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Theory suggests that eutrophication impacts plant community biodiversity by constraining niche differences and increasing competitive inequalities among species, leading to the exclusion of weaker competitors. However, explicit tests of nutrient effects on the strength and direction of inter‐ and intraspecific plant–plant interactions that dictate coexistence are lacking, especially in complex field settings where multiple processes can simultaneously affect plant growth and reproduction.We conducted a field experiment in southeastern Vancouver Island, Canada, using four annual grassland plant species to test how species interactions responded to eutrophication and the covariation of herbivore pressure and neighbour species light‐ and moisture‐use.We found that focal species reproduction was limited by nutrient availability and herbivory, such that the nature of species interactions was context‐dependent. Competitive interactions failed to predict species persistence under these conditions. Instead, facilitation was critical—phytometers grown without neighbours consistently failed to produce seed under herbivore pressure meaning that fecundity significantly increased with neighbour density.Synthesis. Our work demonstrates the importance of species interactions in plant community responses to eutrophication but emphasizes that 'indirect' drivers of plant performance may influence or overwhelm nutrient effects on plant–plant competition. The widespread occurrence of mortality overcome primarily by facilitation highlights the importance of positive density dependence. Overall, our results suggest that knowledge of resource niche overlap may be insufficient to explain plant community responses to eutrophication. This highlights the necessity of considering the broader environmental context when leveraging ecological theory to understand global change effects in empirical settings. [ABSTRACT FROM AUTHOR]

Both tree size and life history variation drive forest structure and dynamics, but little is known about how life history frequency changes with size. We used a scaling framework to quantify ontogenetic size variation and assessed patterns of abundance, richness, productivity and light interception across life history strategies from >114,000 trees in a primary, neotropical forest. We classified trees along two life history axes: a fast–slow axis characterized by a growth–survival trade‐off, and a stature–recruitment axis with tall, long‐lived pioneers at one end and short, short‐lived recruiters at the other.Relative abundance, richness, productivity and light interception follow an approximate power law, systematically shifting over an order of magnitude with tree size. Slow saplings dominate the understorey, but slow trees decline to parity with rapidly growing fast and long‐lived pioneer species in the canopy.Like the community as a whole, slow species are the closest to obeying the energy equivalence rule (EER)—with equal productivity per size class—but other life histories strongly increase productivity with tree size. Productivity is fuelled by resources, and the scaling of light interception corresponds to the scaling of productivity across life history strategies, with slow and all species near solar energy equivalence. This points towards a resource‐use corollary to the EER: the resource equivalence rule.Fitness trade‐offs associated with tree size and life history may promote coexistence in tropical forests by limiting niche overlap and reducing fitness differences.Synthesis. Tree life history strategies describe the different ways trees grow, survive and recruit in the understorey. We show that the proportion of trees with a pioneer life history strategy increases steadily with tree size, as pioneers become relatively more abundant, productive, diverse and capture more resources towards the canopy. Fitness trade‐offs associated with size and life history strategy offer a mechanism for coexistence in tropical forests. [ABSTRACT FROM AUTHOR]

Research on facilitation has not had the advantage of being explicitly underpinned by mathematical population dynamic theory. This has limited understanding of the role of facilitation on ecological dynamics and the integration of facilitation into general theory.I show that facilitative interactions can be classified into four categories likely to be dynamically similar to interaction types for which population‐dynamic theory does exist. This classification provides a useful population‐dynamic foundation for facilitation studies and reveals several fundamental questions unanswered by existing empirical work.While facilitation has typically been excluded from studies of 'competitive' coexistence, I also describe how coexistence theory does not always default to the assumption that species only compete; recent developments in coexistence theory fully accommodate the potentially important influence of facilitation.Simple models of multispecies population dynamics emphasize the importance of reciprocal interspecific interactions, intraspecific interactions, and density dependence for understanding the outcome of facilitative interactions. I point to approaches for aligning future empirical studies of facilitation with these features.Synthesis: Closely aligning at least some future empirical work on facilitation with existing mathematical population dynamic theory will further improve understanding of the effects of facilitation on ecological dynamics. [ABSTRACT FROM AUTHOR]

Interactions between plants and soil microbes can influence plant population dynamics and diversity in plant communities. Traditional theoretical paradigms view the microbial community as a black box with net effects described by phenomenological models.This approach struggles to quantify the importance of plant–microbe interactions relative to other competition and coexistence mechanisms and to explain context dependence in microbe effects.We argue that a mechanistic framework focused on microbial functional groups will lead to conceptual and empirical advances, as demonstrated by extending resource ratio theory to plant–microbe interactions. We review the diverse pathways by which different microbial functional groups can influence plant resource competition. Finally, we suggest approaches to link theory with observations to measure the key parameters of our framework.Synthesis: Our review highlights recent experimental advancements for uncovering microbial mechanisms that alter plant host resource competition and coexistence. We synthesize these mechanisms into a conceptual model that provides a framework for future experiments to investigate the importance of plant–microbe interactions in structuring plant populations and communities. [ABSTRACT FROM AUTHOR]

Plant nutrient acquisition strategies range along a spectrum from autonomous foraging to investment in cooperative foraging through mycorrhizal associations. However, in temperate ecosystems, many plant species encounter contrasted levels of symbiont availability in open fields versus closed forests. Little is known about how fungal partner availability may be associated with intraspecific variation in other root foraging traits in natural settings.Here, we addressed this issue by sampling saplings from two tree species: the arbuscular mycorrhizal (AM) Acer rubrum and the ectomycorrhizal (ECM) Quercus rubra from open fields (AM dominated) and adjacent forest plots (ECM dominated). For each species and environment, we measured morphological, architectural and symbiotic root traits.For the open field, Quercus had greater specific root length (SRL) while Acer had higher AM colonization and root diameter. In the closed forest, the opposite pattern was observed, namely Quercus had higher ECM colonization and Acer greater SRL.Both species showed evidence of a shift towards autonomous root foraging in the habitat with low expected symbiont abundance (open field for Quercus and forest for Acer). Although the confounding effects of site abiotic properties could not be strictly controlled in this study, these results suggest that plants might adjust root foraging traits according to local habitat conditions.Synthesis: Our results shed new light on the intraspecific variation in plant position along the so‐called 'collaboration gradient', and suggest that mycorrhizal symbiont availability, along with other factors such as competition and site properties, may contribute to this variation. [ABSTRACT FROM AUTHOR]

Despite asymmetric competition and a wide array of functional similarities, two ecologically important C4 perennial grasses, Andropogon gerardii and Sorghastrum nutans, frequently codominate areas of the mesic tallgrass prairie of the US Great Plains. A subtle difference in their vegetative reproduction strategies may play a role in preventing the exclusion of S. nutans, the presumed weaker competitor in such regions.While A. gerardii vegetative tiller densities peak in the early growing season and decline thereafter (determinate recruitment), those of S. nutans may continue to increase throughout the growing season (indeterminate recruitment), providing a potential avenue for recovery from more intensive early season competition. However, until now these patterns have only been informally observed in the field.We examined the year‐to‐year consistency of growing season vegetative tiller dynamics (measured as seasonal change in tiller densities) of each grass species from an intact tallgrass prairie in Kansas – a site within the core of both species' distributions – over a period of 8 years. Then, to investigate environmental effects on these dynamics, we examined whether they differ across a Kansas landscape varying in topography, fire management regimes, and the abundances of the study species. Finally, we expanded the investigation of environmental effects on growing season tiller dynamics by observing them at the periphery of the species' distributions in central Colorado, where climatic conditions are dryer and the study species' abundances are reduced.Synthesis. We found that the tiller densities of A. gerardii decline within seasons with striking consistency regardless of spatio‐temporal scale or environmental factors (topography and fire regimes). In contrast, we found the seasonal dynamics of S. nutans tiller densities were dependent on environmental factors, with seasonal tiller density increases occurring only within the Kansas populations but not consistent between years. These observations lay the groundwork for establishing differences in tiller recruitment determinacy as a potentially important yet underappreciated mechanism for promoting coexistence and codominance among perennial plant species. [ABSTRACT FROM AUTHOR]

Mycorrhizal fungi with different nutrient acquisition strategies influence plant species performance and physiology, thereby defining their trophic niche. This might drive resource competition dynamics that cumulatively impact tree species coexistence, but few manipulative experiments have directly tested this.Combining surveys and experiments in a modern coexistence theory framework, we tested how variation in mycorrhizal strategies and nutrient conditions affects plant competitive outcomes. We focused on two genera of co‐occurring tree species with different mycorrhizal states: Acer (arbuscular mycorrhizal, AM) and Populus (dual mycorrhizal, but often considered predominantly ectomycorrhizal, EM).The EM and AM fungal responsiveness in Populus species varied with latitude and nitrogen (N) limitation. Host‐specific soil microbiome conditioning and inorganic N fertilization combined to qualitatively affect coexistence outcomes. Lower N conditions favoured Populus over Acer trees, and N fertilization reversed this outcome for southern species, aligning with regional‐scale forest mycorrhizal transitions.Results from the coexistence experiment also predict competitive exclusion between the tree species pairs, which could arise, in part, from their mycorrhizal differences and is consistent with alternative stable states in dominant forest mycorrhizal strategies. Such bistability appears in natural systems as a bimodal distribution of Populus vs. Acer tree species dominance using long‐term forest inventory data.Synthesis: The magnitude and outcome of microbially mediated competition between Populus and Acer depends on soil nutrient availability, which likely relates to their mycorrhizal differentiation. These findings support the importance of mycorrhizal symbioses for contributing to large‐scale biogeographical patterns of tree species trophic niche separation across soil resource gradients and bistability in forest mycorrhizal structure. [ABSTRACT FROM AUTHOR]

