsindicate that resprout growth of E. australis is positively af-fected by high water availability at the beginning of theresprouting response and negatively so by high soil extractablecation content at later periods. Some of these factors had previ-ously shown to be related, with an opposite sign, to the develop-ment of a relatively larger lignotuber. Indeed, RML and RBmeasured in the second year of resprouting were significantlyand negatively correlated with some indices of biomass alloca-tion to the lignotuber at each site. This indicates that sitesfavouring allocation to the resprouting organ may not favourresprout growth.

Keywords: Disturbance; Fire; Lignotuber; Plant water poten-tial; Resprout growth; Soil fertility.

Resprouting in the Mediterranean-type shrub Erica australisafffected by soil resource availability

Cruz, Alberto; Pérez, Beatriz; Quintana, José R. & Moreno, José M.*

Facultad de Ciencias del Medio Ambiente, Universidad de Castilla-La Mancha, Avda. Carlos III s/n, 45071 Toledo, Spain;*Corresponding author; Fax +34925268840; E-mail [email protected]

Abbreviations: AB = Above-ground biomass; Catextr = soilextractable cations; FB = Foliar biomass; LA = Lignotuberarea; LB = Lignotuber biomass; Ntot = Soil total nitrogen; Pavail= Soil available phosphorus; RA = Root basal area; RB =Resprout biomass; RML = Resprout maximum length; RN =Resprout number; Ypd = Predawn plant water potential.

Nomenclature: Castroviejo et al. (1986).

Introduction

In ecosystems subjected to disturbances such as fire,recovery of plant cover occurs by a series of regene-ration modes ranging from complete plant mortalityfollowed by seedling establishment to complete plantsurvival or resprouting from protected buds withoutrecruitment (Keeley & Zedler 1978; Bond & van Wilgen1996). Successful resprouting may depend on resourcesstored by the plant prior to the disturbance, which,presumably, were diverted from allocation to other func-tions such as vegetative growth and sexual reproduction(Bellingham & Sparrow 2000; Bond & Midgley 2001).Consequently, trade-offs may have evolved in distur-bance prone areas between allocation of resources tostorage favouring vegetative regeneration by resprouting,against allocation to growth and sexual reproductionfavouring seed production and seedling establishment.Nevertheless, we still do not understand the circum-stances under which one strategy may have been se-lected over the other.

Some models indicate that trade-offs between allo-cation to resprouting and seeding may have evolved inresponse to availability of resources, whereby resproutingmay have been favoured in habitats with low resource-availability (Iwasa & Kubo 1997; Bellingham & Spar-row 2000). In Mediterranean-type, fire prone areas thereis evidence that resprouters are more abundant than theircongeners in sites with low resource availability (Midgley1996; Ojeda 1998). However, the relationship betweenthe availability of soil resources and resprouting after adisturbance is poorly known. In Mediterranean-type

642 Cruz, A. et al.

areas, resprouting frequently occurs from specific or-gans such as rhizomes (Malanson & Trabaud 1988) orlignotubers (James 1984). Soil resource availability mayindirectly affect resprouting capacity by controlling therelative size of the resprouting organ. Allocation tosubterranean organs should be larger in sites with lownutrient and water availability (Bloom et al. 1985).From this, it should follow that, relative to above-groundparts of the plant, larger resprouting organs should bemore prevalent in sites with low resource availabilitythan in richer ones. A relatively larger resprouting organmay increase the number of buds that are initiated afterthe disturbance. The number of sprouts, as well as theirgrowth rate, may be supported by the reserves stored inthe lignotuber (Bamber & Mullete 1978; Bowen & Pate1993). In this regard, the development of relativelylarger resprouting organs, eventually richer in storagecompounds, would cause greater resprout density andgrowth at sites with a low resource availability com-pared to those with more available resources. Neverthe-less, the relationship between soil resource availabilityand the relative size of the resprouting organ (thelignotuber) in plants such as Erica australis, a strongresprouter from the western Mediterranean, did not con-form entirely to these predictions: while lignotuberswere relatively larger in low water-availability sites,they were also larger at sites with higher soil pH, whichwere generally more fertile (Cruz & Moreno 2001).

