Plant Ecol DOI 10.1007/s11258-007-9292-y

ORIGINAL PAPER

Responses of seedlings to grass-induced above- and below-ground competition

Juliette M. G. Bloor Æ Paul W. Leadley Æ Laure Barthes

Received: 27 September 2006 / Accepted: 22 March 2007 Springer Science+Business Media B.V. 2007

Abstract The competitive interactions between ined. In contrast, morphological responses to shoot woody seedlings and herbaceous vegetation have competition were limited. In the absence of root received increasing interest in recent years. However, competition, seedlings showed a significant increase little is known about the relative contributions and in the biomass allocated to leaves and a greater leaf underlying mechanisms of above- and below-ground area ratio in response to shoot competition. Our competition between species. We used a novel findings suggest that morphological modifications experimental approach to assess the responses of help to mitigate the negative effects of competition, Fraxinus excelsior seedlings to different combina- but the expression of plasticity may be suboptimal tions of root and shoot competition imposed by the due to resource constraints. The present study also grass Dactylis glomerata under greenhouse condi- highlights the importance of appropriate experimental tions. Seedling growth was significantly reduced by controls and analysis to avoid confounding effects of competition for soil resources, but neither biomass experimental design and ontogeny on the interpreta- nor height were significantly affected by shoot tion of competitive responses. competition for light. Competitive response indices based on biomass confirmed that below-ground Keywords Grasslands Á Growth Á Morphology Á competition was more important than above-ground Root competition Á Woody establishment competition, and indicated that root and shoot competition did not interact to influence plant growth. Fraxinus biomass allocation and seedling traits were Introduction almost all significantly affected by root competition; these responses varied depending on the trait exam- Interspecific competition for light, water or nutrients can be a major force underlying the structure and dynamics of plant communities (Tilman J. M. G. Bloor (&) Á P. W. Leadley Á L. Barthes 1990). Whilst it is generally considered that below- Laboratoire d’Ecologie, Syste´matique et Evolution, UMR ground competition is size-symmetric and competi- CNRS 8079, Universite´ Paris-Sud, Orsay Cedex 91405, France tion for light is size-asymmetric (Weiner 1990), the e-mail: [email protected] relative importance of competition for above- and below-ground resources may depend on the species Present Address: involved, on resource supply, and on the timing and J. M. G. Bloor INRA UR874-Agronomie, 234 Avenue Du Brezet, duration of the interaction (Dillenberg et al. 1993). Clermont Ferrand 63100, France Understanding the competitive interactions between 123 Plant Ecol herbaceous vegetation and woody species is of resources (Wilson and Tilman 1995). To date, particular interest since these interactions may be however, interactions between root and shoot com- critical to community succession (Berkowitz et al. petition among grasses and woody seedlings have 1995, Van Auken 2000; Davis et al. 2005), invasion faced little attention. by exotics (Eliason and Allen 1997; Meiners et al. In this study we used the species Fraxinus 2002) or to the success of forestry and agroforestry excelsior L. and the grass Dactylis glomerata L. as a systems (Noland et al. 2001; Gakis et al. 2004). model system to investigate the relative importance Previous work indicates that large, established of root and shoot competition on early tree seedling woody can modify the composition, distribu- growth in successional communities. Fraxinus excel- tion and productivity of grasses in mixed tree-grass sior is an important pioneer tree species in , communities (Scholes and Archer 1997; Daly et al. and D.glomerata is a vigorously growing tufted 2000; Clavijo et al. 2005), a competitive advantage perennial grass common to a wide variety of grassy attributed to both the mass and growth form of the habitats worldwide. Previous attempts to quantify the woody species (Kochy and Wilson 2000). This effects of above- and below-ground competition have competitive effect of on grasses is most faced criticism for their use of unrealistic designs and pronounced in areas with cool climates and/ or high their lack of appropriate experimental controls productivity (Wilson 1998). In contrast, grasses are (McPhee and Aarssen 2001). We demonstrate that expected to compete successfully against seedlings of tree seedling growth is affected by partition treat- woody species since the high root density and ments associated with the target technique, and use a branched root architecture of grasses in superficial novel experimental approach to assess competitive soil horizons confer a greater capacity for soil effects more accurately. The following questions resource acquisition (Caldwell and Richards 1986; were addressed: (1) How does grass-induced root and Casper & Jackson 1997). In the face of grass-induced shoot competition affect tree seedling growth and below-ground competition, woody seedlings typically morphology? (2) Is there an interaction between root show reduced survivorship and growth, along with and shoot competition? In addition we use our data modifications in seedling root morphology (Davis set to compare the outcomes of different methods for et al. 1999; Jurena and Archer 2003; Harmer and calculating plant competitive responses. Robertson 2003). The competitive advantage of grasses over woody seedlings for the acquisition of light is less clear, since light interception by grasses is Methods constrained by their vertical leaf arrangement (Skal- ova et al. 1999). Nonetheless, grasses may generate Plant material and plant growth chambers significant competition for light when at high density or in fertile conditions (Soussana and Lafarge 1998; The study was carried out under glasshouse condi- Carlen et al. 2002). tions at the University of Paris XI (Orsay, France). Light competition can be strongly influenced by Tree seeds for this experiment were collected from a competition for soil resources, and interactions single Fraxinus excelsior population in Alsace Lor- between root and shoot competition have been raine (NE France) in 2004, and were pre-treated documented among herbaceous species (Wilson according to standard forestry practice in order to 1988; Cahill 2002). A positive interaction between break dormancy (16 weeks at 208C followed by root and shoot competition may arise if one form of 16 weeks at 58C). Dactylis glomerata seed was competition amplifies the effect of the other (e.g. obtained from Arbiotech, St Gilles, France and sown shading reducing a plant’s capacity for nutrient directly. uptake), whereas a negative interaction might be Soil used in the experiment was a loamy topsoil expected if the growth of a nutrient-limited plant is collected in the locality of the University of Paris XI less affected by light limitation (or vice versa) (Cahill (Orsay, France). Deep PVC pots (20 · 15 · 40 cm) 1999). Furthermore, certain researchers argue that were filled with a 10 cm layer of expanded clay there may exist a trade-off between the ability of pellets to improve drainage, then with a 50:50 mix of plants to compete for above- and below-ground sieved topsoil and locally obtained river sand. These 123 Plant Ecol pots were assigned to one of six naturally lit growth chambers (wooden frame and clear plastic walls, 65 · 65 · 100 cm high) set up inside a large glasshouse and ventilated with air taken from outside the glasshouse. Maximum/minimum thermometers set up in the growth chambers indicated that they did a) b) c) d) not differ significantly from each other in their temperature during the experimental period.