Understanding the drivers of species coexistence is essential in ecology. Niche and fitness differences (i.e. how species limit themselves compared to others and species' differences in competitive ability, respectively) permit studying the consequences of species interactions. Yet, the multitude of methods to compute niche and fitness differences hampers cross‐community comparisons. Such shortcoming leaves a gap in our understanding of the natural drivers of species coexistence and whether niche or/and fitness differences capture them.Here, we standardised niche and fitness differences across 953 species pairs to investigate species coexistence across ecological groups and methodological settings (experimental setup, natural co‐occurrence, population model used and growth method). Using data gathered from 29 empirical papers, we asked whether large niche differences, small fitness differences or both explain predicted coexistence. Moreover, we performed an automated clustering algorithm to understand whether different underlying mechanisms drive species interactions. Finally, we tested whether any ecological or/and methodological settings drive these clusters.Species pairs predicted to coexist have larger niche differences but not smaller fitness differences than species pairs predicted not to coexist. Also, species pairs group into two clear clusters along the niche difference axis: those predicted to coexist and those that are not. Surprisingly, ecological or methodological settings do not drive these clusters.Synthesis. Overall, our results show that species coexistence is mainly influenced by mechanisms acting on niche differences, highlighting the importance of sustaining mechanisms that promote niche differences to maintain species coexistence. In addition, our results provide evidence that communities predicted to coexist differ from those that are not in ways that transcend their ecological grouping. [ABSTRACT FROM AUTHOR]

Water is the most limiting resource for plant survival and growth in arid environments, but the diversity of water‐use strategies among coexisting species in dryland communities is not well understood. There is also growing interest in assessing whether a whole‐plant coordination exists between traits related to water‐use and the leaf economic spectrum (LES).We used water stable isotopes (δ2H, δ18O) to quantify water uptake proportions from different soil depths by 24 species in a Mediterranean shrubland. Leaf traits associated with water‐use efficiency, stomatal regulation (δ13C, δ18O) and the LES (SLA, N, P, K concentrations) were also measured. We assessed potential trade‐offs between the above‐mentioned leaf traits, water uptake depth and their relationship with species abundance.We found distinct ecohydrological niche segregation among coexisting species. Bayesian models showed that our shrubland species used a median of 37% of shallow soil water (0–30 cm) and 63% of deep water (30–100 cm). Still, water source proportions varied considerably among species, as shallow soil water‐use ranged from a minimum of 6.4% to a maximum of 68%. Interspecific variability in foliar carbon investment (SLA) and nutrient concentrations was remarkably high, indicating diverse nutrient‐use strategies along the LES. Leaf δ18O, δ13C and δ15N values also differed widely among species, revealing differences in stomatal regulation, water‐use efficiency and nitrogen acquisition mechanisms. After accounting for evolutionary history effects, water uptake depth was coordinated with the LES: species using shallower soil water from fertile topsoil layers exhibited a more acquisitive carbon‐ and nutrient‐use strategy, whereas water uptake from deeper but less fertile soil layers was linked to a more conservative nutrient‐use strategy. Leaf‐level water‐use traits significantly influenced species abundance, as water‐savers with tight stomatal regulation and high water‐use efficiency were dominant.Synthesis. Greater utilisation of water stored in nutrient‐rich topsoil layers favoured a more acquisitive nutrient‐use strategy, whereas a deeper water uptake pattern appeared to constrain access to nutrients. Our findings thus suggest a largely inescapable trade‐off and coordination between soil water uptake depth and carbon‐ and nutrient‐use strategies in low‐fertility drylands. [ABSTRACT FROM AUTHOR]

Competition, niche differences and chance all contribute to community assembly; yet, the role of reproductive interactions between species is often less appreciated. Closely related plant species that share floral form, phenology and habitat often interact through pollination. They potentially facilitate pollinator attraction, compete for pollination services and/or exchange pollen. If reproductive processes are important to co‐occurrence, we predicted that fitness costs of heterospecific pollen transfer or pollen limitation should result in lower rates of co‐occurrence among outcrossing congeners. In contrast, selfers, which may be less exposed to heterospecific pollen, and/or less negatively affected by it, should co‐occur more frequently.Flower size is an excellent proxy for mating system in clovers. Using herbarium records and three independent field datasets, we documented co‐occurrence patterns of Trifolium at 1 m2–1 km2 scales in California. Using a randomization procedure to reshuffle matrices of community membership, we generated null hypotheses for the expected composition of large‐ and small‐flowered species in Trifolium communities of different sizes.Across all spatial scales, large‐flowered outcrossers were over‐represented at sites lacking congeners, but under‐represented in communities with multiple congeners. Conversely, small‐flowered selfers often occupied sites with multiple other Trifolium species. Patterns for plant height and leaf size, which are weakly or strongly correlated with flower size, did not explain co‐occurrence patterns as robustly. Regression analysis and model selection corroborated the null model analyses, indicating that the likelihood of co‐occurrence decreased as flower size, and thus reliance on outcrossing, increased.Synthesis. This study suggests that reproductive traits and processes may be significant contributors to community assembly and co‐occurrence in flowering plants. [ABSTRACT FROM AUTHOR]

Carmona et al. (2019) have recently published the results of a well‐designed experiment examining the effect of functional traits on the outcome of competition. Their main conclusions were that (a) trait hierarchies better predict the strength of competition than trait differences, and (b) the consideration of intraspecific trait variation improves our ability to predict the strength of competition.They applied multi‐model inference by averaging regression coefficients. This approach is strongly criticized because it averages regression coefficients for which the meaning is conditional on the other predictors included in the model.In this paper, their data were re‐analysed using alternative methods. The results only partly support the original conclusions in that trait hierarchy proved to be obviously important in height, but not in the other three traits.Synthesis. Intraspecific trait variation contributes to a better understanding of the relationship between traits and competitive interactions. On the other hand, traits measured in focal individuals may be both drivers and consequences of competition, which makes the interpretation of the relationship more difficult. [ABSTRACT FROM AUTHOR]

Trait diversity is traditionally seen as promoting species richness and ecosystem function. Species with dissimilar traits would partition available resources, increasing niche differences, facilitating coexistence and increasing ecosystem function.Here we first show, using theory and simulations for light‐limited phytoplankton, that combing photosynthetic pigments is indeed a necessary condition for coexistence and stimulates ecosystem function.However, pigment richness does mostly not permit the coexistence of more than two species, and increases productivity at most 40% compared to single‐pigment communities. That is because blending in more pigments leads to coexistence of species with many pigments and therefore flat absorption spectra, which equalizes their fitness but decreases their niche differences. Similarly, seeding species with more variable size leads to an excess of large‐celled species, which does not only decrease fitness differences but also niche differences.Empirical data and additional simulations suggest that pigment richness effects can be stronger during transient dynamics but inevitably weaken with time, that is, pigment richness effects on species richness and function are likely short‐lived.Synthesis. Our results highlight the need to apply coexistence theory to understand the long‐term effects of trait diversity on biodiversity and ecosystem function. [ABSTRACT FROM AUTHOR]

Life‐history trade‐offs are important coexistence mechanisms in plant communities. In particular, a trade‐off between seed quality and seed output may explain coexistence among species with a wide variety of seed sizes in heterogeneous landscapes with varying degrees of local stresses such as shade, drought and browsing.Under spatially implicit formulations of the tolerance‐fecundity trade‐off, species diversity is only bounded by community size and environmental heterogeneity. However, these formulations leave unanswered the question of how dispersal limitation, an important aspect of real communities, impacts the ability of the tolerance‐fecundity trade‐off to support coexistence.Here we show using stochastic cellular automata that coexistence under the tolerance‐fecundity trade‐off in a spatially structured landscape is strongly modulated by the interactions between the spatial scales of species dispersal and environmental variability. Specifically, coexistence in a patchy landscape is higher under short dispersal scales relative to the scale of environmental variation. These conditions allow species to segregate spatially and thus avoid interspecific competition.In addition to dispersal limitation, coexistence under the tolerance‐fecundity trade‐off is promoted by a sharp drop in tolerance as local environmental stress increases, a high overall seed output across species, local homogeneity in environmental conditions and weak age structure in fertility. In particular, coexistence is much higher when species have full tolerance to stress levels below a threshold and zero tolerance above that threshold, with nothing in between. This steep gradient contributes to coexistence because it maximizes niche separation between similar species.Synthesis. When plants compete for space, dispersal limitation and landscape spatial structure can conspire to boost coexistence and diversity. Our results show that the tolerance‐fecundity trade‐off is a strong niche differentiation mechanism when species experience a well‐defined distinction between suitable and unsuitable environments, and supports higher diversity when dispersal scales are shorter than the scales of variation in the environment. We conclude that the trade‐off is more likely to be a relevant coexistence mechanism in the context of large‐scale environmental variation, such as topography, than small‐scale variation, such as in litter cover and canopy shading. [ABSTRACT FROM AUTHOR]

