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Responses of herbs and shrubs to reduced rootcompetition under canopies and in gaps: atrenching experiment in old-growth Douglas-firforests

Briana C. Lindh, Andrew N. Gray, and Thomas A. Spies

Abstract: We tested the effect of root trenching on vegetation in closed-canopy and gap locations in Douglas-fir(Pseudotsuga menziesii (Mirb.) Franco) forests. Based on theory, we expected helowground competition to be intense ina region with low summer rainfall, and trench responses were expected to he greater in the hi gh light environment ofthe gaps. We installed I m deep trenches around study plots and lined the trenches to prevent reinvasion by tree roots.Soil moisture was measured monthly during the growin g season for the first 3 years after trench installation. Vegetationin these trenched plots was compared with control plots 10 years after installation of the plots. Trenched plots with novegetation manipulation avera ged 92% total understory cover, while untrenched plots averaged 47% cover. Contrary toour expectation, both vegetation and soil moisture responses to trenching were greater in areas of high tree canopycover than in gaps. Trenched plots under closed canopies were moister than control plots throughout the growing sea-son. while the trenching effect became apparent in the overall wetter gaps only at the end of the growing season. Weconclude that understory plants at these sites were limited at least as much by belowground competition as by above-ground competition.

Resume : Nous avons teste, sous couvert et dans des ouvertures, l'effet sur la vegetation du creusement dune trancheedans la zone racinaire clans des forets de douglas (Pseudostuga menriesii (Mirb.) Franco). Sur une base theorique. nounanticipions que la competition souterraine serait forte thins une re g ion on la precipitation estivate est faible et que lareaction au traitement serait plus forte dans les ouvertures ou it y a plus de lumiere. Nous avons creuse des trancheesde 1 m de profondeur autour des parcelles experimentales et recouvert les parois pour empecher la reinstallation desracines des arhres. L'humidite du sol a etc mesuree a tour les moil pendant la saison de croissance durant les trois pre-mieres annees apres le creusement des tranchees. La vegetation dans les parcelles experimentales a etc compareecelle des parcelles temoins 10 ans apres le debut de l'experience. Les parcelles experimentales on Ia vegetation n'avaitpas etc perturb& avaient en moyenne un couvert total de 92 % en sous-etage comparativement a 47 % pour ]es parcel-les temoins. Contrairement a nos attentes, tam la vegetation que l'humidite du sol ont davantage reagi an traitementdans les zones avec un fort couvert arhorescent que dans les ouvertures. Les parcelles traitees sous un couvert fermeetaient plus humides que les parcelles temoins pendant toute la saison de croissance Landis que l'effet du traitementdans les ouvertures g lobalement plus humides s'est fan sentir seulement a la fin de la saison de croissance. Nous con-cluons que dans ces sites, la vegetation en sous-etage est limit& au morns autant par la competition souterraine quepar Ia competition epigee.

[Tracluit par la Redaction]

Received 13 December 2002. Accepted 8 May 2003.Published on the NRC Research Press Web site athttp://cifrinc.ca on 16 October 2003.

B.C. Lindh. 1 2082 Cordley Hall Department of Botany andPlant Pathology, Oregon State University. Corvallis, OR97331, U.S.A.A.N. Gray and T.A. Spies. USDA Forest Service, PacificNorthwest Research Station. 3200 SW Jefferson Way,Corvallis, OR 97331. U.S.A.

'Corresponding author (e-mail: blindh@willamette.eclu).