Experimental design

In April 2005 stratified Fraxinus seeds were germi- nated under glasshouse conditions in nursery flats e) f) g) h) filled with comercially available compost. Seedlings were left to grow until 6 May 2005, by which time ANOVA model Shoot Competition the first pair of leaves was expanding and seedlings No Yes were approximately five centimetres tall. A uniform subsample of this seedling stock was selected, and Root No h / d f / b Competition seedlings were then randomly allocated to different Yes g / c e / a experimental treatments and planted individually into the centre of each pot. Prior to transplantation, the compost core around the Fraxinus seedling roots was Fig. 1 Representation of the experimental treatments and the removed. standardised ANOVA model used to determine root and shoot Effects of root and shoot competition on Fraxinus competition in this study. Above- and below-ground partitions are applied in a factorial design to both ash seedlings growing were investigated using a modified Target technique alone (treatments a–d) and to ash seedlings growing with grass which included controls to assess the effects of plant (treatments e–h). Treatments a–d represent controls for partitions on the target ash seedlings (discussed in possible partition effects, and are applied in a pair-wise McPhee and Aarssen 2001). Two above-ground fashion to the corresponding grass treatments partition treatments, two below-ground partition treatments, and two grass competitor treatments were crossed to generate eight experimental treatments, seedling transplantation, each grass-sown pot had 30– each of which was replicated once per growing 35 grass plants and average grass height was 15 cm in chamber in a randomised block design (Fig. 1). One all grass-sown pots. month prior to Fraxinus seedling transplantation, above- and below-ground partitions were installed Plant measurements and seeds of Dactylis were sown into half of the experimental pots in each growth chamber at a Fraxinus seedlings were left to grow in the experi- density of 2,000 seeds/m2, leaving clear a central mental treatments for 11 weeks, and all pots were 8 · 8 cm zone per pot. Above-ground partitions were watered regularly throughout the experimental peri- cones made of chicken wire (15 cm high, diameter at od. No fertilisers were added during the study. base and top of 8 cm and 15 cm, respectively), Midway through the experiment, photosynthetically positioned in the centre of the appropriate experi- active radiation (PAR) measurements taken above the mental pots. Customised plastic tubes (8 · 8 · 25 cm) Fraxinus seedlings indicated an average PAR reduc- were used as below-ground partitions and inserted tion of 35.3 ± 5.0% for seedlings growing with grass into the centre of the experimental pots, such that the in the absence of an above-ground partition (based on base of each tube was in contact with the expanded comparison of instantaneous measurements made clay pellets. The grass began to emerge 10 days later, using gallium–arsenide sensors, JYP 1000, SDEC and the grass-free central zone was maintained by France). No significant difference was detected weeding where necessary. At the time of Fraxinus between the PAR levels recorded above seedlings 123 Plant Ecol either growing alone or separated from grass by an ing following competition. Above-ground competi- above-ground partition. tive response (ACR) was calculated as: ln (‘response On 26 July 2006, all plants were harvested and to shoot competition’/‘response to no competition’). Fraxinus seedlings were carefully disentangled from Belowground competitive response (BCR) was cal- Dactylis plants. Each of the harvested Fraxinus culated as: ln (‘response to root competition’/ seedlings was measured to determine crown diame- ‘response to no competition’). In the absence of an ter, stem length, leaf number and leaf area (using a interaction between root and shoot competition, TCR Delta-T area meter, Delta-T devices Ltd, Burwell, should equal the sum of ACR and BCR (see Cahill UK). In addition, seedling roots were washed and 1999 for full justification). If TCR is less than or scanned to determine total root length (WinRhizo greater than the sum of ACR and BCR, then this 2002, Regent Instrument Inc., Quebec, ). indicates an interaction between root and shoot Seedlings were then oven-dried (608C for at least competition (either positive or negative interactions 72 h) to obtain dry mass values for the roots, stems respectively). We recognise that the use of ratios to and leaves. Plant dry mass values did not include analyse competition data can be problematic (see cotyledon remains as these had dropped off in all Lamb et al. 2006). However, log response ratios have cases. Dactylis plants were separated into root and been shown to be mathematically and statistically shoot material per pot and similarly oven-dried (608C sound (Hedges et al 1999; Oksanen et al. 2006), and for at least 72 h) to obtain dry mass values. are suitable for simple experimental designs where Based on harvest data, eight variables were regression analysis is not possible (Weigelt and calculated per plant for Fraxinus: leaf area ratio Jolliffe 2003). (LAR; total leaf area per plant dry mass), specific leaf Standardised competitive responses for biomass area (SLA; total leaf area per leaf dry mass), specific (as calculated above) were compared to traditional root length (SRL; root length per root dry mass), non-standardised competitive responses to assess the specific stem length (SSL; stem height per stem dry discrepancies between the two techniques. Non- mass), leaf area index (LAI; total leaf area per crown standardised competitive responses were calculated projection area), leaf mass fraction (LMF; leaf dry using a subset of the eight competition treatments mass per plant dry mass), stem mass fraction (SMF; commonly found in competition experiments (treat- stem dry mass per plant dry mass) and root mass mentsa,e,fandgshowninFig.1). These fraction (RMF; root dry mass per plant dry mass). In uncorrected plant variables were then treated follow- addition, root/shoot ratios were calculated on a pot- ing Cahill (1999). For example, ACRNS for biomass basis for Dactylis in the different treatments. is given by: ln (‘biomass when grown with below- ground partitions in the presence of Dactylis’/‘bio- Plant responses to competition mass when grown alone’).