Despite theoretical advances, the ecological factors and functional traits that enable species varying in seed size and fecundity to coexist remain unclear. Given inherent fecundity advantages, why don't small‐seeded species dominate communities?In perennial grasslands, we evaluated whether small‐seeded species are less tolerant of competition from the community dominant bunchgrass than large‐seeded species, but also less vulnerable to seed predation by mice. We also explored whether trade‐offs involving competitive tolerance include two other functional traits, height and leaf mass per area (LMA). We added seeds of 17 forb species to plots where bunchgrass competition and rodent seed predation were manipulated across sites varying in bunchgrass productivity and thus competitive intensity. Seeds were added at densities mimicking interspecific variation in fecundity among target species.Standardizing for differences in fecundity (i.e. seed input, which enabled us to evaluate inherent interspecific differences in susceptibility to biotic interactions), bunchgrass competition more greatly reduced recruitment and establishment of small‐ versus large‐seeded species, whereas rodent seed predation more greatly reduced the recruitment of large‐ versus small‐seeded species. Plant height and LMA were unrelated to the competition effect size.Small‐seeded species abundance decreased across sites increasing in bunchgrass productivity, whereas this was not the case for large‐seeded species. For adult plants but not seedlings, community‐weighted functional trait means (CWM) for seed size, height and LMA increased in plots with versus without bunchgrass competition and the CWM for seed size and height also increased at sites with greater bunchgrass productivity (for adults only). In contrast, rodent seed predation had no significant effects on CWM seed size.At the end of the experiment, adult abundance positively correlated with plant fecundity in plots lacking bunchgrass, indicating the inherent advantages accrued to high fecundity small‐seeded species. However, with bunchgrass competition, abundances were equalized across species due to reduced competitive tolerance of high fecundity small‐seeded species.Synthesis. Our results suggest that coexistence among subordinate forb species varying in seed size and fecundity is in‐part due to a trade‐off involving competitive tolerance and fecundity, mediated by seed size and associated functional traits. [ABSTRACT FROM AUTHOR]

Intraspecific diversity can affect the ecological function of communities. Because community function is the outcome of species interactions, understanding how intraspecific diversity affects species interactions may shed light on the mechanism by which intraspecific diversity affects communities. Competition among and within plant species may be particularly important, as the relative strengths of these interactions can influence species coexistence and community stability.We established a field experiment in a New England salt marsh to determine how plant intraspecific diversity affects both intra‐ and interspecific competition. Focal plants of two species (Spartina alterniflora and Spartina patens) were planted with S. alterniflora neighbours that were either genotypically identical to one another (monoculture) or genotypically distinct (polyculture).After two growing seasons, S. alterniflora performance was enhanced in the presence of polyculture conspecific neighbours compared to monoculture neighbours, whereas S. patens was not strongly affected by diversity of its S. alterniflora neighbours. This suggests that intraspecific competition was weakened by intraspecific diversity, potentially even shifting to facilitation, but we did not find strong evidence of intraspecific diversity impacting interspecific competition. By weakening competition within species, intraspecific diversity may promote coexistence of genotypes within that species.Synthesis. The community‐level effects of intraspecific diversity may be mediated in part by its effect on intraspecific competition/facilitation. Furthermore, stronger intraspecific competition and decreased coexistence resulting from reduced diversity may exacerbate ongoing declines in biodiversity in response to global change. [ABSTRACT FROM AUTHOR]

The persistence of phytoplanktonic diversity in spite of competition for basic resources has long been a source of wonder and inspiration to ecologists. To sort out, among the many coexistence mechanisms suggested by theory and experiments, which ones actually maintain diversity in natural ecosystems, long‐term field studies are paramount.We analysed a large dataset of phytoplankton abundance time series using dynamic, multivariate autoregressive (MAR) models. Phytoplankton was counted and identified down to the genus level, every 2 weeks over 20 years, at 10 sites along the French coastline. MAR models allowed to estimate biotic interaction networks while also accounting for abiotic variables that may drive part of the phytoplankton fluctuations. We then analysed the ratio of intra to intertaxa interactions (a measure of niche differentiation), the frequency of negative versus positive interactions, and how stability metrics (both at the network and genus level) relate to network complexity and genus self‐regulation or abundance.We showed that a strong self‐regulation, with competition strength within a taxon (genus) an order of magnitude higher than between taxa, was present in all phytoplanktonic interaction networks. This much stronger intragenus competition suggests that niche differentiation—rather than neutrality—is commonplace in phytoplankton. Furthermore, interaction networks were dominated by positive net effects between phytoplanktonic taxa (on average, more than 50% of interactions were positive). While network stability (sensu resilience) was unrelated to complexity measures, we unveiled links between self‐regulation, intergenera interaction strengths and abundance. The less common taxa tend to be more strongly self‐regulated and can therefore maintain in spite of competition with more abundant ones.Synthesis. We demonstrate that strong niche differentiation, widespread facilitation between phytoplanktonic taxa and stabilizing covariances between interaction strengths should be common features of coexisting phytoplankton communities in the field. These are structural properties that we can expect to emerge from plausible mechanistic models of phytoplankton communities. We discuss mechanisms, such as predation or restricted microscale movement, that are consistent with these findings, which paves the way for further research. [ABSTRACT FROM AUTHOR]

A major challenge in ecological research is to identify the tolerance of ecological communities to external perturbations. Modern coexistence theory (MCT) has been widely adopted as a framework to investigate the tolerance to perturbations in relative reductions of per capita growth rates, often using metrics that explicitly eliminate the independent role of intrinsic growth rates. More recently, the structural approach (SA) was introduced to investigate the tolerance of communities to perturbations in intrinsic growth rates as a function of the strength of intraspecific and interspecific competition. Because the external perturbations are likely to happen in both intrinsic growth rates and competition strengths, no framework alone can fully disentangle the effects of external perturbations.Here we combine MCT and SA to disentangle the tolerance in coexistence and priority effects of a pair of competing species when subject to perturbations in intrinsic growth rates and competition strengths. Through this combination, we reveal the emergence of a key trade‐off: increasing the tolerance to perturbations in intrinsic growth rates typically decreases the tolerance in competition strengths, and vice versa. Furthermore, this trade‐off is stronger under coexistence than under priority effects.We test this combined framework on competing pairs of 18 California annual plant species. For both coexistence and priority effects, we find that the tolerance to perturbations in intrinsic growth rates is maximized instead of that to perturbations in competition strengths in the studied annual plant communities.Synthesis. Our combined framework of modern coexistence theory and structural approach illustrates that it is possible to disentangle the impact of different external perturbations on the persistence of species. Importantly, our findings show that species interactions may reveal whether communities are dominated either by changes in intrinsic growth rates or by competition strengths. Overall, this combined framework can open a new perspective to understand and predict the response of populations to changing environmental conditions. [ABSTRACT FROM AUTHOR]

Understanding the mechanisms that underlie species assembly is a central concern in community ecology. Abiotic and biotic filters are probabilistic 'sieves' that allow species with certain functional traits to become a part of the community, or not. We manipulated natural plant assemblies in order to identify variations in the timings of biotic and abiotic filters that determine community trait assemblies.We extracted soil portions when the investigated annual plant community was in its seed phase ('community monolith'), thereby maintaining the structure and similar soil characteristics to the field conditions. Community monoliths were subjected to experimental manipulation in terms of the rainfall timing and amount, and perturbations of the biological soil crust (BSC; intact vs. perturbed). We surveyed the experimental community assembly over time based on the functional diversity by considering important functional traits in different life stages.We found that autumn droughts acted as abiotic filters by favouring the germination and establishment of species with greater investment in the root biomass. Under severe droughts (66% water reduction), the experimental assemblies were dominated by species with functional traits adapted to water shortage conditions: high leaf dry matter content, low specific leaf area, small individual size, low reproductive ratio and high root:shoot ratio. We identified two roles of BSCs in annual plant species assemblies: (a) as a biotic filter that limited the establishment of species based on seed size, and (b) as a buffer against water stress conditions by reducing soil evapotranspiration.Synthesis. We demonstrated the importance of the timing and amount of rainfall for shaping annual plant communities, and identified germination filters as the main process that determined community assemblies. Our results suggest that the phenotypic integration of functional traits facilitates resistance to drought during the life cycle. The BSC–annual plant relationship shifted from negative, by acting as a germination filter, to positive, by acting as a buffer in later stages. Climatic fluctuations and fine scale biotic determinants of spatial heterogeneity emerged as sources of changes in the community assembly in time and space to possibly promote species coexistence and trait differences among the communities studied. [ABSTRACT FROM AUTHOR]

Both intra‐ and interspecific differences in traits may modulate interactions between plants. Two mechanisms are hypothesized to regulate these effects: competitive hierarchies and trait dissimilarities, but it is unclear how the prevalence of each might depend on environmental conditions and on intra and interspecific differences.We sowed six replicates of all possible pairwise combinations across eight annual species (including conspecific competition and individuals without competitors), in pots under two levels of fertilization. We measured above‐ and below‐ground traits and total biomass of the focal individuals. We expressed competition as the decrease in biomass of focal individuals relative to biomass without competition, and modelled competition using directional (hierarchy) or non‐directional (dissimilarity) trait differences, with or without intraspecific variability (ITV).We found evidence of different trait hierarchies operating under different fertilization conditions but little support for trait dissimilarities. The negative effect of competition on focal plants increased with the height of the competitors in both of fertilization levels. Further, in unfertilized conditions, plants with lower specific leaf area (SLA) and larger root systems experienced less competition. Including ITV in trait hierarchies substantially improved our ability to predict the intensity of competition. This was partly due to ITV reducing competitive hierarchies.Synthesis. Our results underscore the importance of traits in driving interactions among plants. Competitive relationships between species depend on complex interactions between trait intra and interspecific differences and resource availability. ITV appears to be a mechanism capable of reducing trait hierarchies, and hence the intensity of competition between coexisting plants. [ABSTRACT FROM AUTHOR]