Introduction

The relative importance of aboveground and helowgroundcontrols on understory responses to disturbances of the treecanopy is not well understood. While the response of vegeta-tion to canopy gaps has largely been discussed in terms oflight (Whigham et al. 1993; Pascarclla and Horvitz 1998),the death of canopy trees also influences soil moisture (Grayet al. 2002) and nutrient availability (Matson and Boone1984). Root trenching studies have frequently demonstratedthat growth and establishment of herbs, shrubs, and tree

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seedlings is strongly influenced by belowground competitionfor moisture and nutrients. In a recent review of root trench-ing studies, Coomes and Grubb (2000) cite situations inwhich understory response to gaps was controlled more bylight than by water or nutrients and other situations in whichthe reverse was true. They propose a conceptual frameworkthat predicts the relative importance of root competition as afunction of summer rainfall, soil fertility, and the density ofshade cast by the dominant tree species. Where soils are nu-trient rich and moist throughout the year, vegetation re-sponses to gaps will be controlled more by abovegroundcompetition than by helowground competition. Where soilsare nutrient poor and dry in summer, they predict a large re-sponse to root trenching (release from helow g round compe-tition). Coomes and Grubb (2000) further propose thattrenching effects should be larger in gaps than under dense.closed canopies; they argue that the lack of light under theclosed canopy should prevent a large response to an increasein helowground resource availability.

Few studies have examined responses of understory vege-tation to reduced root competition and canopy openness inthe dense Pseudot.vuga menziesii forests of the PacificNorthwest. The climate in this region is characterized byabundant winter precipitation with summer drou ght and soilsthat are relatively nutrient poor. According to the conceptualframework of Coomes and Grubb (2000), we should expectroot trenching in this region to increase vegetation establish-ment and growth via an increase in soil moisture and nutri-ent levels. The dense shade of the tree canopies would beexpected to prevent large trench responses under closed can-opies, while canopy gaps should show large trench re-sponses. Only two studies have evaluated root competitionin Douglas-fir forests (Christy 1986: Simard et al. 1997).and their results are only partly consistent with each otherand with the predictions of Coomes and Grubb (2000). InOregon. Christy (1986) found that trenching to reduce rootcompetition had a greater effect on growth of juvenile Tsugaheterophylla (RA) Sarg. than did the creation of canopygaps. However, Simard et al. (1997) did not find any effectsof trenching on growth of P menziesii seedlings in BritishColumbia. Neither study found that growing season soilmoisture increased in the trench plots as would be predictedby Coomes and Grubb (2000). Simard et al. (1997), alongwith Hart and Sollins (1998). found increases in soil mois-ture in trench plots in relation to controls only in early fall,after plant growth had ceased. The failure of these studies tofind a soil moisture response is surprising given the summerdrought that is characteristic of P menziesh forests. In con-trast. Gray et al. (2002) did find that growing season soilmoisture was higher in gaps and in trench plots than in con-trols for conifer forests in Washington and Oregon. None ofthese studies reported how differences in soil moisture be-tween trench and control plots varied over the growing seasonor between years. Given the variation observed in vegetativeand soil moisture responses to release from root competitionand canopy shading in this region and the inconsistency ofsome of the results with predictive models, it is clear that ourunderstanding of these processes is incomplete.

No studies have examined the response of the herb andshrub layer to reduced root competition in Pacific Northwest

P. menziesh forests. In contrast with tropical forestunderstories, Pacific Northwest forest understories have rela-tively few tree seedlings; herbs and shrubs make up most ofthe diversity and biomass. McCune (1986), in a grand fir(Abies grandis (Dougl. Ex D. Don) Lindl.) forest inMontana, showed strikin g increases in herb cover (a jumpfrom 7% to 55%) as a result of trenching. In a white pine(Pima strohus L.) forest in New Hampshire. Tuomey andKeinholz (1931) found an eightfold increase in the numberof understory herb and shrub individuals 8 years aftertrenching. Differences persisted 20 years after establishment,with particularly striking establishment and growth of east-ern hemlock (Tsuga canadensi.s . (L.) Can-iere) individuals inthe trenched plot (Lutz 1945). Walter and Breckle (1985) re-viewed the results of several studies published in Russia andGermany. Trenching in boreal spruce (Picea spp.) or Sibe-rian fir (Abies sibirica Ledeb.) forest transformed sparseunderstories into lush carpets dominated by Oxalisacetosella L. (see citations in Walter and Breckle 1985).Trenching studies generally examine vegetative rather thanreproductive responses. perhaps because it is widely heldthat light is the primary resource that limits flowering ofunderstory vegetation (Chazdon 1991: Niesenhaum 1993:Cunningham 1997). The one study that has examined the ef-fect of killing tree roots on flowerin g in an understory herbfound no treatment effect (Hughes et al. 1988). Using obser-vational data, however, St. Pierre (2000) found that soilmoisture and light levels helped explain flowering of twounderstory herb species in experimental canopy gaps.