In order to evaluate Fraxinus responses to root and Statistical analyses shoot competition, plant variables were standardised in a pair-wise fashion to control for possible above- Effects of partitions on Dactylis biomass and root/ and below-ground partition effects following McPhee shoot ratios were tested with a randomised block two- and Aarssen (2001) (Fig. 1). For example, seedling way ANOVA using a fixed effects general linear biomass under root competition was derived as: model. ANOVA was also used to test for effects of ‘biomass when grown with above-ground partitions partitions and grass treatments on Fraxinus plant in the presence of Dactylis’/‘biomass grown with traits. Where necessary, data were transformed prior above-ground partitions only’. to analysis to conform with assumptions of normality Indices of seedling response to grass-induced and homogeneity of variances. Responses of Fraxi- competition were calculated for plant biomass fol- nus morphology to root and shoot competition were lowing Cahill (1999). Total competitive response determined using two-way ANOVA on standardised (TCR) was calculated as: ln (‘response to root + shoot plant variables which were log transformed prior to competition’/‘response to no competition’). Thus, analysis. Since plant variables may vary with plant TCR reflects the proportion of plant biomass remain- size, Fraxinus seedling dry mass was included as a 123 Plant Ecol covariable. This analysis (based on McPhee and and morphology (Table 2). Below-ground partitions Aarssen 2001) generated identical trends to that of had a significant negative effect on Fraxinus dry mass two-way ANCOVA performed on non-standardised and seedling height (Table 3). Presence of below- data with seedling dry mass as a covariable to control ground partitions also resulted in a significant reduc- for possible size effects and partition treatments as a tion in LAR (Table 3) and a shift in biomass covariable to control for possible above- and below- allocation patterns. Seedlings grown with below- ground partition effects (an alternative approach ground partitions had significantly lower LMF but which avoids the use of ratios). Given that the higher RMF compared with seedlings grown without interpretation of ANCOVA with multiple covariables below-ground partitions (Table 3). No significant can be less intuitive (Underwood 1997), we present interactions were observed between above- and only the ANCOVA analyses based on standardised below-ground partitions (Table 2). variables. Finally, Fraxinus competitive response Presence of Dactylis modified the effects of indices were analysed using non-parametric Krus- below-ground partitions on Fraxinus seedlings (Ta- kal–Wallis tests. All statistical analysis was carried ble 3). In the presence of the grass competitor, below- out using Statgraphics Plus 4.1. ground partitions had a significant positive effect on

Fraxinus dry mass and seedling height (F1,15 = 119.29, P < 0.001 and F1,15 = 286.79, P < 0.001, Results respectively). The barriers between Dactylis and Fraxinus roots resulted in a significant increase in

Effects of partition treatments on Dactylis Fraxinus LAR and LMF (F1,15 = 86.89, P < 0.001 glomerata and F1,15 = 258.53, P < 0.001, respectively). Below- ground partitions were also associated with a signif-

Presence of above- and below-ground partitions had icant reduction in seedling RMF and LAI (F1,15 = no significant effect on either plant dry mass or on 100.15, P < 0.001 and F1,15 = 26.05, P < 0.001 root/shoot ratios of Dactylis on a pot-basis (Table 1). respectively). Average dry mass ranged between 11.3 g and 13.1 g in the different treatments. Average root/shoot ratios Effects of root and shoot competition on Fraxinus varied between 0.48 and 0.58. excelsior

Effects of partition treatments on Fraxinus Root competition had a significant negative effect on excelsior the biomass and height of Fraxinus seedlings (Fig. 2,

F1,15 = 186.3, P < 0.001 and F1,15 = 200.09, P < 0.001 In the absence of Dactylis, above-ground partitions respectively). This effect was similar irrespective of had no significant effect on any of the Fraxinus the presence or absence of shoot competition (indi- seedling morphological traits measured (Table 2). cated by non-significant interactions between root However, the presence of below-ground partitions and shoot competition in both cases, F1,15 = 4.11 and had a significant effect on Fraxinus seedling growth 0.08, respectively, P > 0.05). Shoot competition had