The number of co‐flowering species, floral density and floral trait diversity can be major determinants of pollinator‐mediated plant–plant interactions in a community. However, evaluation of how each one of these co‐flowering components affects the pollination success of a single focal plant species, and how these effects vary at different spatial scales, is lacking.Here, we evaluated the effects of functional diversity (flower morphology and colour), taxonomic diversity (reflecting potential sampling effects) and flower density (conspecific and heterospecific), on the pollinator environment (i.e. visitation rate and pollinator diversity) and pollination success (i.e. pollen load size and number of pollen tubes per style) of Cakile edentula (Brassicaceae). We applied structural equation models (SEMs) at the floral‐neighbourhood (plot level) and community‐wide scales to uncover the factors that mediate co‐flowering community effects on C. edentula pollination success.We found that co‐flowering community effects at the community level are more important than fine‐scale floral‐neighbourhood differences in mediating plant pollination success in our study species. Increasing plant functional diversity decreased pollinator visitation rate but increased the diversity of pollinator functional groups visiting C. edentula flowers. Taxonomic diversity negatively affected pollinator diversity suggesting that other unmeasured floral traits may be relevant or that single‐species effects (sampling effects) may be important. Overall, our results suggest that functional floral trait diversity in a community may be the most important factor influencing pollination success of species in a community. We also found evidence for intra‐ and interspecific pollinator competition mediated by flower density, but none of these effects seemed to have a significant impact on pollination success.This study is an important step towards understanding the complexity of co‐flowering community effects on the pollination success of individual plant species at multiple spatial scales. This study further reveals the potential importance of plant functional diversity in a community in helping predict competitive and facilitative interactions in co‐flowering communities.Synthesis. Floral density and taxonomic and functional co‐flowering diversity are important drivers of pollination success in flowering plants. The effects of the co‐flowering diversity on the pollination success of plant species can largely depend on the spatial scale being studied. Only evaluating the outcomes of pollinator‐mediated plant–plant interactions at multiple stages of the pollination process can lead to a complete understanding of their ecological consequences in nature. [ABSTRACT FROM AUTHOR]

Niche availability, niche overlap and competitive ability are key determinants of the distribution and abundance of species. However, quantifying each of these components is difficult because it is not always possible to identify or measure relevant environmental gradients (niche axes) along which species might partition or compete for niche space.We describe a method that uses seed addition experiments to quantify the number of 'safe‐sites' (microsites suitable for a species to recruit from seed) at a location and show how this method can be used to quantify niche availability, niche overlap and competitive ability. We illustrate our approach using two seed addition experiments in grassland.In the first experiment, we added seeds of one native and two exotic grass species, alone and in mixture, to plots that were arrayed along a gradient of soil moisture availability. We show that the three species partitioned safe‐sites, implying that all three species could locally co‐occur through niche partitioning, in part due to different responses to moisture availability.In the second experiment, we added seeds of three commonly co‐occurring native grass species, alone and in mixture, to plots with no obvious environmental gradients. One species out‐competed two others for site occupancy, allowing us to quantify both the degree of niche overlap and the relative ability of each species to compete for safe‐site occupancy without a priori knowledge of the niche axes. Results from both experiments demonstrate the fine‐scales at which species can partition niches to facilitate co‐occurrence.Synthesis. By conceptualising a plot of ground as containing a limited number of microsites that are safe for the recruitment of a given species, and using seed addition experiments to measure both the number of safe‐sites and degree of safe‐site overlap among species, we show how niche availability, niche overlap and competitive ability can be quantified at fine‐scales without a priori knowledge of niche axes. Our approach allows questions about niche availability and competition for shared niche space to be empirically tested, and to examine how these processes vary along environmental gradients to shape species distributions and patterns of co‐occurrence. [ABSTRACT FROM AUTHOR]

One of the central questions in ecology is why some species are abundant while others are rare. In plant communities, some studies show that rare plant species are rare because they suffer stronger negative density‐dependent effects from pathogens compared to abundant plant species. Moreover, such pathogen effects are also suggested to be related to phylogenetic distance among plant species. However, the importance of pathogens has been commonly inferred by treating the entire pathogen community as a "black box" without explicitly characterizing pathogen richness and community composition.Here, we test two predictions. First, if rare plant species are more susceptible to pathogens, we predict that rare plant species are associated with a higher richness of specialists (i.e., pathogens that attack only a single plant species) and/or the total pathogen community. If phylogenetically distinct plant species are less susceptible to pathogens, we predict that plant species with higher phylogenetic distinctiveness (i.e., a measure of how phylogenetically distant a species is from other co‐occurring species) are associated with a lower richness of phylogenetic specialists (i.e., pathogens that attack closely related plant species) and/or the total pathogen community.We conducted a survey of the root‐associated pathogen communities from 45 plant species in a subtropical forest. We showed that approximately 40% and 25% of the pathogens were specialists and phylogenetic specialists respectively. In contrast to our first prediction, the richness of the total pathogen community but not the richness of the specialists was found to be positively related to plant species abundance, indicating that rare plant species suffer less from pathogens. Consistent with our second prediction, both the richness of the phylogenetic specialists and the total pathogen community were found to be negatively related to plant species phylogenetic distinctiveness. Furthermore, these correlations were stronger at the earlier plant life stages examined.Synthesis. We found that the root‐associated pathogen communities were less diverse in rare plant species and plant species with few close relatives. These associations varied across multiple plant life stages, suggesting that the strength of the above‐ground–below‐ground interactions change dynamically across plant life span. [ABSTRACT FROM AUTHOR]

Climate change is predicted to have profound consequences for multispecies coexistence, and thus, patterns of biological diversity. These consequences will be mediated by direct and indirect impacts of environmental change on species' vital rates and interactions. While the impacts of environmental change on individual species has received much attention to date, the consequences for coexistence mediated by changes in the strength and direction of multispecies interactions are not as well understood.To investigate how coexistence dynamics may be sensitive to environmental change, we conducted a field experiment in a diverse semi‐arid annual plant system. We imposed a water manipulation treatment in two sites that vary in aridity and associated rainfall. Focusing on four common annual plant species in these sites, we quantified the fecundity (seed production) of individuals in response to a gradient of intra‐ and interspecific competitor densities and aridity. We then used these fecundities to parameterize an annual plant population model and examine the influence of aridity and species identity on resultant coexistence dynamics (as a function of stabilizing niche differences and fitness inequalities).While the responses of some vital rates and competitive impacts to watering varied somewhat predictably across sites, coexistence metrics encapsulating changes in these vital rates and interaction strengths did not. Fitness inequalities among our focal species were driven largely by differences in sensitivity to competition, which were almost always much greater than the magnitude of stabilizing niche differences. These findings were surprising given observational evidence suggesting that these species do coexist at local scales in these natural communities.Synthesis. Our study is one of the first to explicitly consider the influence of environmental variation on the individual components of coexistence outcomes. We show that environmental change has the ability to influence coexistence not only through direct pathways (i.e., vital rates), but also indirect pathways (i.e., species interactions). Despite the consistency of many of the responses of these individual components to environmental variation, their combined influence on predictions of both current and future coexistence remains unclear. Our study is among the first to explicitly consider the influence of environmental variation on individual components of coexistence outcomes. We show that environmental change may influence coexistence not only through direct pathways (i.e., vital rates), but also indirect pathways (i.e., species interactions). Despite consistent responses of some components to environmental change, their combined influence on coexistence predictions remains unclear. [ABSTRACT FROM AUTHOR]

Abstract: Coexistence and diversity in plant communities depend upon outcomes of plant competition. Competition and coexistence can be mediated by abiotic soil nutrient differences as well as by soil microbial communities. The latter effects occur through various mechanisms including negative plant–soil feedbacks, when plants foster the build‐up of specialized pathogenic microbes, which ultimately reduce conspecific, but not heterospecific, densities. Microbial mutualists can have generalized associations with host plants, and by associating with multiple species might affect coexistence by conferring different levels of benefit to hosts. We examined the effects of abiotic differences and soil microbial communities, including mutualistic nitrogen‐fixing rhizobial bacteria, on coexistence processes and asked whether these interactions inform patterns of co‐occurrence in natural communities. We measured plant–soil feedbacks in the greenhouse for three native Trifolium species that either highly co‐occurred or were spatially repulsed at our field site. Using size‐fractioned soil microbial inocula prepared from field‐collected soils, we explored the effect of soil microbes on nodulation and the outcome of competition (relative interaction intensity). We also examined the effects of soil origin (home vs. away soil) on the outcome of competition between species. Soil microbes had strong positive effects on plant growth and nodulation. Microbes in general reduced the strength of plant competition relative to competition in sterilized soil and altered the relative strength of interactions with conspecific vs. congeneric neighbours, which often occurred in ways predicted to enhance coexistence. In one pair of highly co‐occurring Trifolium, competition was strong in sterilized soils, but these species facilitated one other in the presence of the full microbial community. The net effect of these microbes in general reduced competition while also increasing performance over that in sterilized soil, suggesting a role for soil microbial mutualists in host coexistence. Synthesis. Our results demonstrate that interactions between plants and diverse soil communities can alter plant–plant interactions and plant–soil feedbacks. They may increase niche differences and equalize fitness differences in ways consistent with observed co‐occurrence in the field. [ABSTRACT FROM AUTHOR]