In this study, we extend research into root competitionand gap effects by examining the response of cover andflowering of forest herbs and shrubs to trenching in old-growth P menziesii forests in the Pacific Northwest. Our ob-jectives are (i) to compare the cover of herbs and shrubs intrenched and untrenched plots in canopy gaps and beneathclosed canopies: (ii) to compare the number of floweringrarnets in these treatments; and (iii) to evaluate changes insoil moisture over the growing season and among years inrelation to the treatments.

Materials and methods

Our two-study sites were in mid-elevation old-growth for-ests on the west side of the Cascade Mountains in northernOregon and southern Washington. Both were mixed-speciesstands with a dominant canopy of P menziesii and asuhcanopy of T heterophylla. Recent (last 25 years) averageannual precipitation at the H.J. Andrews Experimental For-est site (44°15'N, 122°15'W) and at the Wind River Experi-mental Forest (45°50'N, 122'00'W) is about 220 cm, withless than 10% of that falling during the months June—September.

Trenched plots (helowground gaps) were installed to com-plement an experimental canopy gap study at the H.J. An-drews and Wind River sites (Gray and Spies 1996). Theseplots were installed in 1990 and monitored for 4 years forsoil moisture response (Gray et al. 2002). Vegetation in theplots had never been quantified until the present study.

Our experimental design was a split-plot, with two wholeplots (closed-canopy or naturally occurring gap) at each of

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the two sites (H.1. Andrews and Wind River). Within eachwhole plot, four factorial combinations of trenching(trenched or control) and clipping (clipped or not) were as-signed randomly to four adjacent (separated by 2-5 m) sub-plots. While we did not measure initial plant cover, subplotswere selected to he visually homogeneous. Treatment effectscould potentially be confounded with differences in initialcover: however. random assignment of treatments to plotssought to avoid this problem. Trenched plots were createdby severing tree roots to a depth of 1 m around the perimeterof a 3 m x 3 m area. Plots did not include sapling or adulttrees 'but did include occasional tree seedlings less than30 cm tall. The trenches were lined with 0.5 mm stainlesssteel mesh on up- and down-slope sides and black plastic onother sides to prevent the reinvasion of tree roots. The clip-ping treatment was applied during the first 3 years of the ex-periment and then discontinued; the objective of thetreatment was to estimate understory vegetation effects onsoil moisture. All plants (herbs, shrubs. and tree seedlings)were clipped to ground level. After clipping was discontin-ued, some existing plants recovered and many new plantsseeded in. Although the clipping treatment per se is not thefocus of this paper, clipped plots were included in this anal-ysis to increase the sample size over which trench effectscould be estimated.

We sampled the plant communities in these plots in thesummer of 2000, 10 years after installation. Sampling wascarried out in a 2 m x 2 in plot set within the 3 m x 3 intreatment plot to avoid edge effects. Cover of each plant spe-cies was estimated visually and added to yield a total covervalue, which for some plots exceeded 100%. We alsocounted the number of flowering ramets for each species;because different species flowered in each plot, numbers foreach species were added to give the total number of flower-ing ramets. Growing season soil moisture in the plots wassampled monthly in May—October of 1991, 1992, and 1993.Volumetric soil moisture was determined using time domainreflectometry (model 1502C, Tektronix Inc., Beaverton,Ore., U.S.A.; Gray and Spies 1995; Gray et al. 2002). Twoparallel probes extended vertically 45 cm into the soil ineach plot; the value reported for each plot is the averagefrom two sets of probes. In this paper, we present themonthly trends in soil moisture and the relationship betweenve getation and soil moisture response; a previous analysisprovided a test of si gnificance of the effect of trenching onsoil moisture (Gray et al. 2002).