In addition, water and soil nutrients may also directlylimit resprout growth, as these resources usually controlprimary productivity in Mediterranean-type ecosystems(Specht 1969; Mooney 1981). Growth of resproutingplants may be less sensitive to these limitations, due totheir favourable root : shoot ratio and reduced competi-tion in post disturbance environments (DeSouza et al.1986; Midgley 1996). Fertilization does not usually in-crease growth of resprouts in post disturbance environ-ments but irrigation may do (Matlack et al. 1993; Castell& Terradas 1994). It suggests that resprout growth maybe limited by a low water supply during the early stages ofregeneration after a disturbance. However, no direct rela-tionships have been established between variations in soilfertility or water availability and growth of resprouts aftera disturbance. This is important, not only to understandthe evolutionary pressures on resprouting, but also toevaluate how changes in resources across a landscapemay impact a community in which species with differentregeneration strategies may co-exist, as is common inMediterranean-type ecosystems.

This paper analyses the production and growth ofresprouts following the experimental removal of theabove-ground tissue in the lignotuberous, Mediterra-nean-type shrub, Erica australis along a gradient of soilresource-availability (water and mineral nutrients). Our

expectation was that, because of the favourable root :shoot ratio of resprouting plants and enhanced soil nutri-ent and water availability in recently disturbed environ-ments, soil resource availability may have little directimpact on the growth rate of resprouts shortly afterbeing disturbed. Consequently, if variation in resproutingbetween sites was to occur, it would be the result ofprevious storage allocations to the resprouting organ(lignotuber).

Methods

Species and study sites

Erica australis is a lignotuberous resprouting shrub,up to 2.5 m tall which occurs in subhumid climates andsiliceous soils (Rivas-Martínez 1979). It is a commonand dominant shrub in the western part of the IberianPeninsula and resprouts vigorously after fire (Ojeda etal. 1996; Calvo et al. 1998).

We selected 13 sites in the province of Cáceres(Central-Western Spain), covering a wide range of sub-strate types in which E. australis is present: fluvialsands of granitic origin (site A1), granites (sites G1, G2and G3), quartzites (sites C1, C2 and C3), slates (sitesP1, P2 and P3) and Pliocene sediments of quartziticorigin (known in Spanish as ‘rañas’, sites R1, R2 andR3). These substrates covered a range as large as possi-ble with respect to soil conditions. The distance betweensites was less than 80 km and sites were selected to besimilar in altitude (from 300 to 600 m a.s.l.), slope andaspect. Sites were selected so that no visible fire scarswere present to ensure that no recent fires had occurred.The climate in the study sites, determined by data col-lected from seven meteorological stations, has a meanannual temperature between 15 and 16 ∞C and annualrainfall of 800-1100 mm. Rainfall during April-May1993, the period just before plant manipulations, wasrelatively high, i.e. 175 mm on average. However, only70 mm fell in the four-month period June to September1993. During 1994, rainfall was concentrated in the firsthalf of the year (500 mm from January to May), whereasonly 14 mm fell during summer (June to September).

Plant manipulation

We randomly selected six E. australis individuals ineach site (total 78 plants), covering a wide range of plantsizes. For each plant we measured the two largest diam-eters of the lignotuber (D1 and D2), perpendicular to eachother. For each plant we calculated the lignotuber area(LA) as p * D1 * D2/4, which is a measurement relatedto the overall size of the plant (Moreno et al. 1999).