Abstract: Coexistence between plant species is well known to depend on the outcomes of species interactions within an environmental context. The incorporation of environmental variation into empirical studies of coexistence are rare, however, due to the complex experiments needed to do so and the lack of feasible modelling approaches for determining how environmental factors alter specific coexistence mechanisms. In this article, we present a simple modelling framework for assessing how variation in species interactions across environmental gradients impact on niche overlap and fitness differences, two core determinants of coexistence. We use a novel formulation of an annual plant population dynamics model that allows for competitive and facilitative species interactions and for variation in the strength and direction of these interactions across environmental gradients. Using this framework, we examine outcomes of plant–plant interactions between four commonly co‐occurring annual plant species from Western Australian woodlands. We then determine how niche overlap and fitness differences between these species vary across three environmental gradients previously identified as important for structuring diversity patterns in this system: soil phosphorus, shade and water. We found facilitation to be a widespread phenomenon and that interactions between most species pairs shift between competitive and facilitative across multiple environmental gradients. Environmental conditions also altered the strength, direction and relative variation of both niche overlap and fitness differences in nonlinear and unpredictable ways. Synthesis. We provide a simple framework for incorporating environmental heterogeneity into explorations of coexistence mechanisms. Our findings highlight the importance of the environment in determining the outcome of species interactions and the potential for pairwise coexistence between species. The prevalence of facilitation in our system indicates a need to improve current theoretical frameworks of coexistence to include noncompetitive interactions and ways of translating these effects into explicit predictions of coexistence. Our study also suggests a need for further research into determining which factors result in consistent responses of niche overlap and fitness differences to environmental variation. Such information will improve our ability to predict outcomes of coexistence, invasion events and responses of whole communities to future environmental change. [ABSTRACT FROM AUTHOR]

Abstract: Species are shifting their ranges, for example to higher elevations, in response to climate change. Different plant species and soil microbiota will likely shift their ranges at different rates, giving rise to novel communities of plants and soil organisms. However, the ecological consequences of such novel plant–soil interactions are poorly understood. We experimentally simulated scenarios for novel interactions arising between high‐ and low‐elevation plants and soil biota following asynchronous climate change range shifts, asking to what extent the ability of plants to coexist depends on the origin of the soil biota. In a greenhouse experiment, we grew pairs of low‐ (Poa trivialis and Plantago lanceolata) and high‐ (Poa alpina and Plantago alpina) elevation plant species alone and against a density gradient of con‐ or heterospecific neighbours. Plants grew on sterilized field soil that was inoculated with a soil community sampled from either low or high elevation in the western Swiss Alps. We used the experiment to parameterize competition models, from which we predicted the population‐level outcomes of competition in the presence of the different soil biota. In the absence of neighbours, three of the four species produced more biomass with the low‐elevation soil biota. As a result of generally similar responses across plant species, soil biota tended not to affect plant interaction outcomes, with the low‐elevation species generally predicted to competitively exclude high‐elevation species irrespective of the soil biota origin. However, the low‐elevation grass Poa trivialis was only able to invade communities of Poa alpina in the presence of a low‐elevation soil biota. This suggests that, at least in some cases, the outcome of novel competitive interactions between plants following climate change will depend on whether shifts in the distribution of plants and soil organisms are asynchronous. Synthesis. Our results indicate that the changing soil communities that plants encounter during range expansion can influence plant performance. However, this is only likely to alter expectations for the ability of plants to coexist following climate change if plant species respond differently to the change in the soil community. [ABSTRACT FROM AUTHOR]

Abstract: Propagule size and number often vary by several orders of magnitude among co‐occurring plant species. Explaining the maintenance of this variation and understanding how propagule size contributes to coexistence remain a central challenge for community ecologists. The dominant paradigm is that a competition–colonization trade‐off maintains interspecific variation in seed size, but empirical support is limited and other coexistence mechanisms, such as size‐dependent seed predation, have not been examined. We examined how seed size, fecundity, and other functional traits of 18 co‐occurring perennial forbs trade‐off with both vulnerability to rodent seed predation and competitive response to a community‐dominant perennial bunchgrass. We added seeds of these species to 1 m2 plots at 10 sites where we factorially manipulated rodent seed predation and competition from the community dominant, Festuca campestris. For a given plot, seeds of each species were added at densities that reflected the fecundity (i.e., per capita seed production) for each species. Moreover, we varied total seed density among plots by adding seeds at one of five relative densities (0, 0.25. 0.5, 1.0, and 1.5 times each species’ fecundity) to examine how fecundity affected overall recruitment for each species. There was a trade‐off between seed size and fecundity, as expected, but larger seed size was also associated with greater plant height, lower C/N ratios, and lower water use efficiency, suggesting that seed size represented a “resource acquisitive” trait syndrome (as quantified by trait principal component scores). In the field experiment, the suppressive effects of seed predation on seed recruitment rate increased with increasing seed size. In contrast, smaller seeded species with less resource‐acquisitive traits were more negatively affected by competition from F.  campestris than were species with more resource acquisitive traits. Synthesis. While the competition–colonization trade‐off has been the predominant mechanism thought to maintain coexistence among species that vary in fecundity and seed size, our work suggests that susceptibility to rodent seed predation and competitive response to community dominants represent alternative mechanisms that can differentially influence plant recruitment of species based on their seed size and associated traits. [ABSTRACT FROM AUTHOR]

Abstract: Competition–defence trade‐offs have long been thought to promote plant coexistence and increase species diversity. However, insights from modern coexistence theory clarify that such trade‐offs alone cannot sustain coexistence. Coexistence also requires stabilising niche differences and the ability of competitor populations to persist under consumer pressure. Despite the breadth of potential consequences of competition–defence trade‐offs, we have little mechanistic understanding of how they affect diversity in natural communities. We investigated the effects of seed harvesting by ants on coexistence in an annual plant community. We parameterised a model of plant competitive population dynamics with data from two field experiments: (a) plant demographic rates and competition coefficients determined by growing plants alone and against intra‐ and interspecific competitor density gradients; (b) plant fitness losses to ant consumers determined by measuring seed removal from experimental depots. We tested for a trade‐off between a plant species’ demographic potential and its susceptibility to consumption and then determined the bounds of ant effects on pairwise and multispecies coexistence by comparing model projections with and without seed consumption. Ants harvested seeds of all plant species, but they strongly preferred the competitively superior large seeded species, inducing a competition–defence trade‐off. Unequal seed loss to ants changed competition‐based fitness hierarchies, affecting both the number and identities of plant species pairs that were predicted to coexist compared to a scenario without ants. The trade‐off most often prevented coexistence by severely disadvantaging the superior competitor or excluding species directly through overconsumption, and a simulated reduction in the overall consumption rate opened few additional opportunities for coexistence. Ant exacerbation of average fitness differences was particularly disruptive to multispecies coexistence, where niche differences were insufficient to stabilise the coexistence of plant triplets and quadruplets. Synthesis. Our results show that the presence of a competition–defence trade‐off in a community with stabilising niche differences does not always increase diversity. Instead, the full range of diversity outcomes—positive and negative changes in species number and changes in the identity of the dominant—are possible. Taken together, our results support the emerging paradigm that consumers have wide‐ranging impacts on plant diversity and suggest that variation in consumer pressure may be an important driver of large‐scale diversity patterns. [ABSTRACT FROM AUTHOR]

Abstract: Plant pathogens reduce the performance of their hosts and therefore may contribute to ecological mechanisms of coexistence. In Chesson’s framework, pathogens contribute to stabilizing mechanisms when they intensify negative intraspecific interactions, such as density‐dependent disease. Additionally, pathogens contribute to equalizing mechanisms when they reduce differences in performance among species. Life‐history trade‐offs predict higher susceptibility to pathogens in rapidly growing species, which could equalize performance among fast‐ and slow‐growing species in the presence of pathogens. In a coastal prairie in California, we studied the impact of leaf diseases on the performance of 17 co‐occurring species of Trifolium and Medicago (“clovers”). We transplanted clovers in randomized arrays into the natural prairie community in 3 years of common garden experiments. We quantified infection rates by isolating fungi from leaves, and we measured disease severity as per cent leaf area damaged. In a fungicide experiment, we measured the impact of infection on biomass and survival. We assessed whether disease on transplants was positively related to natural abundance of that species in the surrounding community, which we monitored over 5 years. We assessed life‐history trade‐offs by testing whether more rapidly growing species were more susceptible to pathogens. Rank abundance of clover species was stable over 5 years despite marked environmental fluctuations. Across hosts, fungal infection was not linearly related to density, although transplants of species that were locally absent showed lower and more variable infection. Disease severity was greater for more abundant species, but in only 1 of 3 years, and response to fungicide was not stronger in more abundant species. As predicted by life‐history trade‐offs, faster‐growing species experienced greater fungal infection. However, the impact of that infection was less negative, not more negative, on faster‐growing species. Our results suggest that life‐history trade‐offs in plant–pathogen interactions may influence equalizing mechanisms among species in this guild, but the combined effects of greater infection with greater tolerance may limit rather than promote coexistence. We also found modest evidence that density‐dependent disease may contribute to stabilizing mechanisms. Lack of host specificity, rapid evolution of host use and temporal variation in climatic conditions may all influence the role that pathogens play in coexistence of these closely related plants. [ABSTRACT FROM AUTHOR]

Abstract: Interactions with symbiotic microbes, such as mycorrhizal fungi, have the potential to greatly influence plant growth, but it is unclear whether field variation in symbiont availability is common and, if so, sufficient to influence interspecific plant competition. In a greenhouse experiment using natural field soils, I varied the timing of ectomycorrhizal inoculation and the presence of an arbuscular mycorrhizal plant competitor, and measured their effects on pine seedling growth. I found that ectomycorrhizal colonization was absent in some field soils, and that in soils without mycorrhizal inoculum, delayed arrival of ectomycorrhizal spores progressively reduced pine seedling growth and favoured growth of the competitor. Competition had significant negative effects on pine seedling growth, but the competition effect was much smaller than the effect of delayed mutualist arrival. Synthesis. The importance of mycorrhizal spore arrival time on pine growth suggests that plants may experience mutualist limitation more frequently than previously expected, and the relative magnitude of seedling responses to mycorrhizal fungi and competing plants show that in some systems mutualism is likely of equal or greater importance compared with interspecific competition in affecting plant community assembly. [ABSTRACT FROM AUTHOR]

