Neighbour Biomass Research Articles

Nitrogen‐fixing plants increase soil nitrogen and neighbouring‐plant biomass, but decrease community diversity: A meta‐analysis reveals the mediating role of soil texture

Abstract Several recent regional studies have cast doubt on the widespread assumption that nitrogen‐fixing plants (N‐fixers) act as facilitators of neighbouring plant communities. We conducted a meta‐analysis to synthesize the effects of N‐fixers on plant communities and to understand how ecological context moderates these effects. We analysed studies that assessed paired effects of N‐fixers and non‐fixers on soil N, neighbouring‐plant (non‐fixer) biomass and plant community diversity; ecological moderators included climate, soil texture and N‐fixer growth form and invasive status. N‐fixers led to higher soil N and neighbouring‐plant biomass, but lower community diversity compared to non‐fixers. The effect of N‐fixers on neighbouring‐plant biomass was strongly mediated by soil texture; N‐fixer invasive status and growth form were also significant mediators of the facilitative effects of N‐fixers. Synthesis: N‐fixer effects lie on a continuum between facilitation and suppression that is moderated by intrinsic and extrinsic processes, and this analysis provides insight into how these factors moderate the effects of N‐fixers. Overall, N‐fixers facilitate neighbour biomass but suppress diversity, though high variation in these effects can be explained in part by ecological context.

Indirect genetic effects are shaped by demographic history and ecology in Arabidopsis thaliana.

The phenotype of an individual can be affected by the genes of its conspecifics through indirect genetic effects (IGEs). IGEs have been studied across different organisms including wild and domesticated animals and plants, but little is known about their genetic architecture. Here, in a large-scale intraspecific interaction experiment, we show that the contribution of IGEs to the biomass variation of Arabidopsis thaliana is comparable to values classically reported in animals. Moreover, we identify 11 loci explaining 85.1% of the variability in IGEs. We find that positive IGE alleles (that is, those with positive effects on neighbour biomass) occur both in relict accessions from southern Eurasia and in post-glacial colonizers from northern Scandinavia, and that they are likely to have two divergent origins: for nine loci, they evolved in the post-glacial colonizers independently from the relicts, while the two others were introgressed in the post-glacial colonizer from the relicts. Finally, we find that variation in IGEs probably reflects divergent adaptations to the contrasting environments of the edges and the centre of the native range of the species. These findings reveal a surprisingly tractable genetic basis of IGEs in A. thaliana that is shaped by the ecology and the demographic history of the species.

The relative importance of drought stress and neighbor richness on plant–plant interactions shifts over a short time

Exploring how plant–plant interactions between species and their neighbors vary with biotic and abiotic factors is vital to understanding community assembly mechanisms in the context of global changes. In this study, using a dominant species (Leymus chinensis (Trin.) Tzvel.) in the semiarid Inner Mongolia steppe as the target species and ten other species as neighbors, we carried out a microcosm experiment to evaluate how drought stress, neighbor richness and season affected the relative neighbor effect (Cint) (the ability to reduce growth of target species). The factor of season interactively affected the effect of drought stress or neighbor richness on Cint. In the summer, drought stress decreased Cint directly and indirectly by decreasing SLA hierarchical distance and neighbor biomass. In the subsequent spring, drought stress increased Cint, and neighbor richness increased Cint directly and indirectly by increasing neighbor community functional dispersion (FDis) and neighbor biomass. Specifically, SLA hierarchical distance was positively associated with neighbor biomass, while height hierarchical distance was negatively associated with neighbor biomass in both seasons, which increased Cint. These findings show that the relative importance of drought stress and neighbor richness on Cint shifted over seasons, which provides strong empirical evidence of how plant–plant interactions respond to changes in biotic and abiotic factors in the semiarid Inner Mongolia steppe over a short-term time. Furthermore, this study provides novel insight into community assembly mechanisms in the context of climatic aridity and biodiversity loss in semiarid regions.