Cover and flowerin g, response variables were analyzed ina split plot ANOVA with canopy cover as the whole plot fac-tor and trenching and clipping as subplot factors. The analy-sis was carried out using PROC MIXED in SAS (version 8.SAS Institute Inc. 1999) with trenching, clipping, and can-opy openness as fixed effects and site as a random block ef-fect. The approximate normality of the cover data supporteda parametric analysis. The number of flowering ramets waslog-transformed to improve normality.

ResultsPlots were dominated by herbs and low shrubs, with occa-

sional tall shrubs and tree seedlings present. Trenched plotshad higher vegetation cover than controls in both the intact

Fig. 1. Response of understory vegetation to trenchin g and clip-pin g (averaged over canopy openness). Sample size for each baris four (an open- and closed-canopy plot at each of two sites).Error bars are one standard error.

Control Trench Clip Trench+Clip

vegetation (unclipped plots) and recovering vegetation(clipped plots) (Fig. I. F1161 = 64.74, p = 0.0002). Trenchedplots (unclipped) had 92% cover, while control plots had47% cover. Trench plots in gaps had about 30 percentagepoints higher plant cover than trenched plots under closedcanopy, providing an estimate of response to abovegroundgaps.

Cover in the trenched plots was composed of the fernPolystichum munition (Kaulf.) K. Presl (Polypodiaceae) andthe low shrub Berberis nervosa Pursh (Berberidaccae) at theH.J. Andrews gap area, the herb Coptis laciniata A. Gray(Ranuneulaceae) at the Andrews closed-canopy area, and theherb Achlys triphylla (Sm.) DC. (Berberidaceae) at both can-opy densities at Wind River. These species were also presentin the control plots, but at lower abundance. The total num-ber of flowering ramets (of all species) tended to he higherin trenched than in control plots (26 vs. 16; F[1.61 = 4.40, p0.08). At the H.J. Andrews site, most flowering ramets be-longed to the herb Tiarella trilohata L. (Saxifragaceae), andat Wind River, to the herb Achlv.c t•iphylla.

The effect of trenching on soil moisture was evidentthroughout the growing season and in all 3 years (Fig. 2).Each year varied in the initial soil moisture measurement,but the pattern of drying over the summer and the relativeposition of the treatments were the same in all years. Thetrenched plots exhibited the highest soil moisture; trenchedplots had hi gh and similar soil moisture regardless of canopydensity. In the gap areas. the control plots started at the samesoil moisture as the trenched, but dried faster to end the sea-son four percentage points drier. In the closed canopy areas,the control plots were much drier than the trenched plots(eight percentage points), and that difference was alreadypresent in May at the time of the first measurement. Statisti-cal evidence for an effect of clipping on soil moisture wasweak but suggestive (F11,11 -= 5.32, p = 0.06). Consideringonly untrenched plots, clipped plots had four percentagepoints higher soil moisture than unclipped: however, intrenched plots, clipping had no effect.

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Fig. 2. Soil moisture measured throughout the growing season inunclipped plots. Each point is the average of two values (onefrom each site). Circles (0) are closed-canopy control plots, tri-angles (II) are gap control plots, pluses (+) are closed-canopytrench plots. and crossed lines (x) are gap trenched plots.

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Like soil moisture, the vegetation responded much moreto trenching in the closed canopy area. The average differ-ence in plant cover between trenched and control plots wasmuch greater in closed-canopy than gap areas (64 percentagepoints versus 27 percentage points, considering only thoseplots with unclipped vegetation: see Fig. 3). The plot pairsthat show the largest difference in plant cover associatedwith trenching also show the largest differences in soil mois-ture (Fig. 3).