Phenotypic variation controls the species interactions which determine whether or not species coexist. Long-standing hypotheses in ecology and evolution posit that phenotypic differentiation enables coexistence by increasing the size of niche differentiation. This hypothesis has only been tested using macroscopic traits to date, but niche differentiation, particularly of microscopic organisms, also occurs at the molecular and metabolic level., We examined how phenotypic variation that arises at the level of gene expression over evolutionary time affects phytoplankton species interactions and coexistence., We predicted that similarity in gene expression among species would decline with phylogenetic distance, and that reduced similarity in gene expression would weaken competition, increase facilitation and promote coexistence., To test this, we grew eight species of freshwater green algae in monocultures and bicultures for 46 days in a laboratory microcosm experiment. We quantified the strength of species interactions by: (i) fitting Lotka-Volterra models to time-series densities and estimating interaction coefficients, and (ii) calculating relative densities that compare species' steady-state densities in biculture to those in monoculture. We used Illumina high throughput sequencing to quantify the expression of 1253 families of homologous genes, including a set of 17 candidate genes that we hypothesized a priori to be involved in competition or facilitation., Synthesis. We found that closely related species had greater similarity in gene expression than did distantly related species, but as gene expression became more similar, species experienced weaker competition or greater facilitation, and were more likely to coexist. We identified gene functional categories that were uniquely differentially regulated in association with particular species interaction types. Contrary to common thinking in ecology and evolution, similarity in gene expression, and not differentiation, was associated with weaker competition, facilitation and coexistence. [ABSTRACT FROM AUTHOR]

Interspecific interactions have a fundamental role in plant population dynamics, as they may set the conditions for species coexistence. Parasitic plants, like dwarf mistletoes, offer the opportunity to study competition for resources that are different from those consumed by most plants, allowing for a better understanding of the interaction., We explored how interspecific interactions between two dwarf mistletoe species ( Arceuthobium), co-infecting the same host species (even sharing the same individual tree of Pinus hartwegii) affect their infection dynamics at two different stages of population development (colonization of new hosts and subsequent growth), and if heterogeneity in resource availability (host density and size structure) affects these interactions. For that purpose, we integrated these processes into a spatially explicit model of density-dependent population growth., We found that self-regulation (density-dependence) was strong for both species; however, the intensity and sign of interspecific interactions changed depending on host size and demographic process. Population growth in Arceuthobium globosum was reduced by competition, except for smaller hosts where A. globosum growth was facilitated by Arceuthobium vaginatum. Arceuthobium vaginatum was facilitated by A. globosum regardless of host size. Colonization of new hosts by A. globosum was enhanced by previous infection by the other species, showing intraguild facilitation., Demographic importance of interactions depended on stand structure: in homogeneous, low-density forests, facilitation predominates, increasing the population sizes of both species, whereas the opposite occurs in heterogeneous and dense forests. Both species achieved stable coexistence, fulfilling the invasibility criterion because each mistletoe species can invade a forest that is already infected by the other species., Synthesis. Despite the fundamentally different mechanisms underlying the interactions between mistletoes compared with non-parasitic plants, our results reveal that their behaviour at the population level is similar. Stabilizing mechanisms, like strong self-limiting population growth, allow dwarf mistletoe coexistence. Interactions shift as populations develop, and they depend largely on environmental factors such as forest structure. Intraguild mutualism is shown as a relevant process for colonization of new spaces, highlighting the complexity of competitive/facilitative interactions between parasitic plants, a formerly unexplored subject. Interactions can only be fully understood when integrating all their components at the population level. Analysing these interactions may contribute to the understanding of plant-plant interactions in general, and convey interesting implications for forest management. [ABSTRACT FROM AUTHOR]

Understanding the factors that drive commonness and rarity of plant species and whether these factors differ for alien and native species are key questions in ecology. If a species is to become common in a community, incoming propagules must first be able to establish. The latter could be determined by competition with resident plants, the impacts of herbivores and soil biota, or a combination of these factors., We aimed to tease apart the roles that these factors play in determining establishment success in grassland communities of 10 alien and 10 native plant species that are either common or rare in Germany, and from four families. In a two-year multisite field experiment, we assessed the establishment success of seeds and seedlings separately, under all factorial combinations of low vs. high disturbance (mowing vs mowing and tilling of the upper soil layer), suppression or not of pathogens (biocide application) and, for seedlings only, reduction or not of herbivores (net-cages)., Native species showed greater establishment success than alien species across all treatments, regardless of their commonness. Moreover, establishment success of all species was positively affected by disturbance. Aliens showed lower establishment success in undisturbed sites with biocide application. Release of the undisturbed resident community from pathogens by biocide application might explain this lower establishment success of aliens. These findings were consistent for establishment from either seeds or seedlings, although less significantly so for seedlings, suggesting a more important role of pathogens in very early stages of establishment after germination. Herbivore exclusion did play a limited role in seedling establishment success., Synthesis: In conclusion, we found that less disturbed grassland communities exhibited strong biotic resistance to establishment success of species, whether alien or native. However, we also found evidence that alien species may benefit weakly from soilborne enemy release, but that this advantage over native species is lost when the latter are also released by biocide application. Thus, disturbance was the major driver for plant species establishment success and effects of pathogens on alien plant establishment may only play a minor role. [ABSTRACT FROM AUTHOR]

Plant communities show two general responses to gradients of soil resources: a decrease in species richness at high levels of resource availability and an associated shift in species composition from small and slow-growing species to large and fast-growing species. Models attempting to explain these responses have usually focused on a single pattern and provided contradicting predictions concerning the underlying mechanisms., We use an extension of Tilman's resource competition model to investigate the hypothesis that both patterns may originate from the size-asymmetric nature of light exploitation by competing plants. The only mechanism producing changes in species richness and species composition in our model is mortality due to competition., Under the framework of the model, asymmetric light exploitation is a necessary and sufficient condition to obtain the empirically observed responses of species richness and species composition to soil resource gradients. This theoretical result is robust to relaxing the simplifying assumptions of the model., Our model shows that the traits enhancing competitive superiority depend on the mode of resource exploitation: under symmetric exploitation, competitive superiority is achieved by tolerance of low resource levels, while under asymmetric exploitation, it is achieved by the ability to grow fast and attain a large size. This result indicates that a long-standing debate concerning the traits that enhance competitive superiority in plant communities (the 'Grime-Tilman debate') can be reduced into a single parameter of our model - the degree of asymmetry in resource competition., The model also explains the observed shift from below-ground to above-ground competition with increasing productivity, the associated increase in the asymmetry of competitive interactions and the increasing likelihood of competitive exclusion under high levels of productivity. None of these patterns could be obtained under symmetric competition in our model., Synthesis. The ability of the model to explain a wide range of observed patterns and the robustness of these predictions to its simplifying assumptions suggest that the size asymmetry of competition for light is a fundamental factor in determining the structure and diversity of plant communities. [ABSTRACT FROM AUTHOR]

Positive interactions among plants can increase species richness by relaxing environmental filters and providing more heterogeneous environments. However, it is not known if facilitation could affect coexistence through other mechanisms. Most studies on plant coexistence focus on negative frequency-dependent mechanisms (decreasing the abundance of common species); here, we test if facilitation can enhance coexistence by giving species an advantage when rare., To test our hypothesis, we used a global data set from drylands and alpine environments and measured the intensity of facilitation (based on co-occurrences with nurse plants) for 48 species present in at least 4 different sites and with a range of abundances in the field. We compared these results with the degree of facilitation experienced by species which are globally rare or common (according to the IUCN Red List), and with a larger data base including over 1200 co-occurrences of target species with their nurses., Facilitation was stronger for rare species (i.e. those having lower local abundances or considered endangered by the IUCN) than for common species, and strongly decreased with the abundance of the facilitated species. These results hold after accounting for the distance of each species from its ecological optimum (i.e. the degree of functional stress it experiences)., Synthesis. Our results highlight that nurse plants not only increase the number of species able to colonize a given site, but may also promote species coexistence by preventing the local extinction of rare species. Our findings illustrate the role that nurse plants play in conserving endangered species and link the relationship between facilitation and diversity with coexistence theory. As such, they provide further mechanistic understanding on how facilitation maintains plant diversity. [ABSTRACT FROM AUTHOR]

Theory suggests spatial heterogeneity can facilitate species co-occurrence at fine scales, but environmental data are rarely collected at sufficiently high resolution to test this empirically. Whilst there is emerging evidence that subtle variation in soil hydrology represents a fundamental fine-scale niche axis within plant communities, this is largely derived from studies of soil hydrology in isolation from other environmental factors., We assessed the comparative importance of fine-scale hydrological niche differentiation for species co-occurrence using a high-resolution study of soil hydrology and other edaphic variables, coupled with a long-term (24 years) data set of herbaceous plant plots in a heathland community in south-east Australia., For the analysis, we employed novel latent variable models (LVMs), which offer an explicit, model-based approach to partitioning out the different drivers of species co-occurrence patterns. Whilst the regression component of an LVM models the species-specific environmental responses, the latent variable component can be used to identify residual patterns of co-occurrence, which may be attributable to unmeasured factors and/or biotic interactions., Relative to a host of plant resources, non-resource factors and 'unmeasured' latent variables, soil hydrology emerged as the best predictor of negative co-occurrences within the community, with the dominant species exhibiting strongly differentiated responses across a comparatively narrow moisture gradient. Nevertheless, strong species-specific responses to environmental variability only emerged at scales greater than those at which plants may be expected to compete for resources, throwing doubt on the direct role of spatial heterogeneity as a mechanism for local-scale coexistence., Synthesis. This study confirms the vital role of hydrological niches for the maintenance of within-community plant diversity, but also highlights the need for more rigorous analysis of scale dependencies to better understand the underlying coexistence mechanisms at play. In addition, it illustrates the inferential gains made possible with model-based approaches to the analysis of species co-occurrence. r code illustrating model fitting and inference is provided as a supplement. [ABSTRACT FROM AUTHOR]