The E. australis plants included in the experiment hadvalues for lignotuber area (LA) between 50 and 600cm2. We cut the aerial biomass of each selected plant atthe level of the lignotuber in late spring 1993 (May 27to June 7) and surrounded all plants with a chicken-wire fence to prevent grazing by rabbits and goats.Clipping treatments were applied in late spring, at thecommencement of the fire season. The clipping treat-ment was intended to simulate the removal of the aerialparts that could be produced by an early season firewith null intensity, thus avoiding the possible interac-tions with fire behaviour characteristics. To limit thepossible differential effect in the resprout responsecaused by competition with neighbours, the biomass ofall plants rooted in a 1.5 m radius circle around thetarget E. australis plants were removed at the begin-ning of the experiment.

Resprout production and growth

Measurements of number and growth of resproutsafter clipping were made four times: in December1993 (early winter, six months after clipping), April1994 (early spring, 10 months) and January and July1995 (winter and summer, 20 and 25 months afterclipping, respectively). At each sampling date wecounted the number of resprouts (RN) and measuredthe length of the largest resprouting shoot (resproutmaximum length, RML) for each plant. Furthermore,we measured the length of 10 randomly chosenresprouts from each plant and calculated the meanresprout length. However, this last variable was highlycorrelated at either the individual or the populationlevel with RML at all the sampling dates and was thusexcluded from the final analysis. RML was assumed torepresent the growth rate of the resprouts at eachsampling date, RN represented the number of activateddormant buds. In July 1995, all the biomass of resprouts(RB) produced by each plant was cut off, taken to thelaboratory, oven-dried (70 ∞C for 48 h) and weighed.In one of the sites (G2) the wire fences had beenbroken and plants showed evident signs of browsing.Therefore, the final sampling date of this site wasexcluded from the analysis.

Soil fertility

To evaluate soil fertility we collected five soilsamples (0-10 cm depth) from each study site. Sampleswere air-dried and passed through a 2-mm mesh, afterwhich the samples from each site were combined. Thefollowing analyses were made: pH (1 : 2.5 soil/water),extractable cation concentration (Catextr = sum of Na,K, Ca and Mg, after extraction by shaking 5 g of soil in

100 ml of ammonium acetate at pH of 7.0, and determi-nation of Ca and Mg by atomic absorption and of Naand K by flame photometry), total nitrogen (Ntot, in aCHN auto-analyser after total combustion) and avail-able phosphorous (Pavail, after extraction with sulphu-ric and hydrochloric acid and determination accordingto Olsen & Sommers 1982). An additional estimate ofsoil fertility was obtained by growing Avena sativaplants from seed for 12 wk in 10 soil samples (400 geach) obtained from each site. Pots were periodicallywatered and maintained close to saturation. After thistime, plants were harvested, oven-dried at 70 ∞C for 48h and weighed. The mean biomass (above plus below-ground) produced by Avena plants in the soils of eachsite was used as a relative index of soil fertility.

Plant water potential

Shoot predawn water potential (Ypd) was measuredin terminal branches of four additional, non-disturbedE. australis individuals per site, with a Scholandertype pressure chamber. Measurements were made dur-ing summer 1993 (19 to 29 July). During July 1993rainfall was only 3 mm and the period without rainslasted for at least 20 days before water potential meas-urements. Mean values of predawn water potentials ateach site were used as relative indices of water avail-ability during the summer drought period, at whichtime resprout growth may be more sensitive to lowwater availability.

Data analysis

The significance of among site differences (siteeffect) for the different variables describing the pro-duction and growth of resprouts (resprout number RN,resprout maximum length RML and resprout biomassRB) was tested separately for each sampling date byANCOVA tests, using LA (ln-transformed) as thecovariable. In those cases in which the effect of thecovariable was not significant, site effects on resproutvariables were tested by one-way ANOVA. Before theapplication of the statistical tests, all the variables weretransformed by obtaining the square root (RN) or thenatural logarithm (RML and RB).