Plant communities, populations and individuals have distinct responses to short‐term warming and neighbour biomass removal in two montane grasslands

AbstractAimsClimate change will impact plant communities and populations but also individual plant performance. Most predictive models of community responses to climate change ignore individual‐level biotic interactions despite their known importance for community diversity and functioning. Here, we consider plant fitness and diversity responses to climate change associated factors at three organisational levels: communities, populations and individual plants, to increase our understanding of how plant communities respond to climate change.LocationMontane grassland, Tasmania, Australia.MethodsIn two plant communities, we manipulated temperature using open‐top chambers and removed random and dominant species biomass. Two years after experimental manipulations, we assessed the impact of treatments on species diversity, community‐ and population‐level functional traits and individual plant fitness.ResultsSpecies diversity was affected by warming in one of the two communities, while community‐level functional trait diversity metrics were unaffected by treatments. Mean community trait values were strongly impacted by dominant species biomass removal in both communities, notably increasing specific leaf area (SLA) and specific root length. SLA showed the strongest population‐level trait response, with higher values found in warmed plots and lower values found in dominant species biomass removal plots. Neighbours had a stronger competitive effect on individual plant fitness in warmed compared to unwarmed conditions at the higher‐elevation site and facilitation was common in both communities.ConclusionsWe demonstrated that over short time scales, plant communities respond to experimental warming and biomass removal across multiple organisational levels. Competitive and facilitative interactions played a significant role in determining fitness outcomes, but competitive interactions dominated under warmed conditions. We highlight the importance of local‐scale biotic interactions in mediating individual responses to warming and recommend their inclusion in future studies of how climate change will impact the long‐term structure and function of plant communities through short‐term impacts on individual plant fitness.

Which traits predict pairwise interactions in a mountain grassland?

AbstractAimThe outcome of interaction between plant individuals is often expected to be determined by differences in their sizes with other traits playing minor roles. However, such findings typically come from manipulative experiments in experimental gardens. Consequently, little is known about the relative roles of size vs other traits in the field, particularly in communities other than forests. Here we aimed; (a) to determine the relative role of size‐related traits in pairwise interactions between common species in a species‐rich grassland community; and (b) to determine which other traits predict outcomes of such interactions in the field, and which traits predict mean competitive effects and responses of individual species.LocationThe Krkonoše Mts., Czech Republic.MethodsWe fitted dynamic models with explicit terms for pairwise interactions on a 31‐year long data series of spatially explicit ramet counts of 19 coexisting species in a mountain meadow. We compared these fits with those having total neighbour biomass as the only competition term to determine whether accounting for pairwise interactions increases model fit. Further we linked estimates of the parameters of pairwise interactions with species traits, including size‐related traits.ResultsComparison of models with and without pairwise predictors showed that pairwise interactions provided better fits only in less than half of the cases. The main determinant of competitive interactions was leaf area and, to a lesser extent, mean ramet biomass; there was an additional signal of difference in rooting depth.ConclusionPlant size and overall biomass have overwhelming effects on species‐to‐species interactions even in unproductive grassland communities, despite low differentiation of plant species along the size axis there. However, the weak albeit detectable signal of predictors not related to size of pairwise interactions points to possible mechanisms of within‐community niche differentiation, which can be captured by inter‐specific difference in functional traits and tested experimentally.

Both vacant niches and competition-trait hierarchy are useful for explaining the invasion of Caragana microphylla into the semi-arid grassland

Trait-based coexistence theories are useful to explain community processes. The aim was to distinguish which theory played a more important role in explaining the invasion mechanism of Caragana microphylla into communities in the Eurasian semi-arid grassland region. We conducted a water condition (WC) × neighbouring plant diversity (NSD) experiment, measured functional dispersion (FDis) and biomass of neighbours, absolute and hierarchical distances between neighbours and C. microphylla for plant height, specific leaf area and leaf dry matter content (LDMC), and explored the relative effect of each predictor on neighbour-effect intensity index (NIntA) on C. microphylla. Significant effect of NSD was direct or mediated by cascading effects between FDis and biomass of neighbours, significant effect of WC was direct or mediated by LDMC hierarchical distance, shoot biomass of neighbours mediated indirect effects of both NSD and WC on the NIntA on C. microphylla. None of the trait absolute distances was employed to explain the variability of the NIntA on C. microphylla. The reduction of NSD or water could decrease the inhibitory effects of neighbours on C. microphylla by increasing vacant niches or increasing the LDMC hierarchical distance between neighbours and C. microphylla, respectively, suggesting that vacant niches and competition-trait hierarchy played more important roles than competition-trait similarity in explaining shrub invasion processes in the Eurasian semi-arid grassland region.