1. Despite the fundamental significance of water to plants and the persisting question of how competing species coexist, this is the first review of hydrological niches. We define hydrological niche segregation (HNS) as: (i) partitioning of space on fine-scale soil-moisture gradients, (ii) partitioning of water as a resource and/or (iii) partitioning of recruitment opportunities among years caused by species specializing on particular patterns of temporal variance of water supply (the storage effect). 2. We propose that there are three types of constraint that lead to the trade-offs that underlie HNS. (i) An edaphic constraint creates a trade-off between the supply to roots of O2 on the one hand vs. water and nutrients on the other. (ii) A biophysical constraint governs gas exchange by leaves, leading to a trade-off between CO2 acquisition vs. water loss. (iii) A structural constraint arising from the physics of water-conducting tissues leads to a safety vs. efficiency trade-off. 3. Significant HNS was found in 43 of 48 field studies across vegetation types ranging from arid to wet, though its role in coexistence remains to be proven in most cases. Temporal partitioning promotes coexistence through the storage effect in arid plant communities, but has yet to be shown elsewhere. In only a few cases is it possible to unequivocally link HNS to a particular trade-off. 4. Synthesis. The field and experimental evidence make it clear that HNS is widespread, though it is less clear what its precise mechanisms or consequences are. HNS mechanisms should be revealed by further study of the constraints and trade-offs that govern how plants obtain and use water, and HNS can be mechanistically linked to its consequences with appropriate community models. In a changing climate, such an integrated programme would pay dividends for global change research. [ABSTRACT FROM AUTHOR]

Disturbance can function to maintain diversity within forest communities; however, specific mechanisms and the relationship to productivity are not well understood., We examined these linkages in forest ground-layer plant communities using a replicated, manipulative field experiment. Treatments included a range of gap sizes and untreated controls. We assessed spatial and temporal responses over the first three years following gap creation., Light transmittance and soil water content increased with gap size, while rates of colonization and species richness increased after a critical threshold. Subsequent increases in productivity were associated with declines in species richness, increased rates of local extirpation and a unimodal relationship between species richness and productivity at the individual quadrat scale (4 m2)., The richness and productivity of vines, shrubs and especially graminoids, increased within 200-380 m2 gaps treatments. However, the productivity of forbs and tree seedlings did not, showing possible drought sensitivity overriding treatments. Spatial and temporal partitioning of gaps occurred as a result of interactions between species traits and environmental conditions. Significantly, productivity and richness showed complex relationships with canopy structure., Synthesis. Our results show that richness increases to an asymptote after a critical threshold in disturbance severity initially. Decreases in species richness over time associated with increases in productivity may eventually result in the unimodal relationship predicted by the intermediate disturbance hypothesis. However, species composition continues to differ with canopy gap size, suggesting a range of canopy gap sizes is required to maintain the greatest diversity of plant species over broader spatial and temporal scales. [ABSTRACT FROM AUTHOR]

Negative plant-soil feedback occurs when the presence of an individual of a particular species at a particular site decreases the relative success of individuals of the same species compared with those other species at that site. This effect favours heterospecifics thereby facilitating coexistence and maintaining diversity. Empirical work has demonstrated that the average strengths of these feedbacks correlate with the relative abundance of species within a community, suggesting that feedbacks are an important driver of plant community composition. Understanding what factors contribute to the generation of this relationship is necessary for diagnosing the dynamic forces that maintain diversity in plant communities., We used a spatially explicit, individual-based computer simulation to test the effects of dispersal distance, the size of feedback neighbourhoods, the strength of pairwise feedbacks and community wide variation of feedbacks, community richness, as well as life-history differences on the dependence of relative abundance on strength of feedback., We found a positive dependence of relative abundance of a species on its average feedback for local scale dispersal and feedback. However, we found that the strength of this dependence decreased as either the spatial scale of dispersal and/or the spatial scale of feedback increased. We also found that for spatially local (i.e. relatively small) scale interaction and dispersal, as the mean strength of feedbacks in the community becomes less negative, the greater the increase in abundance produced by a comparable increase in species-specific average feedback. We found that life-history differences such as mortality rate did not generate a pattern with abundance, nor did they affect the relationship between abundance and average feedback., Synthesis. Our results support the claim that empirical observations of a positive correlation between relative abundance and strength of average feedback serve as evidence that local scale negative feedbacks play a prominent role in structuring plant communities. We also identify that this relationship depends upon local scale plant dispersal and feedback which generates clumping and magnifies the negative feedbacks. [ABSTRACT FROM AUTHOR]

1. Understanding the mechanisms governing coexistence is a central goal in ecology and has implications for conserving and restoring communities, yet the high diversity in many plant communities is difficult to explain. Theory suggests that plant-soil feedbacks (PSF) can lead to frequency-dependent coexistence by suppressing conspecifics more than heterospecifics, potentially helping to explain high-diversity plant communities. In addition, species-specific population dynamics, including the rate at which individuals are replaced in a population or population turnover rate, may influence coexistence outcomes. 2. We have created a rigorous test of the coexistence predictions of theory by generating a soil heterogeneity experiment in the field and testing for mutual invasibility by establishing resident populations, then experimentally invading them. Experimental tests of mutual invasibility can demonstrate coexistence because, if species are able to invade one another's populations when at low density, they should exhibit long-term coexistence. We use pairs of congeners in this experiment that coexist at small spatial scales, sometimes within cm, at our field site. 3. We demonstrate that invader individuals established better in congener's soils than in conspecific soils, consistent with plant-soil feedback mediated coexistence. This effect was often mediated by competition with established resident plants. 4. Further, we show that soil heterogeneity interacted with the population turnover rate of the resident population to influence invasibility (P < 0.10), consistent with the theoretical prediction that a plant's population dynamics will interact with heterogeneity to influence coexistence. 5. Synthesis. Plant-soil feedbacks (PSF) can in theory lead to frequency-dependent coexistence, and reciprocally, negative feedback effects in glasshouse experiments are often consistent with this prediction. We provide the first field test of mutual invasibility structured by PSF, demonstrating that PSF can lead to coexistence when they create a patchy, or heterogeneous, soil environment. This work suggests that understanding the influence of PSF on diversity necessitates understanding the spatial scale at which soil heterogeneity emerges in the field. Thus, high diversity might be maintained in plant communities by heterogeneity created by plants' influence on the soil, and this outcome depends strongly on population dynamics. [ABSTRACT FROM AUTHOR]

There is an ongoing debate about whether communities are closely integrated and bound together via interactions such as competition or facilitation, or are disintegrated and dominated by chance. We still lack community-wide data on the intensities of interactions and randomness, and measurements of their impacts on community structure., Using a long-term data set, we sought to measure the effects of interactions and stochasticity in structuring a highly diverse (>100 species) semi-arid grassland plant community, testing for positive and negative interactions at different stages of population growth of all species., During the colonization of new patches, most species were facilitated or inhibited by several others. These opposite effects can potentially have a large effect on species abundances, but they were correlated and cancelled out at the community level. Nevertheless, competition during colonization was strong enough to cause poor competitors to have small population sizes., The subsequent phase of population growth (increase and subsequent change in numbers within occupied patches) was mainly driven by intraspecific density dependence, and we found little evidence for interspecific interactions., Model results showed that stochasticity and recurrent colonization of transient, favourable patches maintained diversity, keeping poor competitors from becoming extinct., Synthesis: Our results, taken together with recent studies on tropical forests, suggest that weak interactions among established plants may be a general phenomenon, but that local interactions during colonization are important drivers of community composition. Most of the variance in species abundance in our community was explained by intraspecific competition and stochasticity, with interspecific interactions playing a minor role due to their overall weakness, interaction changes over ontogeny, and the cancellation of opposite-sign interactions when all the species in the community are considered. Despite this, some species were rare seemingly because they cannot withstand interspecific competition. Thus, to untangle the effects of interactions on community structure, future research should focus on interactions occurring at different phases of population growth and on whole communities. [ABSTRACT FROM AUTHOR]

Taller plant species can pre-empt solar energy and suppress growth of subordinate species in vegetation stands, which is described through one-sided competition. Yet, in much of the world's vegetation species of different statures coexist. This study aims to clarify the mechanisms underlying this apparent paradox., We quantified how co-occurring species and individuals intercepted and used light for growth in a mature, warm-temperate evergreen forest. This was performed by determining the 3D distribution of foliage and light with a ground-based lidar system in combination with nondestructive measurements of plant growth., Taller trees intercepted light more efficiently per unit of above-ground biomass than shorter trees did (=higher light interception efficiency, LIE). However, taller trees tended to have lower biomass production per unit light interception (=lower light use efficiency, LUE). Reduced LUE in taller trees was associated with their higher biomass allocation to nonphotosynthetic organs and probably with over-saturated light intensity for photosynthesis at high canopy positions. Due to the increased LIE and decreased LUE with tree heights, a trade-off between LIE and LUE was found, and this trade-off resulted in trees of different statures having similar relative growth rates., Synthesis. Light competition drives trees to grow taller, and the light interception efficiency is higher in taller trees; however, this benefit comes at a cost of decreased efficiency of light use for growth. This trade-off allows trees of different statures to grow at proportionally comparable rates and may promote coexistence of tree species in one-sided light competition. [ABSTRACT FROM AUTHOR]