To determine the relationships between variablesdescribing resprouting and those related to soil re-sources, we followed three steps. First, we regressedthe resprout variables (RN, RML and RB) for all indi-viduals at the different sampling dates against lignotuberarea. This procedure was used to determine the amountof variability of each resprout variable explained bylignotuber size. Second, we calculated the mean valueof the residuals of such relationship for the six plants

644 Cruz, A. et al.

from each site: this represents the mean of the devia-tions from the values predicted by lignotuber size. Third,we regressed the mean of the residuals for each site on thevariables describing soil resource availability of the sites(pH, Catextr, Ntot, Pavail, mean Avena biomass in the bio-assay and Ypd) by stepwise multiple regression. Thisprocedure was selected because it determined what com-bination of soil variables accounted for a greater amountof variance of the different resprouting variables at eachsampling date, while ignoring redundant variables. Ypdmeasured in July 1993 was used as an independent vari-able only for resprouting variables corresponding to thefirst sampling date (December 1993).

To determine the relationship between previousallocation to the lignotuber and resprouting response,we calculated the Pearson correlation coefficients be-tween the resprouting variables (mean site values ofthe residuals of the RN, RML and RB LA regressions)measured at the different sampling dates and someindices of relative allocation to the lignotuber at eachsite. These indices were the mean values of thelignotuber biomass (LB) adjusted for the mean valuesof three different plant dimensions (above-ground bio-mass AB; foliar biomass FB and root basal area RA),obtained from linear regressions based on ten differentE. australis plants at each site (Cruz & Moreno 2001).The mean values of LB adjusted for the mean value ofAB, FB or RA values represented the mean lignotubersize at each site for a given mean value of plant sizeand were indicative of the relative allocation of bio-mass to the lignotuber at each site (Cruz & Moreno2001). If resprouting response is favoured by a greaterdevelopment of the resprouting organ in relation to thewhole plant size, we would expect to find a positivecorrelation between RN, RML or RB and the adjustedmean values of LB.

Results

The soils of the study sites were acidic, poor inextractable cations, with moderately low total nitrogenconcentrations and deficient in available phosphorus(Table 1). Soil pH and Ntot values were highly uniformamong sites. These soils were more variable with re-spect to Catextr and Pavail values (Table 1). Variations inCatextr among the sites were apparently caused by theirdifferences in Ca concentration, which constitutes themain fraction of Catextr (Table 1). Indeed, Ca concentra-tion was the cation more closely correlated with Catextr(R2 = 0.99; n = 13). In July 1993 Ypd values were highlyvariable among sites, suggesting a different degree ofwater stress endured by E. australis during the firstsummer after clipping (Table 1). These variables indi-cated potentially important among site differences inwater and nutrient availability for E. australis plantsduring the course of the experiment.

Emergence and growth of resprouts began shortlyafter clipping. By July 1993, presence of resprouts wasobserved in plants from all the sites. RN was highest inmost sites 10 months after clipping (Fig. 1), decreasingthereafter. By this time (April 1994), resprout numberdiffered by a factor of four among the sites. However,site effects on RN were significant only at six and 25months after clipping (ANCOVA P < 0.05, Table 2).

RML increased continuously throughout the two-year period, reaching a mean value of 80 cm, 25 monthsafter clipping (Fig. 2). Differences between sites formean RML values ranged from nearly six-fold six monthsafter clipping, to two-fold 20 months after clipping. Siteeffects on RML were highly significant at all samplingdates (ANOVA/ANCOVA P < 0.001, Table 2).

RB measured 25 months after clipping varied nearlyeight-fold among sites. Site effect on RB was statisti-cally significant (ANCOVA P < 0.001, Table 2, Fig. 3).

Table 1. Mean, minimum, maximum values and coefficients of variation (CV %) of the variables indicative of soil fertility and plantwater potentials of Erica australis plants in the study sites (n =13).