Table 1 ANOVA of plant dry mass and root/shoot ratio for Dactylis glomerata grown in above- and below-ground partition treatments Source Dry mass Root/shoot ratio

Df FP Df FP

Block 5 1.93 0.149 5 2.73 0.060 Above-ground partition (cone) 1 0.51 0.486 1 0.03 0.867 Below-ground partition (tube) 1 3.58 0.080 1 0.03 0.857 Cone · Tube 1 0.33 0.572 1 2.20 0.159 Residual 15 15

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Table 2 ANOVA of plant traits for Fraxinus excelsior seedlings grown alone with above- and below-ground partitions. F values are presented; values shown in bold are significant to 0.05 Plant trait Source

Block (df = 5) Above-ground Below-ground Above · belowground partition (df = 1) partition (df = 3) partition (df = 3)

Dry mass 6.34 0.27 32.34 2.74 Height 2.35 0.79 32.70 4.14 LAR 2.97 0.11 10.29 2.75 SLA 1.81 0.32 0.01 1.13 LMF 1.07 0.03 18.41 0.52 SMF 0.49 1.23 4.93 0.31 RMF 0.75 0.27 7.28 0.17 SSL 2.49 0.00 0.44 0.27 SRL 1.69 2.49 0.01 1.19 LAI 2.34 2.26 0.00 1.56

Table 3 Dry mass, morphology and allocation patterns of target Fraxinus excelsior seedlings grown in above- and below-ground partition treatments either alone or in combination with Dactylis glomerata. Means and standard errors are shown (n =6) Plant trait Fraxinus No Above-ground Below-ground Above + below ground treatment partitions partition partition partition

Dry mass (g) Alone 3.35 ± 0.48 3.94 ± 0.50 2.26 ± 0.37 1.95 ± 0.27 Competition 0.34 ± 0.03 0.31 ± 0.03 1.09 ± 0.10 1.34 ± 0.11

Height (cm) Alone 23.5 ± 1.2 27.8 ± 1.8 18.1 ± 2.3 16.4 ± 1.1 Competition 5.1 ± 0.3 4.9 ± 0.2 18.6 ± 1.0 15.5 ± 1.0

LAR Alone 99.4 ± 6.4 91.8 ± 3.6 80.9 ± 4.2 85.9 ± 4.0 2 À1 (cm g ) Competition 60.7 ± 2.6 62.1 ± 4.4 99.9 ± 5.7 85.3 ± 3.4

SLA (cm2 gÀ1) Alone 273 ± 11 268 ± 10 264 ± 3 278 ± 12 Competition 290 ± 7 281 ± 10 307 ± 12 286 ± 7

LMF (g gÀ1) Alone 0.44 ± 0.02 0.42 ± 0.01 0.36 ± 0.02 0.36 ± 0.01 Competition 0.22 ± 0.01 0.24 ± 0.02 0.39 ± 0.01 0.35 ± 0.01

SMF (g gÀ1) Alone 0.21 ± 0.01 0.22 ± 0.01 0.23 ± 0.01 0.24 ± 0.01 Competition 0.26 ± 0.01 0.26 ± 0.01 0.27 ± 0.01 0.25 ± 0.01

RMF (g gÀ1) Alone 0.36 ± 0.02 0.36 ± 0.01 0.41 ± 0.02 0.40 ± 0.01 Competition 0.52 ± 0.02 0.54 ± 0.03 0.34 ± 0.01 0.40 ± 0.01

SSL (cm gÀ1) Alone 36.1 ± 3.4 34.5 ± 4.3 37.0 ± 5.8 39.2 ± 4.8 Competition 60.8 ± 3.7 59.4 ± 6.6 65.8 ± 5.9 49.8 ± 6.4

SRL (cm gÀ1) Alone 2,680 ± 190 2,782 ± 249 2,430 ± 236 3,000 ± 243 Competition 2,344 ± 183 2,819 ± 243 3,148 ± 178 2,753 ± 218