The spatial distribution of species is mediated by a combination of biotic interactions and environmental conditions. Understanding the relative importance of these factors and how they interact is particularly important for predicting the spread of non-native species and their impact on resident communities., We used a 3-species Lotka- Volterra model parameterized with field and experimental data to understand the potential for continued spread by an introduced, non-native, dune-building beach grass ( Ammophila breviligulata) and whether this invasion will result in coexistence or displacement in the resident beach grass communities (native Elymus mollis and introduced, non-native Ammophila arenaria) of the US Pacific Northwest., We also used the model to investigate to what extent different rates of ocean-driven sand supply mediate the outcomes of beach grass species interactions., Our model suggests that A. breviligulata could invade and dominate dunes across the range of sand supply rates observed in the region. We found that sand supply influenced intra- vs. interspecific interactions, the strength of positive and indirect effects among beach grasses and the long-term abundances of the beach grass species themselves., Of the two non-natives, A. breviligulata is the inferior dune-building species. Thus, our results suggest that further invasions by A. breviligulata could reduce the coastal protection services afforded by tall dunes currently dominated by A. arenaria., Synthesis: In systems with strong environmental forcing and stressful conditions such as coastal dunes, environmentally mediated positive and indirect species interactions can govern invasion success and long-term native-non-native coexistence. In doing so, these interactions ultimately shape community structure and ecosystem function. Understanding the joint effects of environmental forcing and species interactions on community assembly is particularly important in cases where species introductions can alter ecosystem services, such as coastal protection, which are vulnerable to the effects of climate change. [ABSTRACT FROM AUTHOR]

Plant species can influence soil biota, which in turn can influence the relative performance of plant species. These plant-soil feedbacks ( PSFs) have been hypothesized to affect many community-level dynamics including species coexistence, dominance and invasion., The importance of PSFs in exotic species invasion, although widely hypothesized, has been difficult to determine because invader establishment necessarily precedes invader-mediated PSFs. Here, we combine a spatial simulation model of invasion that incorporates PSFs with a meta-analysis that synthesizes published case studies describing feedbacks between pairs of native and exotic species., While our spatial model confirmed the link between positive soil feedbacks ('home' advantage) for exotic species and exotic species spread, results were dependent on the initial abundance of the exotic species and the equivalence of dispersal and life history characteristics between exotic and native species., The meta-analysis of 52 native-exotic pairwise feedback comparisons in 22 studies synthesized measures of native and exotic performance in soils conditioned by native and exotic species. The analysis indicated that the growth responses of native species were often greater in soil conditioned by native species than in soil conditioned exotic species (a 'home' advantage). The growth responses of exotic species were variable and not consistently related to species soil-conditioning effects., Synthesis. Overlaying empirical estimates of pairwise PSFs with spatial simulations, we conclude that the empirically measured PSFs between native and exotic plant species are often not consistent with predictions of the spread of exotic species and mono-dominance. This is particularly the case when exotic species are initially rare and share similar dispersal and average fitness characteristics with native species. However, disturbance and other processes that increase the abundance of exotic species as well as the inclusion of species dispersal and life history differences can interact with PSF effects to explain the spread of invasive species. [ABSTRACT FROM AUTHOR]

Understanding the processes by which species sort themselves into communities remains a central puzzle for attempts to maintain biodiversity. It remains unclear whether any single assembly process is generally dominant or whether the influence of contrasting processes varies in a predictable way relative to biotic and abiotic gradients. Abundance-weighted niche overlap between species provides a powerful means of contrasting two major assembly processes - niche complementarity and environmental filtering. We examined mean overlap for four vegetative functional traits, relative to that expected when abundances were randomly allocated to species co-occurring in experimental plots in a wet meadow. This provided a test of whether any single assembly process prevailed for the meadow as a whole and across all traits. The effects of mowing, fertilization and dominant species removal, and associated gradients of Simpson's dominance and biomass on the niche overlap of plots, were also examined. Niche overlap was higher than expected at random for three of the four traits studied (height, leaf and stem dry matter content, leaf C:N ratio). However, niche overlap was lower than expected for specific leaf area. Mowing was the treatment with the greatest effect on both niche overlap and biomass, with overlap significantly lower in the absence of mowing for three of the traits, while biomass was lower in mown plots. For three of the traits there was evidence of a significant decrease in overlap with increasing biomass, but not increasing dominance. None of the significant mowing effects on overlap remained when the effect of biomass had been removed. Synthesis: These results suggest that the importance of niche differences between species in structuring grassland communities should increase with increasing biomass and decrease with disturbance in grassland communities. They also emphasize that contrasting community assembly processes may occur for different niche axes, even within a single community. [ABSTRACT FROM AUTHOR]

Models of local stable coexistence require negative feedbacks, i.e. intraspecific interactions must be more negative than interspecific interactions. However, most competition experiments, often done in the glasshouse, have found evidence for competitive hierarchies. Measurement of interactions under realistic field conditions is necessary to assess their contribution to community dynamics, and explicit measurement of intermediaries thought to be important in interactions may allow studies to account for any variation in experimental results. In this study, we compare conspecific and heterospecific interactions in a field experiment in a dry sand prairie in Michigan. We study the four dominant species at two different stages, germination and adult growth. Using seed addition and adult transplant experiments, we ask whether plants perform best in natural field monocultures of conspecifics, heterospecifics or no neighbour plots. We also measure abiotic environmental characteristics associated with each neighbourhood type and test whether performance can be explained by environmental effects. We hypothesize that plants will create competitive hierarchies because our experimental design is similar to classic competition experiments. Neighbour species created consistent hierarchies in their effects on germination of all four target species, which is likely due to light limitation. However, interestingly, adult plant biomass for two of the three species (one species did not survive) was lower in conspecific monocultures compared to heterospecific or no neighbour plots, thus producing negative feedbacks. For two species, the effects of neighbours on adult growth are likely due to reduction of light and soil nitrate; for the third, however, resources could not explain the pattern that conspecific interactions were more negative than heterospecific. Synthesis. These results suggest that patterns in the relative strength of conspecific and heterospecific competition depend on life-history stage. Moreover, resource uptake could explain some, but not all, of the interactions among species, suggesting that other factors such as microbial communities or other forms of niche partitioning may play a role and that field experiments are necessary to gauge their relative importance. [ABSTRACT FROM AUTHOR]

1. Hemiparasitic plants, such as Rhinanthus species, have substantial effects on community composition and biomass. For example, the presence of parasites often increases diversity but reduces the combined biomass of hosts and parasites by c. 25% compared with unparasitized controls. We present and test a simple model of the host–parasite interaction in which parasite growth rate is a function of host growth rate that offers a new explanation for why hemiparasitic plants reduce ecosystem productivity. 2. The model predicts that the combined mass of the host–parasite system is always less than the mass of the host grown alone, because the combined biomass is dependent only on host growth rate, which is reduced by the parasite. The model also predicts that the parasite should adopt an intermediate virulence to maximize its own performance, but that the optimum virulence depends on host growth characteristics. 3. The key assumption of the model is that parasite growth rate and hence parasite biomass is tightly coupled to host growth rate. We tested this assumption by measuring the performance of Rhinanthus alectorolophus, a widespread hemiparasitic annual plant, on nine common European grass species. First, we determined size-corrected growth rates for the grasses by fitting power-law growth curves to multiple-harvest data on host individuals grown without Rhinanthus. Second, we grew Rhinanthus on each of the grass species and related Rhinanthus final biomass to the grass species’ growth rates. 4. Rhinanthus performance was strongly correlated with the growth rate of the host grass species, thus validating a key assumption of our model. However, Rhinanthus biomass on three of the nine grass species differed significantly from the value predicted based on host growth rate alone, suggesting that grass species differ in their resistance to parasitism. 5. Synthesis. Parameterizing such models of the host–parasite relationship could help to explain variation in Rhinanthus performance on different hosts, variation in the effects of hemiparasites in grasslands of different productivity and differences in virulence among parasite populations. [ABSTRACT FROM AUTHOR]

1. Studying the spatial pattern of plants may provide significant insights into processes and mechanisms that maintain species richness. We used data from a fully mapped 25-ha temperate forest plot at Changbaishan (CBS), north-eastern China, to conduct a community-wide assessment of the type and frequency of intra- and interspecific spatial association patterns. We analysed complex scale effects in the patterning of large trees of 15 common species. First, we tested for overall spatial patterning at 6, 30 and 50 m neighbourhoods and classified the types of bivariate association patterns at these spatial scales (analysis 1). We then explored small-scale (0–20 m) association patterns conditioning on the larger-scale pattern (analysis 2) and tested for positive large-scale (50–250 m) association patterns (analysis 3). 2. Analysis 1 provided ample evidence for non-random spatial patterning, and the type and frequency of spatial association patterns changed with scale. Trees of most species pairs co-occurred less than expected by chance and positive associations were rare in local neighbourhoods. Analysis 2 revealed a separation of scales in which significant small-scale interactions faded away at distances of 10–15 m. One third of all species pairs showed significant and mostly negative bivariate small-scale association, which occurred more often than expected by chance between species sharing attributes such as family, fruit type and habitat association. This suggests the occurrence of competitive interactions. Analysis 3 showed that only 8% of all species pairs co-occurred at large scales. 3. Comparison of our results with an analogous study conducted in the species-rich tropical forest at Sinharaja, Sri Lanka, revealed several structural similarities including the dominance of segregation and partial overlap in the overall patterning (analysis 1) and the separation of scales (analysis 2). However, species pairs at CBS showed considerably more significant negative small-scale associations (31% vs. 6% at Sinharaja). 4. Synthesis. The techniques presented here allow for a detailed analysis of the complex spatial associations in species-rich forests and have the potential to reveal indicative patterns that may allow researchers to discriminate among competing hypotheses of community assemblage and dynamics. However, this will require comparative studies involving a large number of plots. [ABSTRACT FROM AUTHOR]