Mean Minimum Maximum CV (%)

Soil fertilityBioassay (mg Avena) 381.6 121.4 983.0 73.4pH 5.2 4.8 5.9 7.6Ntot (%) 0.22 0.13 0.37 28.2Pdisp (mg/100g) 1.04 0.29 4.29 115.0Catextr (mg/100g) 86.45 29.05 186.45 55.8

Extractable Ca (mg/100 g) 61.66 14.82 144.75 61.0Extractable Mg (mg/100 g) 11.42 2.92 26.10 61.6Extractable K (mg/100 g) 11.53 3.55 20.00 44.8Extractable Na (mg/100 g) 1.83 1.40 2.55 21.9

Plant water potentialsYpd (MPa) July 93 – 1.6 – 4.3 – 0.4 69.2


RB measured 25 months after clipping was significantlycorrelated with RML measured at the same samplingdate (r = 0.70, P < 0.05). However, RB was poorly corre-lated with RN measured 25 months after clipping (r = 0.37,P > 0.05). This suggests that resprout biomass may be moredependent on the elongation rate of the resprouts than onthe number of emergent buds.

The variability of the different resprout variables thatcould be explained by lignotuber size (represented by LA)varied: correlation coefficients (r) were moderately highand significant for RN, moderately low and significant forRB and low but not significant for RML (Table 3). Theresiduals of the relationships between resprouts andlignotuber area produced some significant models when

Fig. 1. RN (resprout number)(mean ± SE) ofErica australis at the different sites at differenttimes (months) after clipping: A. 6 mo; B. 10mo; C. 20 mo and D. 25 mo (n = 6). Eachcolumn represents a different substrate type.

Table 2. Results of the ANCOVA or ANOVA tests for significance of the among-site differences in RN (square root), RML (ln cm)and RB (ln g) of Erica australis plants at different times after clipping. LA (ln cm2) was the covariable. ANOVAs were applied in caseof no significant effect of the covariable.

ANCOVA ANOVACovariable effect Site effect Site effect

df F P df F P df F P

RN (No.) 6 mo 1 20.03 *** 12 2.82 **10 mo 1 14.91 *** 12 1.25 n.s.20 mo 1 6.68 * 12 1.42 n.s.25 mo 1 24.02 *** 11 2.42 *

RML (cm) 6 mo 1 1.13 n.s. - 12 7.14 ***10 mo 1 2.35 n.s. - 12 8.88 ***20 mo 1 8.13 ** 12 4.52 *** -25 mo 1 5.34 * 11 7.67 *** -

RB (g) -25 mo 1 16.89 *** 11 5.99 *** -

n.s.= P > 0.05; * = 0.05 ≥ P > 0.01; ** = 0.01 ≥ P > 0.001; *** = P £ 0.001.

646 Cruz, A. et al.

Fig. 2. Maximum resprout length (RML)(mean ± SE, cm) of Erica australis at thedifferent sites at different times (months)after clipping: A. 6 mo; B. 10 mo; C. 20mo; D. 25 mo (n = 6). Each column repre-sents a different substrate type.

Fig. 3. Resprout biomass (RB) (mean ± SE, g) of Ericaaustralis at the different sites 25 months after clipping (n = 6).Each column pattern represents a different substrate type.

Table 3. Correlation coefficients between RN (square root),RML (ln cm) and RB (ln g) at different sampling dates and thelignotuber area LA (ln cm2). Regressions were calculated bypooling into a single data set all the individuals from thedifferent sites.

Correlation (r) n Significancewith LA

RN (No) 6 mo 0.50 78 (***)10 mo 0.56 78 (***)20 mo 0.42 78 (***)25 mo 0.63 72 (***)

RML (cm) 6 mo 0.07 78 (n.s.)10 mo 0.10 78 (n.s.)20 mo 0.20 78 (n.s.)25 mo 0.17 72 (n.s.)RB (g)25 mo 0.36 72 (**)

n.s.= P > 0.05; * = 0.05 ≥ P > 0.01; ** = 0.01 ≥ P > 0.001; *** = P £ 0.001.

regressed by steps against soil properties. Soil pH and soilPavail were negatively correlated with the number ofresprouts (Table 4), indicating that the emergence ofresprouts was greater in sites with more acidic substratesand/or lower available P. Ypd also accounted for a signifi-cant amount of the variance of RN six months afterclipping (Table 4). Correlation was positive, suggestingthat resprout emergence was greater at those sites withhigher water availability during summer.