LAI (cm2 cm2) Alone 0.70 ± 0.05 0.69 ± 0.06 0.78 ± 0.09 0.61 ± 0.08 Competition 1.28 ± 0.13 1.07 ± 0.04 0.77 ± 0.09 0.76 ± 0.05

123 Plant Ecol

Fig. 2 Plant biomass, 2 2 height and morphology for

Fraxinus excelsior s 1.5 1.5 s t a seedlings grown in four h g i m 1 1

competition treatments. e y r Data have been H

D 0.5 standardised as ratios to 0.5 remove above- and below- 0 0 ground partition effects (see None Root Shoot Full None Root Shoot Full text). Means and standard error are shown; n =6 2 2 1.5 1.5 I R A A 1 1 L L 0.5 0.5 0 0 None Root Shoot Full None Root Shoot Full

2 2 1.5 1.5 A F

L 1

M 1 S L 0.5 0.5 0 0 None Root Shoot Full None Root Shoot Full

2 2 1.5 1.5 F L M 1 S 1 S S 0.5 0.5 0 0 None Root Shoot Full None Root Shoot Full

2 2 1.5 1.5 F L M 1 R 1 S R 0.5 0.5 0 0 None Root Shoot Full None Root Shoot Full

no significant effect on seedling biomass or height Five out of eight seedling morphological traits

(F1,15 = 0.31 and 2.24, respectively, P > 0.1). How- measured showed a significant effect of root compe- ever, seedlings facing shoot competition alone did tition (Table 4). Root competition was associated show a tendency towards decreased biomass and with a significant increase in RMF, LAI and SSL increased height (Fig. 2). (Fig. 2). Root competition also had a significant

123 Plant Ecol

Table 4 ANOVA of plant traits for target Fraxinus excelsior and below-ground partition effects and ln-transformed prior to seedlings grown in competition with Dactylis glomerata, with analysis. F values are presented; values shown in bold are root and shoot competition as factors and Fraxinus seedling dry significant to 0.05 mass as a covariate. Data were standardised to remove above- Plant trait Source of variation

Block (df = 5) Dry mass (df = 1) Shoot competition (df = 1) Root competition (df = 1) Shoot · root competition (df = 1)

LAR 1.60 0.33 4.88 20.38 6.49 SLA 1.35 0.50 2.90 0.85 0.72 LMF 3.92 0.21 1.75 34.09 10.12 SMF 1.09 5.28 1.04 3.12 0.30 RMF 0.57 1.11 3.26 16.13 2.65 SSL 0.67 7.91 1.38 5.67 0.20 SRL 0.42 2.44 0.52 3.53 1.70 LAI 7.07 3.90 0.23 9.57 0.79 negative effect on LAR and LMF (Fig. 2). Fraxinus Fraxinus leaf and root morphology were not seedling morphology was less responsive to shoot affected by seedling size (Table 4). However, competition; only LAR and LMF showed significant seedling biomass had a significant negative effect effects of shoot competition, and these effects varied on both SSL and the biomass allocated to stems depending on the presence or absence of root (Fig. 2). competition (significant root/shoot competition inter- actions, Table 4). In the absence of root competition, Competitive response indices for Fraxinus shoot competition was associated with an increase in excelsior biomass LAR and LMF, but in the presence of root compe- tition, no such increases were detected (Fig. 2). Total competitive response (TCR) and below-ground competitive response (BCR) were of a similar magnitude, both demonstrating a strong negative effect of competition on Fraxinus biomass (*90% 1 biomass reduction in all cases, Fig. 3). This result held true irrespective of the calculation used (stan- e

s dardised versus non-standardised biomass data);

n 0.75 o values for TCR or BCR did not differ from TCRNS p s and BCR , respectively (Kruskal–Wallis, e NS r