DiscussionOur data show clear differences in understory vegetation

cover, flowering, and growing season soil moisture associ-ated with the removal of competition by tree roots. Thisfinding matches our expectation that below ground competi-tion should be important in summer-dry regions. Our resultsclarify that even in a closed-canopy old-growth coniferousforest, belowground competition may be more importantthan li ght limitation. At our sites, the difference in plantcover moving from control to trench under a closed canopy(45 percentage points) was larger than that associated withmoving from closed-canopy to gap areas, where light levels

Fig. 3. Vegetation cover and soil moisture in control andtrenched plots. Each point is the value from one sample plot;open symbols correspond to control plots, filled symbols totrenched plots, and lines connect adjacent pairs of control andtrenched plots. Soil moisture values arc from October 1991. thedriest month in the first year followin g trenching.

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should be optimal for understory plants (30 percentagepoints).

Both vegetation and soil moisture responses to trenchingwere much higher in the closed-canopy areas than in thegaps. Our finding contrasts with the prediction of Coomesand Grubb (2000) that trench responses would be greater ingaps. In our sites, the gaps probably had lower root densitiesthan the closed canopy sites. Experimental gaps nearby hadmuch lower fine root densities in mineral soil than did con-trol plots (Vogt et al. 1995). Severing the less numerousroots in our gap areas would have had less effect onbelowground resources than severing the dense carpet ofroots in the closed-canopy area. Further evidence for lowerroot densities in the gap is provided by the fact that the dif-ference in soil moisture between control and trenched plotswas already present in May in the closed-canopy area, whileit developed gradually over the summer in the gaps. The de-pendence of the trenching effect on canopy environment(and by extension, root density) may explain why other re-searchers found trenching effects on soil moisture only at. theend of the growing season (Simard et al. 1997: Hart andSollins 1998) if their sites had lower root densities than ours.

The magnitude of vegetation response that we observedcorrelated well with the magnitude of effect that trenchinghas on soil moisture (sec Fig. 3). Our vegetation responsemay be due to soil moisture alone or to a concomitant in-crease in the availability of nutrients, which we did not mea-sure. However, in a nearby site. Hart and Sollins (1998)found that trenched plots had total nitrogen amounts indis-tinguishable from control plots, although trenched plots didhave higher rates of nitrogen mineralization. Since increasedwater availability can increase both the supply of availablenitrogen and the ability of the plant to take it up (Chapin etal. 1987; Rie gel et al. 1992), it is not easy to separate the ef-fects of water and nutrients.

Findings from trench studies carried out in temperate andtropical regions suggest responses to both soil moisture andnutrients (Coomes and Grubb 2000). Studies from wet coni-

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fer forests in Europe have suggested that nutrients mediatethe trenching response (reviewed in Walter and Breckle1985). Walter and Breckle (1985) suggest that at the ariditylimit of European tree species, understories tend to bedepauperate because on these sites the trees are using all theavailable water and leavin g none for the understory. In theopen, dry Pants ponderosa Dougl. ex P. & C. Laws. forestsof eastern Oregon. Riegel and coworkers (1992) showed thatunderstories were limited by belowground resources ratherthan light. They found increases in both water and nitrogenlevels as a result of trenching and concluded that plants wereresponding to both. In fact, we should expect plants in forestunderstories to he simultaneously limited by water, nutri-ents, and light and to be able to respond to increased abun-dance of any of these resources (Chapin et al. 1987). Studiesin tropical forests have produced comparable results, withtrenching having zero or negative effects on plant perfor-mance in nutrient-rich wet sites (Denslow 1991; Ostertag1998) and positive effects in nutrient-poor (Coomes andGrubb 1998) or dry sites (Gerhardt 1996).