Residuals of the RML- LA relationships were posi-tively and significantly related with plant Ypd values inJuly 93, six months after clipping (Table 4). This rela-tionship suggests that initial growth of resprouts wasgreater in sites with a lower water stress during the firstmonths after clipping. No significant model was ob-tained for RML residuals 10 months after clipping. At 20and 25 months after clipping, multiple regression mod-els for RML were highly significant, with determination


coefficients of 0.91 and 0.82, respectively (Table 4). Inboth cases, the variable Catextr (negative value) ac-counted for the greatest amount of the variance of RML,suggesting that the growth of resprouts during the sec-ond year after clipping was greater in sites with lowextractable cation contents. Alternative regressions withthe respective concentrations of cations (Ca, Mg, K andNa) as independent variables, instead of the sum (Catextr),indicated that Ca concentration was the cation nega-tively related with RML (data not shown). Some vari-ables related to soil fertility were positively correlatedwith RML: Avena biomass produced in the bioassay (arelative index of soil fertility) and soil Ntot. Soil pH wasnegatively correlated with RML 19 mo after clipping. Acombination of soil Catextr (negative) and Ntot (positive)also accounted for 75% of the variance of RB-LAresiduals (Table 4). These results suggest that growth ofresprouts of E. australis was initially (results for the firstsix months) increased by a greater availability of water.However, at later stages growth of resprouts was posi-tively related to soil nitrogen and negatively to extract-able cations.

Residuals of RN-LA regressions were not signifi-cantly correlated with any of the different indices of rela-tive lignotuber biomass at any sampling date (Table 5).This indicates that the mean number of resprouts producedby the plants at each site was not favoured by having arelatively greater allocation to the lignotuber. Residualsof RML-LA regressions were initially (first year ofresprouting) not related with the indices of relative

lignotuber biomass (Table 5). However, residuals ofRML- and RB-LA regressions during the second year ofresprouting were significantly correlated with eitherAB, FB or RA based lignotuber biomass indices. Thisindicates that growth of resprouts was clearly greater inthose sites that had proportionally smaller lignotubers.

Table 4. Stepwise regression models of the mean site residuals of the LA-resprout variables relationships of Erica australis measuredat different times on some variables related to soil resource availability of sites (n = 13).

Independent variables (1) Regression model R2 P

Y = Residuals of RN-LA 6 months X1: Ypd J93 (MPa) (F = 16.85) Y = 21.03 + 1.57 (X1) – 3.57 (X2) 0.70 0.002

X2: Soil pH (F = 10.46)10 months X1: Soil pH (F = 6.51) Y = 12.57 – 2.44 (X1) 0.37 0.02720 months - - - -25 months X1: Soil Pavail (F = 7.15) Y = 0.66 – 0.67 (X1) 0.42 0.023

Y = Residuals of RML-LA 6 months X1: Ypd J93 (MPa) (F = 12.31) Y = 0.73 + 0.44 (X1) 0.53 0.00510 months - - - -20 months X1: Soil Catextr (%) (F = 75.11) Y = 0.63 – 4.10(X1) + 0.46 (X2) + 1.32 (X3) – 0.14 (X4) 0.91 <0.001

X2: Bioassay (ln g) (F = 30.27)X3: Soil Ntot (%)(F = 16.66)X4: Soil pH (F = 6.92)

25 months X1: Soil Catextr (%) (F = 34.32) Y = -0.26 – 10.0 (X1) + 2.73 (X2) + 0.42 (X3) 0.82 0.002X2: Soil Ntot (%) (F = 16.54)X3: Bioassay (ln g) (F = 10.75)

Y = Residuals of RB-LA25 months X1: Soil Catextr (%) (F = 25.73) Y = 0.25 – 10.0 (X1) + 3.58 (X2) 0.75 0.002

X2: Soil Ntot (%) (F = 4.46)

(1) Partial F value for each independent variable in brackets.