Effect of insect herbivory on plant community dynamics under contrasting water availability levels

Abstract Plant diversity is impacted by multiple global change drivers but also by altered biotic interactions with antagonist and mutualist organisms that can potentially affect species coexistence. With a 2‐year, outdoor mesocosm experiment in realistic mesic grassland communities, we explored the role of insect herbivory in impacting plant community dynamics under contrasting levels of water availability simulating altered rainfall regimes. We selected a grasshopper species (Calliptamus italicus L.) feeding predominantly on forbs while avoiding grasses. High water availability reduced species coexistence, boosting productivity while decreasing individual plant survival. At the community level, herbivores were not able to promote species coexistence but asymmetrically influenced grasses and forbs, reducing forb biomass under high water availability. Herbivores shaped individual plant responses to both abiotic conditions and individual‐neighbours’ interactions. Herbivores influenced focal plant survival by altering the effect of neighbouring plants, mitigating the negative effect of high neighbour biomass at low water availability and exacerbating it at high level of water availability. Synthesis. Altered rainfall has the capacity to change the relative strength of plant–plant interactions and also to determine the effects of herbivores on grassland communities. The complexity of the interactions between plants and herbivores and the observed context dependence indicate the need to incorporate multiple biotic and abiotic drivers to fully understand the mechanisms underlying plant dynamics and species coexistence in a changing world.

Community traits affect plant–plant interactions across climatic gradients

Plant abundances and demography often vary along gradients of environmental stress, and neighboring plants can amplify or diminish such variation. We asked to what degree the effects of neighboring plants on a focal species can be explained by the traits and abundances of species in the surrounding community. We studied a common understory herb, Trientalis latifolia, across climatic gradients created by topography in the Siskiyou Mountains, southwestern Oregon. We compared Trientalis fitness along these gradients with and without neighbor removal, and asked whether the effects of neighboring plants could be predicted by their community‐weighted trait values and abundances. Environmental conditions alone did not explain whether neighbors had competitive or facilitative effects on Trientalis. However, the environment interacted with neighbor traits and biomass to influence neighbor effects: at cool, higher elevations, high neighbor biomass was associated with stronger facilitative effects, while at warm, lower elevations, high neighbor dissimilarity from Trientalis was associated with stronger competitive effects. We suggest that covariation and interactions among environmental and community characteristics are key to understanding species performance along climatic gradients.

Daily environmental conditions determine the competition–facilitation balance for plant water status

SummaryPlants compete with their neighbours for a finite set of limiting resources, and this decreases individual plant performance, growth and survival. However, neighbouring plants also affect each other in positive ways.Positive facilitative effects can occur when neighbouring plants ameliorate harsh abiotic conditions (temperature, wind and high irradiation). Thus, when environmental conditions are severe, the importance of facilitation may increase. The co‐occurrence and masking effects of competition and facilitation among neighbouring plants have made it difficult to tease them apart in the past.We planted bur oak acorns (Quercus macrocarpa) into an experimental diversity gradient in a centralMNgrassland that provided a gradient in plant biomass. We predicted that greater biomass of neighbours would increase both competition and facilitation as measured by impacts on the minimum leaf water potential reached on any given day. Under moderate conditions, competition should predominate, but under hot/dry conditions, facilitation should become more important. We measured temperature, humidity and soil moisture in these plots for two growing seasons, as well as oak seedling leaf water potential across a range of daily conditions.On cool/humid days, plant interactions were dominated by competition for soil water: leaf water potentials of juvenile oaks were lower in plots with greater herbaceous biomass (and higher diversity). Conversely, on hot/dry days, facilitation of the microclimate determined the net effect of plants on their neighbours: leaf water potentials of juvenile oaks were higher in plots with higher herbaceous diversity and biomass.Synthesis. In terms of plant water status, plant interactions among neighbours can flip from net negative (competition) to net positive (facilitation) depending on daily abiotic conditions. The relative importance of both positive and negative interactions for plant water status may affect the overall performance of plants over time.