Understory plants in summer-dry coniferous forests facelimiting levels of several resources because the trees whosecanopies shade the understory also hear roots that depletelevels of water and nutrients in the soil. Smith and Huston(1989) hypothesized that plants cannot develop strategies tosimultaneously and efficiently use low levels of water andlight. Shade-tolerant plants should develop large thin leavesat the expense of a limited root system. while drought-tolerant plants should have large belowground allocation andthick evaporation-resistant leaves. Coomes and Grubb (2000)suggested that plants that do tolerate both stresses have thesmall leaves of a drou g ht-adapted plant and grow extremelyslowly as a result of their very limited light capture. Of theherbs that responded most vigorously to trenching in theclosed-canopy sites in our study. one appears tolerant ofboth drought and shade, while the other is a shade toleratorthat evades drought through early senescence in the summer.Coptis laciniata, the dominant species in the shade plots atthe H.J. Andrews site, exhibits small, thick, evergreen leavesthat probably resist evaporation well. It is highly clonal,slow growing, and able to occupy even the densest forest.AchLys triphylla, which dominated the trenched plots atWind River. has large. thin leaves more typical of a shade-adapted plant. Perhaps as a result, it appears very vulnerableto drought. It occupies the wettest sites within coniferousforests and uses a "spring ephemeral" strategy. otherwiserare in western coniferous forests, to evade the dry late sum-mer period.

We found suggestive evidence that the number of flower-ing ramets was hi gher in trenched plots than in controls; thiseffect was stronger in the closed-canopy areas than in gaps.This result suggests that for the species flowering in ourplots (primarily Tiarella trifoliata L. and Achlys triphylla),not only vegetative growth but sexual reproduction is limitedby belowground resources. This experimental result supportsthe observational work of St. Pierre (2000). who found thatTiarella a-it-Ohara fecundity was explained better by soilmoisture than by light levels in gaps.

As mentioned above. previous work on effects of trench-in g in P menziesii forests has focused on understory trees

rather than shrubs and herbs (Christy 1986; Simard etal. 1997) and has found mixed results. Working only a fewkilometres away from one of our study sites. Christ y (1986)found that root trenchin g around suppressed Tsuga hetero-phyla juveniles increased their growth. Simard et al. (1997)transplanted P. tnenziesn seedlin gs into trenched and controlplots and found that trenching had no significant effect ontotal growth; stunting of height (vs. stem diameter) growthin the trenched seedlings suggested a negative effect of root.trenching. Simard ct al. (1997) attributed this lack of posi-tive response to two factors: (I) soil moisture may not havebeen limiting in her system because of significant summerrainfall and (ii) trenching lowered mycorrhizal diversity onthe roots of the seedlings and precluded mycorrhizal connec-tions between the seedlings and overstory trees. Since theunderstory herbs and shrubs that responded to trenching inour study are probably mostly vesicular—arbuscular mycor-rhizal (Brundrett and Kendrick 1.988), they are unlikely toshare mycorrhizal species or direct hyphal connections withthe overstory; trenching would thus not have any deleteriouseffect. Interestingly. tree seedlings established and orewfairly well in our trenches, suggesting either that these spe-cies (primarily Abies spp. and Tsuga heterophylla) do notrely on mycorrhizal innoculum or hyphal connections fromthe canopy trees or that the benefit provided b y trenchingoutweighed the loss of mycorrhizae.

Our results suggest that in the summer-dry P menziesiiforests that we studied, belowground competition may play alarge role in explaining the patterns of understory—overstoryinteractions. Even under dense shade. root trenching pro-duced dramatic increases in understory plant cover. Denselystocked young coniferous forests, referred to as closed can-opy or "dark". often have depauperate herb layers. Althoughlight limitation probably plays a role, young forests have thehighest root densities of any stand age (Vogt et al. 1983),and the even spacing of these forests will result in fewbelowground gaps. The characteristic patchiness of old-growth forest understories (Franklin at al. 1981) may hebetter explained by patchiness of belowground rather thanaboveground resources. Above- and below-ground gaps willbe con-elated but not necessarily totally overlapping in spaceor time; this lack of overlap may help explain observeddisjunctions between canopy cover and understory ve geta-tion response.

Acknowledgements

We thank C. Halpern, P. Muir. and two anonymous refereesfor helpful comments on previous versions of this manuscriptand C. Pereira for help with statistics. Funding was providedby National Science Foundation grant BSR8909038 and by aNational Science Foundation graduate fellowship to B.C.Lindh.

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