Table 5. Correlation coefficients between mean values oflignotuber biomass (LB) adjusted for the mean values of otherplant dimensions (AB, FB, RA) (x-y) and resprouting variables(residuals of RN, RML and RB regressed against La) measuredat different sampling dates (n = 13, except for 25 mo afterclipping, when n =12).

Lignotuber relationship (x-y)AB-LB FB-LB RA-LB

Residuals of RN-LA 6 mo –0.13 –0.21 0.1310 mo 0.08 –0.07 0.0320 mo 0.01 –0.14 0.0125 mo 0.06 –0.04 –0.20

Residuals of RML-LA 6 mo –0.24 –0.13 0.1110 mo –0.46 –0.37 –0.1220 mo –0.88(***) –0.68(*) –0.56(*)25 mo –0.87(***) –0.78(**) –0.58(*)

Residuals of RB- LA25 mo –0.73(**) –0.67(*) –0.46

n.s.= P > 0.05; * = 0.05 ≥ P > 0.01; ** = 0.01 ≥ P > 0.001; *** = P £ 0.001.

648 Cruz, A. et al.

Discussion

This paper documents that resprouting by E. australisdiffered significantly among sites at various times ofmonitoring during the two years of study. Differences inmean resprout biomass between sites at the end of thisperiod were as great as eight-fold. Variations inresprouting vigour after a fire for a given species haveusually been reported at the stand level and commonlyattributed to differences in fire severity (Moreno &Oechel 1991) or plant size (Rundel et al. 1987; Moreno& Oechel 1993). Reports on variations in resproutingvigour along larger spatial scales are less common in theliterature (Plumb 1961; Keeley & Keeley 1981; Malanson& O’Leary 1985; Keeley 1998). A significant amount ofthe variability in resprouting found in this study couldbe explained by several variables related to soil fertility.After two years of treatment implementation, theresprouts of E. australis had grown faster and producedgreater biomass in sites with relatively high soil totalnitrogen and low soil extractable cation contents. Totalsoil N per se might not be a good indicator of soil Navailability; however, it is often highly correlated withsoil N potential mineralization, as shown for some Medi-terranean-type soils (Marion et al. 1981; Marion &Black 1988; Schlesinger 1997). The positive effect ofsoil total nitrogen is therefore not surprising, because Nis considered a limiting factor of plant productivity inMediterranean ecosystems (Specht 1969; Carreira &Niell 1992). The negative effect of extractable cations isindicative of a more vigourous resprouting in low-cal-cium soils, and can be interpreted as a particular nutri-tional effect for plants of the genus Erica. Although wehave no direct studies of soil effects on nutrition by E.australis, some evidence suggests that enzymatic activ-ity and growth of specific ericoid mycorrhiza may befavoured in soils with low pH (Leake & Read 1989,1990). Indeed, this species is particularly frequent, andhas a higher vegetative and reproductive performance,on acidic and low-calcium soils (Ojeda et al. 2000). Alow soil pH value also shown here to favour the produc-tion of a larger number of resprouts, reinforcing the ideathat acidic and low-calcium soils may be favourable forthis species. A generalization of the effects reportedhere over the resprouting of E. australis could lead tothe conclusion that resprouting vigour would vary alongsubstrate type patches in heterogeneous landscapes af-fected by extensive disturbances such as fire, because ofdifferences in soil fertility. The effect of experimentaltotal removal of above-ground biomass on the plants inthis study is similar to that expected to be caused by awildfire. Fires may also cause an additional change innutrient availability, usually immediately after a fire(Christensen & Muller 1975; Marion et al. 1991), which

could alter the differences in soil fertility between sub-strate patches. However, this fertilization effect is usu-ally short-lived (Christensen 1987; Trabaud 1990) andthe effect of soil fertility on resprouting reported hereseems to be more permanent, being evident two yearsafter clipping. This suggests that differences in soilconditions may control resprouting after a disturbancesuch as fire.