e 0.5 H6,6 = 1.64 and 0.33, respectively, P > 0.1 in both v i t

i cases). In contrast, the above-ground competitive t e

p response differed depending on the use of standar-

m 0.25 dised or non-standardised biomass data (Fig. 3). Use o

C of standardised data generated a negligible ACR, whereas ACR suggested a strong effect of above- 0 NS Shoot Root Full ground competition (*65% biomass reduction). Standardised biomass data indicated no interaction Competitive form between root and shoot competition; values for TCR Fig. 3 Mean competitive response (antilog) for Fraxinus did not differ from the sum of BCR and ACR excelsior seedlings grown with Dactylis glomerata. Light bars (Kruskal–Wallis, H6,6 = 1.26, P > 0.1). However, the are competitive responses based on data which have been same calculation based on non-standardised biomass standardised to remove partition effects, dark bars are competitive responses based on non-standardised data. Stan- data suggested a negative interaction between root dard errors are shown; n =6 and shoot competition. TCRNS was greater than the 123 Plant Ecol

sum of BCRNS and ACRNS (Kruskal–Wallis, availability, plant growth form or the stand structure H6,6 = 7.41, P < 0.01). present (Schwinning and Weiner 1998; Cahill 1999). Establishing woody seedlings may tend to compete size-symmetrically with grass competitors Discussion early in development, but interactions between root and shoot competition could occur as woody Effects of root and shoot competition on Fraxinus seedlings become more dominant in the vegetation seedling growth size hierarchy.