Competitive effect and response of savanna tree seedlings: Comparison of survival, growth and associated functional traits

Questions What is the effect of neighbour competition on the survival, growth and biomass of mesic and humid savanna tree species? Can competitive effect and response be linked to plant functional traits? Location Neil Tainton Arboretum, University of KwaZulu-Natal, Pietermaritzburg, ZA. Methods Using a target–neighbour design, all combinations of four humid and four mesic savanna tree seedlings were tested in a greenhouse experiment, to establish the effects of neighbourhood competition on target plant performance. The competitive response and effect of each species was quantified, and regressed against several functional traits to determine which traits are predictive of competitive ability. Results We found that neighbour density negatively affected the survival of mesic tree seedlings but not humid tree seedlings. Mesic plants were able to maintain their relative growth rates (RGRs) despite increasing neighbourhood competition. The RGRs of humid species significantly decreased as the density of neighbours increased, indicating that competition may be a factor affecting plants in savannas receiving more than 650 mm mean annual rainfall. We used the quantile regression method to show that the biomass of humid tree seedlings decreased with increased neighbour biomass, when considering competitive response only. We also found that resource-acquiring traits (such as maximum RGR, plant height and root biomass) were positively correlated with the competitive effect of mesic savanna seedlings, while specific leaf area captured the competitive response of humid savanna seedlings to neighbour competition. Conclusions Competitive interactions between savanna trees negatively affect survival and growth rate, although the competitive effect or response of a particular species depends on its adaptation to either high- or low-resource environments. Our results show significant relationships between plant traits and competitive ability, which may be used to predict competitive interactions between tree seedlings from humid and mesic savannas.

Competitive effect and response of savanna tree seedlings: comparison of survival, growth and associated functional traits

AbstractQuestionsWhat is the effect of neighbour competition on the survival, growth and biomass of mesic and humid savanna tree species? Can competitive effect and response be linked to plant functional traits?LocationNeil Tainton Arboretum, University of KwaZulu‐Natal, Pietermaritzburg, ZA.MethodsUsing a target–neighbour design, all combinations of four humid and four mesic savanna tree seedlings were tested in a greenhouse experiment, to establish the effects of neighbourhood competition on target plant performance. The competitive response and effect of each species was quantified, and regressed against several functional traits to determine which traits are predictive of competitive ability.ResultsWe found that neighbour density negatively affected the survival of mesic tree seedlings but not humid tree seedlings. Mesic plants were able to maintain their relative growth rates (RGRs) despite increasing neighbourhood competition. The RGRs of humid species significantly decreased as the density of neighbours increased, indicating that competition may be a factor affecting plants in savannas receiving more than 650 mm mean annual rainfall. We used the quantile regression method to show that the biomass of humid tree seedlings decreased with increased neighbour biomass, when considering competitive response only. We also found that resource‐acquiring traits (such as maximum RGR, plant height and root biomass) were positively correlated with the competitive effect of mesic savanna seedlings, while specific leaf area captured the competitive response of humid savanna seedlings to neighbour competition.ConclusionsCompetitive interactions between savanna trees negatively affect survival and growth rate, although the competitive effect or response of a particular species depends on its adaptation to either high‐ or low‐resource environments. Our results show significant relationships between plant traits and competitive ability, which may be used to predict competitive interactions between tree seedlings from humid and mesic savannas.