The combination of soil variables that explained thecontrol of resprouting changed with time after clipping.Plant shoot water potentials measured in summer ac-counted for a significant amount of the variance ofmaximum resprout length and resprout number meas-ured at the end of the first drought period after clippingthe plants. Assuming that summer predawn plant waterpotentials were good indicators of the relative wateravailability at the sites, this suggests that resprout emer-gence and growth during summer and early autumnwould be greater at sites with better water availability.However, soil fertility did not account for any of thevariance of the maximum length of resprouts during thefirst months after clipping. This indicates that low wateravailability may be a more limiting factor than low soilfertility on growth rates of the resprouts of E. australis,at least during the early periods of recovery under thesummer drought. Plants from Mediterranean-typeshrublands usually show a greater growth enhancementin response to irrigation than to fertilization, either inundisturbed (McMaster et al. 1982; Kummerow et al.1982) or disturbed areas (Castell & Terradas 1994). Inthe studied shrubland communities, fires usually occurduring summer (Vázquez & Moreno 1998) and signifi-cant resprout growth may occur during drought periods.Soil water availability seems to be the primary limitingfactor of the early resprout growth of E. australis. Thisfinding is important because a rapid growth rate of theresprouts during the first summer after a disturbance maybe critical for ensuring resprouting success, as this maydetermine the rapid re-occupation of space (Bellingham& Sparrow 2000). A reduction in the amount of rainfallfor the Mediterranean Basin, with the subsequent length-ening of the drought period, as predicted by severalfuture scenarios of climate change (Parry 2000), maynegatively affect the resprouting capacity due to a re-duction in the early growth of resprouts.

Current models referring to the trade-offs of re-sprouting or seeding assume that a vigourous resproutingresponse would be favoured by a greater allocation tostorage in the resprouting organs, such as the lignotuber.A larger lignotuber should allow more vigourousresprouting, hence rendering the plant less dependent onexternal resources. However, the results reported heresuggest that the production of resprout biomass in E.australis was not directly related to the size of the


resprouting organ. Firstly, although the lignotuber areawas highly related to resprout number, it was poorlyrelated to resprout biomass or maximum length. Resproutbiomass seemed to be mainly related to the growth rateof the resprouts, as suggested by the equivalence in thefactors which affected both resprout biomass and maxi-mum length, and by the positive and significant correla-tion between these variables. Lignotuber size did notaccount for any portion of the variance of maximumresprout length. On the other hand, resprout biomasswas poorly related to resprout number, which seemed tobe determined by lignotuber size. Second, we observeda consistent pattern of negative and significant correla-tions between some indices of biomass allocation to thelignotuber at each site and resprouting response (elon-gation and biomass) in the second year of resprouting.In other words, sites in which lignotubers tended to beproportionally larger in this study showed a lessvigourous resprouting. Indeed, some of the factors fa-vouring greater production and growth of resprouts(soils with acidic pH, low extractable cation concentra-tion and high water availability) had been shown to berelated to the development of proportionally smallerlignotubers in the same study areas (Cruz & Moreno2001). Assuming that the size of the lignotuber wouldbe positively related with fire survival (Auld 1990;Moreno & Oechel 1991; Quintana 1999), the effects ofsoil resources on resprouting success might be complex.Site productivity may affect resprouting, but its effectson the development of the resprouting organ and onresprout production may not be coincidental. In fact,sites favouring a larger development of the resproutingorgan, the lignotuber, may lead to a less vigourousresprouting response.

Acknowledgements. This research was supported by the Span-ish SGPN I+D (NAT90-0542), and a FPI-grant to AlbertoCruz by the Spanish Ministry of Education and Science. Wethank Fernando Ojeda for his helpful comments on an earlierversion of this paper. We also thank Angel Velasco for fieldcollaboration.

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Received 6 August 2001;Revision received 24 May 2002;

Accepted 24 May 2002.Coordinating Editor: C. Leuschner.

Resprouting in the Mediterranean-type shrub Erica australis afffected by soil resource availability

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