Root competition from Dactylis had significant Effects of root and shoot competition on Fraxinus negative effects on Fraxinus seedling growth, both seedling morphology in terms of dry mass and height. In the present study, root competition was essentially for soil Patterns of biomass allocation and architecture in nutrients since all treatments were watered on a response to variation in resource availability are well regular basis; nutrient analysis indicated that control documented, but surprisingly few studies have Fraxinus seedlings had twice the nitrogen concen- described plant morphology in response to competi- tration than that of seedlings in the full competition tion (Cahill 2003). We found that grass-induced root treatment (data not shown). However, competition competition increased Fraxinus seedling allocation to for soil water has also been shown to be critical to roots at the expense of leaves, and was associated woody seedling growth and survival in grassland with an increase in seedling SSL. The changes in root systems (Gordon et al. 1989; Weltzin and McPh- allocation observed are consistent with optimality erson 1997; Davis et al. 1999; Picon-Cochard et al. theory, which suggests that plants allocate biomass in 2001). The relative contributions of soil nitrogen order to maximise capture of those resources in most and soil water to grass-induced root competition limiting supply and to maintain carbon gain (Bloom may vary according to the species and habitat et al. 1985). Although such changes in root allocation involved (Burton and Bazzaz 1995; Kochy and in response to root competition have sometimes been Wilson 2000). attributed to changes in plant size rather than to Unlike competition for below-ground resources, adaptive plasticity (Reynolds and Antonio 1996; shoot competition had minimal effects on Fraxinus Cahill 2003, but see Shipley and Meziane 2002), growth. Competitive response indices (standardised we found no significant size effects on biomass for partition effects) demonstrated no significant allocation. In contrast, the modifications in SSL effect of shoot competition on seedling biomass observed probably reflect differences in ontogeny compared with a 90% biomass reduction in the rather than an adaptive response to competition since presence of root competition. These results are in we found a significant effect of seedling biomass on agreement with studies on establishing woody stem traits. shrubs and herbaceous vegetation which show that Root traits may influence plant competitive below-ground competition usually exerts a greater ability, and grass-induced root competition has been impact on competing species than above-ground associated with changes in fine root growth, rooting competition (Aerts et al. 1991; Van Auken and depths and root morphology of woody seedlings Bush 1997). In addition, effects of root and shoot (Dawson et al. 2001; Harmer and Robertson 2003; competition on seedling biomass were shown to be Curt et al. 2005). Contrary to what might be independent, implying that competition between expected, we found no significant effects of root grass and woody seedlings is size-symmetric. Evi- competition on SRL, a root trait linked to the dence for interactions between root and shoot efficiency of soil resource acquisition. However, the competition in the literature is mixed (e.g. Wilson magnitude and significance of root competition 1988; Cahill 2002; Song et al. 2006), and it has effects on tree seedling root structure has been been suggested that the probability of detecting found to vary among species (Harmer and Robert- asymmetry in above- and below-ground competitive son 2003). Furthermore, patterns of root morphol- responses may depend on plant density, resource ogy at the whole-plant level may mask effects of 123 Plant Ecol root competition on different size-classes of roots Comments on experimental design (Picon-Cochard et al 2001) or on roots in different soil horizons (Cheng and Bledsoe 2004). Greenhouse experiments have long been used to Previous studies which have measured the effects demonstrate plant interactions; the controlled condi- of root competition on leaf traits in woody seedlings tions of greenhouses minimise extrinsic variability, have generated conflicting