Biomass explains the intensity of facilitative – not competitive – interactions: three intraspecific tests with annuals

Abstract. Despite efforts to discern the role of plant size in resource competition, the circumstances under which size-dependent plant-plant interactions occur are still unclear. The traditional assumption is that competition intensifies with increasing neighbour size. However, recent studies suggest that the size (biomass) dependence of competitive interactions is strongest at very low biomass levels and becomes negligible after a certain threshold neighbour biomass has been reached. We searched for the generality of such patterns for three common annual plant species in Israel. We monitored target and neighbour biomass along their entire lifecycle using an even-aged, intraspecific and intrapopulation competition screenhouse experiment under water-limited conditions. For all focal species, neighbour presence had a net negative effect on vegetative biomass at harvest. However, this was not explained by increasing neighbour biomass over time, as a consistent pattern of size-dependent facilitative, rather than competitive, interactions was observed at all life stages. We explain these observations in terms of co-occurring aboveground facilitation and dominant belowground competition for water. Since our findings are the first of their kind and contradict theoretical predictions of biomass dependence of net negative interactions, we advocate further experiments addressing size dependence in interactions among plants. In particular, theoretical models addressing size dependence of positive interactions must be developed.

Towards a more exact definition of the importance of competition - a reply to Freckleton et al. (2009)

Summary 1. Freckleton, Watkinson & Rees (2009) criticize a recent paper by ourselves in this journal (Brooker & Kikvidze 2008) as well as our earlier work on competition importance (Brooker et al. 2005). In response, here we clarify our ideas with the aim of defining more clearly the key points of scientific debate, specifically (i) the definition of the importance of competition and (ii) its measurement. 2. Freckleton, Watkinson & Rees (2009) interpret the classic paper by Welden & Slauson (1986) such that importance as a concept relates to long-term, population-level consequences of competition. However, we consider competition importance to be the proportional impact of competition relative to the overall impact of the environment, and our index Cimp expresses changes in competition importance – as defined by ourselves – along productivity gradients. We argue that our definition more accurately reflects the work of Welden & Slauson, as well as a more recent use of the concept (Grace 1991), which precedes the work of Freckleton & Watkinson (2001). 3. We highlight that Cimp was never proposed as a general index of competition importance, but is readily applicable in certain circumstances. Notably, our index and the approaches to measuring competition importance as set out by Freckleton, Watkinson & Rees (2009) are not unrelated. 4. We also discuss some recent additional responses to both our (2008) paper and that by Freckleton, Watkinson & Rees (2009), including applications of the concept of competition importance. Although the authors of these papers may not have used our index Cimp, they follow the same definitions for the overall concept of competition importance as ourselves. 5. Synthesis. We conclude that the complex topic of biotic interactions, including the specific issue of the importance of competition, invites a range of approaches. Importantly, these approaches can be complementary and not conflicting. Here, we propose what we see as a sensible resolution to the current debate concerning the definition of competition importance, a resolution which is backed by the original source article, literature precedent and current usage.

Species‐specific positive effects in an annual plant community

Plant facilitation studies commonly test the nurse‐plant hypothesis wherein an adult shrub species enhances the establishment of associated herbaceous species under its canopy. Using field and glasshouse experiments, this hypothesis is extended by testing the following four predictions: (1) nurse‐plant effects can occur between species with similar life‐forms and phenologies (2) positive effects are species specific, (3) the outcome of interactions is life‐stage dependent, and (4) facilitative interactions among annuals are primarily commensal. In the Negev Desert in Israel, the response of an annual plant community to removal of relatively larger annuals, Erodium laciniatum, Erucaria pinnata and Trifolium tomentosum, was tested in the field and in the glasshouse. Removal of these dominants was applied early in the growing season, immediately after germination but before establishment of seedlings, and again mid‐season following establishment and growth to adults. In both the field and glasshouse, the presence of E. laciniatum increased establishment and survival to reproduction of neighbouring plants. These positive effects were life‐stage dependent with more positive effects occurring early in the season, and there was no cost of facilitation to E. laciniatum. This positive effect was species specific in that neither E. pinnata nor T. tomentosum had an effect on the plant community. There was also a cost of association with the nurse E. laciniatum in that biomass of neighbours was reduced. These experiments demonstrate that the positive effects typically detected at larger scales between species of different life‐forms are also occurring at finer spatial scales amongst annuals. This study clearly supports the predictions made in the facilitation literature that effects are species‐specific and highly life‐stage dependent.