results (Picon-Cochard allow repeatability and a greater degree of precision et al. 2001; Fotelli et al. 2005). We found no evidence in examining plant interactions than is often possible of competition-induced changes in SLA, but root in the field (Gibson et al. 1999; Freckleton and competition had significant negative effects on LAR. Watkinson 2000). Of course, greenhouse experiments This decrease in LAR resulted from a greater can be criticised on their lack of realism, and we investment of carbon in stems and roots and a recognise that our short-term study may provide suppression of leaf area growth. In natural vegetation, limited information on the long-term dynamics such altered morphology may constrain the ability of between neighbours (but see Howard and Goldberg plants to respond when competition later decreases 2001). Under natural conditions, Fraxinus seedlings due to disturbance (Burton and Bazzaz 1995). would most likely face strong competition from Effects of grass-induced shoot competition on perennial old genets of Dactylis, and might be Fraxinus morphology were only apparent in the expected to show declines in seedling survival as absence of root competition. The increase in LAR well as growth (Wardle 1961; Marigo et al. 2002). and biomass allocation to leaves observed enhance Nonetheless, our results offer a valuable insight into the ability of Fraxinus seedlings to intercept light the effects of grass competition at a critical stage in in the face of reduced light availability. Such the regeneration of woody plants. morphological plasticity could mitigate the negative Studies that attempt to separate root and shoot effects of above-ground competition on seedling competition invariably use partitioning techniques, growth and explain the absence of shoot competi- with partitions made of different materials and in tion effects on plant biomass found in this study. different arrangements depending on the experimen- Why does seedling response to shoot competition tal design (Wilson 1988). Although our control depend on the absence of competition for below- treatments indicated that wire mesh cones had no ground resources? One possibility is that seedlings significant effect on the growth or morphology of require a minimum of soil resources in order to individual Fraxinus seedlings, we found significant express phenotypic plasticity in response to light. effects of below-ground barriers on woody seedling Clear evidence suggests that woody seedlings development. The reduced seedling growth and require a minimum of light resources in order to modifications in Fraxinus seedling morphology respond to soil nutrient changes (reviewed in observed are consistent with reduced rooting volumes Coomes and Grubb 2000). It seems reasonable that and resources associated with the presence of below- the converse might also be true; work on herba- ground barriers (McConnaughay and Bazzaz 1991; ceous species provides support for this idea (Peace Casper and Jackson 1997). Furthermore, we found and Grubb 1982; Wilson and Tilman 1995), but discrepancies between the results of competitive data for woody seedlings is lacking. Alternatively, response indices depending on whether or not effects seedlings that are strongly limited by soil resources of partitions were taken into account. Whilst the use may not actually experience shoot competition. For of indices for measuring competition remains a both woody and herbaceous species, respiration and subject of much debate (Weigelt and Joliffe 2003; photosynthetic rates have been shown to mirror Lamb et al. 2006; Oksanen et al. 2006), our results nitrogen uptake rates and leaf nitrogen concentra- highlight the importance of adequate experimental tions (Reich et al. 1998; Wright et al. 2005). controls in order to avoid confounding the effects of Consequently plants experiencing severe nutrient partitions with the effects of competition. shortage should have a lower light demand than plants at high nutrient supply thanks to their lower Acknowledgements We thank Annick Ambroise, Sandrine respiration and photosynthetic rates. Fontaine, Jean-Louis Mabout and Lionel Saunois for technical assistance and help with plant harvesting. This study was

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