Measuring the components of competition along productivity gradients

Summary Controversy surrounds the measurement of competition intensity. Moreover, when biomass varies systematically along productivity and other environmental gradients, common indices of competitive outcome mask important ecological interactions. This study presents two indices derived from how neighbours interact with target plants. The first, relative crowding, increases directly with the abundance of neighbours present and decreases inversely with the potential size and vigour of the target plant itself. The second, interaction strength, is the integral of suppression of the target by neighbours over the range of neighbour abundance. Relative crowding and interaction strength are derived independently, but when multiplied produce the commonly used relative competitive index, showing the biological underpinnings of the relative competition index in terms of crowding and strength of interaction. Since the new indices of relative crowding and interaction strength explicitly account for the amount of neighbour biomass, they serve as a valid method to track the effects of changing habitat conditions on the components of competition. The new indices are applied to three published data sets. In each case, relative crowding increased with standing crop. In one case competition was reported as unchanged along a productivity gradient, whereas the new indices show that relative crowding and interaction strength both had significant patterns, but their effects were counteracting. These results do not fit current theories of competition. Further empirical studies are needed to see if competition theory needs revision. Separating the mechanisms of competition into relative crowding and strength of interaction reveals previously hidden patterns that help bring to light underlying processes of competition along productivity gradients.

Variation in plant performance in a grassland: Species‐specific and neighbouring root mass effects

AbstractQuestion: What characteristics of local biotic neighbourhood is the best proxy of competitive effects experienced by plants in a herbaceous community: (1) total above‐ground biomass, (2) root mass or (3) relative above‐ground abundance of selected species?Location: Grassland at ca. 1100 m a.s.l. in the Krkonoše Mts., northern Czech Republic.Methods: We implanted two phytometer species, Antho‐xanthum alpinum and Festuca rubra, into a mountain grassland, and examined their response to local variation in (1) total above‐ground biomass, (2) root mass at three soil depths, and (3) relative abundance of individual species above‐ground. Results: Performance of both phytometer species was determined much more consistently by the mass of neighbouring roots and by species composition of neighbours than by the total above‐ground biomass. The two phytometer species showed different responses to these parameters. The most important relationships were (1) negative relationship between performance of Anthoxanthum and mass of neighbouring roots at 0–3 cm, (2) positive relationship between performance of Festuca and mass of neighbouring roots at 3–6 cm, and (3) negative relationship between performance of Festuca and relative abundance of Festuca in the neighbourhood.Conclusions: Neighbouring root mass and above‐ground species composition are better determinants of biotic interactions than total above‐ground biomass of neighbours in the studied mountain grassland. However, the relationships found are not necessarily due to variation in competitive intensity but can be due to other hidden factors as well, e.g. local availability of resources.

Factors determining plant–neighbour interactions on different spatial scales in young species‐rich grassland communities

AbstractIn naturally colonised species‐rich grassland communities, we examined the properties of a plant's aboveground neighbourhood that affect its performance (aboveground biomass). To this end a range of neighbourhood parameters were measured: number, biomass and species richness of the neighbours, number and biomass of the conspecific neighbours, and light availability at the base of the target plant. We also determined at which neighbourhood size the strongest target plant–neighbour interactions occurred, and whether conspecific neighbours affected competitively stronger or weaker target species differently. Target plant performance varied with target identity, and was significantly affected by light availability and the number of neighbouring plants (neighbourhood density). Depending on the target species, there was also an effect of total neighbour biomass on plant performance. The target plants were most strongly affected by their neighbours within a 3‐cm distance, which could account for 78% of the variance in target biomass. Number or biomass of the conspecific neighbours did not contribute to the explanation of target performance in any of the target species. Whereas in an 8‐cm neighbourhood the amount of light penetration was the strongest predictor of target performance, the number of neighbours was more important in a 3‐cm neighbourhood. These experimental results might be useful to extend existing neighbourhood competition models for one or two species to multi‐species competition models.

Competitive effect is a linear function of neighbour biomass in experimental populations of Kochia scoparia

1 Neighbour size and distance are confounded in most studies of plant growth and competition at the individual level. To investigate the effects of neighbour size on competitive effect, we grew target Kochia scoparia individuals surrounded by six equidistant, even-aged conspecific neighbours. We varied neighbour size by sowing groups of neighbours at different times, and we also varied the sowing time of the target plants to generate variation in both neighbour and target size during the process of competition. We analysed the growth of target plants over two time intervals as a function of their own size and the size of their neighbours at the beginning of the interval. 2 When competition became intense, the relative growth rate of target plants was primarily determined by the size of their neighbours. There was a negative linear relationship between the relative growth rate of target plants over an interval and the biomass of their neighbours at the beginning of the interval. The size of the target plant itself did not make a significant additional contribution to predicting its growth rate. There is a limit on the growth in biomass of the population (target + neighbours), and growth of individuals occurs within this constraint. Local biomass density, which can be primarily determined by neighbouring individuals, can be much more important for an individual's growth than its own size. 3 There was no evidence of size-asymmetric competition. The size of neighbours was the primary determinant of a target plant's relative growth rate, but the effect of a given amount of neighbour biomass was the same for neighbours larger and those smaller than the target plant.

Differential genetic influences on competitive effect and response in Arabidopsis thaliana

Summary Competition plays an important role in structuring populations and communities, but our understanding of the genetic basis of competitive ability is poor. This is further complicated by the fact that plants can express both competitive effect (target plant influence upon neighbour growth) and competitive response (target plant growth as a function of a neighbour) abilities, with these ecological characteristics potentially being independent. Using the model plant species Arabidopsis thaliana, we investigated patterns of intraspecific variation in competitive effect and response abilities and their relationships to other plant traits and resource supply rates. Both competitive effect and response were measured for 11 genotypes, including the Columbia ecotype and 10 derived mutant genotypes. Plants were grown alone, with intragenotypic competition, and with intergenotypic competition in a replicated blocked design with high nutrient and low nutrient soil nutrient treatments. We quantified competitive effect and response on absolute and per‐gram bases. Competitive effect and response varied among genotypes, with the relative competitive abilities of genotypes consistent across fertilization treatments. Overall, high rates of fertilization increased competitive effect and competitive response abilities of all genotypes. Both competitive effect and response were correlated with neighbour biomass, though genotype‐specific traits also influenced competitive response. At the genotype level, there was no correlation between competitive effect and response in either fertilization treatment. Overall patterns in competitive response appeared consistent among inter‐ and intragenotypic competition treatments, indicating that a target genotype's response to competition was not driven by the genetic identity of the competitor. These findings indicate that within A. thaliana, there is the potential for differential selection on competitive effect and response abilities, and that such selection may influence different sets of plant traits. The concept of a single competitive ability for a given plant is not supported by these data, and we suggest continued recognition of these dual competitive abilities is essential to understanding the potential role of competition in influencing intra‐ and interspecific processes.

Invasion in grassland gaps: the role of neighbourhood richness, light availability and species complementarity during two successive years

Summary We examined how the performance of three grass species, inserted as invaders in the gaps of synthesized communities, was affected by percentage light transmittance in these gaps and by the species richness and productivity of the surrounding neighbour plants. We also investigated whether these influences changed through time. In both the first and the second season following invader establishment, realized growth (invader leaf length) was positively related to percentage light transmittance (which was the best predictor for invasibility), and negatively related to neighbour biomass, in all species examined. The richness–invasibility relationship became significant only in year 2, when increasing neighbourhood richness enhanced complementarity (demonstrated by means of Imax, an index for assessing the degree of complementary resource use), which in turn negatively affected invader leaf length. In the first year such a relationship had not yet developed, probably because the communities were still young and plant interactions were still limited. However, even in year 2 the richness effect saturated at low richness, with no mixture over‐yielding the best two‐species mixture. The data suggest that moderately productive, species‐rich communities, preferably with some broad‐leaved species, and with a nutrient input as low as possible, will offer the best defence against invasions.