Spatial heterogeneity in soil particle size: does it affect the yield of plant communities with different species richness?

Aims Soil heterogeneity is ubiquitous in many ecosystems. We hypothesized that plant communities with higher species richness might be better adapted to soil heterogeneity and produce more biomass than those with lower richness. This is because there is niche differentiation among species and different species can complement each other and occupy a broader range of niches when plant species richness is high. However, no study has tested how soil particle heterogeneity affects the yield of plant communities, and whether such effects depend on the spatial scale of the heterogeneity and the species richness within the communities.Methods In a greenhouse experiment, we sowed seeds of four-species or eight-species mixtures in three heterogeneous treatments consisting of 32, 8 or 2 patches of both small (1.5mm) and large quartz (3.0mm) particles arranged in a chessboard manner and one homogeneous treatment with an even mixture of small and large quartz particles.Important findings Biomass production was significantly greater in the communities with high species richness than those with low species richness. However, soil particle heterogeneity or its interactions with patch scale or species richness did not significantly affect biomass production of the experimental communities. This work indicates that plant species richness may have a bigger impact on plant productivity than soil particle heterogeneity. Further studies should consider multiple sets of plant species during longer time periods to unravel the potential mechanisms of soil heterogeneity and its interactions with the impacts of species richness on community yield and species coexistence.     

Legacy effects of soil homogenization on tallgrass prairie restoration: toward resolved understanding of the relationship between soil heterogeneity and plant species diversity

Within plant communities, niche‐based species sorting can occur among distinct soil patches (microsites), increasing coexistence and diversity. Microsite edges (microedges) may also offer additional niche space. Therefore, in recently abandoned croplands, which often have uniform soils caused by a legacy of tillage (soil homogenization), the plant species diversity of future restoration efforts may be reduced. We conducted an experiment during the early establishment phase (3 years) of a tallgrass prairie restoration on former cropland to determine if soil homogenization decreases species diversity and alters community composition, and if microedges offer additional niche space. Heterogeneous plots with sand‐ or woodchip‐enriched patches were compared to plots made up of the same components, but distributed homogeneously, and pits and mounds were compared to flat topsoil. Homogenization decreased diversity in flat topsoil plots relative to pit plots and increased diversity in woodchip plots. In both cases, the treatments with the lowest canopy cover and greatest plant density had the greatest diversity. Sand and topographic homogenization decreased diversity, but when a drought occurred in year two, the effect was suppressed in the sand treatment and magnified in the pit plots. Microedges had properties unique from adjacent patches. Overall, variability in heterogeneity–diversity relationships was affected by interactions with plant growth patterns and environmental conditions. Our results indicate that while the addition of contrasting soil microsites has the potential to promote increased diversity in grassland restoration on former cropland, the patch components and design must be optimized to achieve this management goal.     

Canopy cover type,and not fine-scale resource availability,explains native and exotic species richness in a landscape affected by anthropogenic fires and posterior land-use change

Anthropogenic fires and land-use change, including the conversion from native to exotic species canopies, are two major types of disturbances that strongly affect the functioning of forest ecosystems around the world. These disturbances alter the resource availability for plants, which may lead to changes in species richness. Here we examined the relative effects of canopy cover type, light availability and soil nutrient (N and P) availability on species richness, including invasive species, at different post-fire plant systems. Additionally, we tested the resource heterogeneity hypothesis (RHH) for plant diversity, which proposes that diversity is higher in habitats with spatially heterogeneous resources. We evaluated four different canopy cover types, including mature and second-growth Nothofagus pumilio forests, treeless prairie, Pinus sylvestris afforestations, all of which were converted from mature N. pumilio forests. Using generalized mixed-effects model correlations, we determined (1) the relative influence of canopy cover type, light and soil nutrient availability on understory species richness and (2) the relationship between species richness and resource heterogeneity. We found that canopy cover type was the factor that best explained species richness, much more than fine-scale light and soil nutrient availability. Additionally, we found that the more homogeneous the light environment the higher the number of exotic species (mainly found in the prairie where the highest light intensity occurred), which is contrary to what the RHH states. In conclusion, canopy cover type, a stand-scale driver, and not fine-scale resource (light, N and P) availability, was most important for explaining native and exotic (including invasive) species understory richness in a landscape affected by anthropogenic fires and posterior land-use change.     

Vegetation responses to different spatial patterns of soil disturbance in burned and unburned tallgrass prairie

Pocket gopher (Geomyidae) disturbances are created in spatiallypredictable patterns. This may influence resource heterogeneity and affectgrassland vegetation in a unique manner. We attempt to determine the extent towhich density and spatial pattern of soil disturbances influence tallgrassprairie plant community structure and determine how these disturbances interactwith fire. To investigate the effects of explicit disturbance patterns we createdsimulated pocket gopher burrows and mounds in various spatial patterns.Simulated burrows were drilled into the soil at different densities inreplicated plots of burned and unburned prairie. Separate plots of simulatedmounds were created in burned and unburned prairie at low, medium, or high mounddensities in clumped, uniform, or random spatial dispersions. In both burned and unburned plots, increased burrow density decreasedgraminoid biomass and increased forb biomass. Total-plant and graminoid biomasswere higher in burned than unburned plots while forb biomass was higher inunburned plots. Total-plant species richness was not significantly affected byburrow density or burning treatments, but graminoid species richness increasedin unburned plots and forb species richness increased in burned plots. Plant species richness was temporarily reduced directly on mounddisturbances compared to undisturbed prairie. Over time and at larger samplingscales, the interaction of fire and mound disturbance patterns significantlyaffected total-plant and graminoid species richness. The principal effect inburned and unburned prairie was decreased total-plant and graminoid speciesrichness with increased mound disturbance intensity. Although species richness at small patch scales was not increased by anyintensity of disturbance and species composition was not altered by theestablishment of a unique guild of disturbance colonizing plants, our studyrevealed that interactions between soil disturbances and fire alter the plantcommunity dominance structure of North American tallgrass prairie primarily viachanges to graminoids. Moreover, these effects become increasingly pronouncedover time and at larger spatial sampling scales.     

Relationships between spatial environmental heterogeneity and plant species diversity on a limestone pavement

No empirical studies have examined the relationship between diversity and spatial heterogeneity across unimodal species richness gradients. We determined the relationships between diversity and environmental factors for 144 0.18 m² plots in a limestone pavement alvar in southern Ontario, Canada, including within-plot spatial heterogeneity in soil depth, microtopography and microsite composition. Species richness was unimodally related to mean soil depth and relative elevation. Microsite heterogeneity and soil depth heterogeneity were positively correlated with species richness, and the richness peaks of the unimodal gradients correspond to the maximally spatially heterogeneous plots. The best predictive models of species richness and evenness, however, showed that other factors, such as ramet density and flooding, are the major determinants of diversity in this system. The findings that soil depth heterogeneity had effects on diversity when the effects of mean soil depth were factored out, and that unimodal richness peaks were associated with high spatial heterogeneity in environmental factors represent significant contributions to our understanding of how spatial heterogeneity might contribute to diversity maintenance in plant communities.     

Vole disturbances and plant diversity in a grassland metacommunity

We studied the disturbance associated with prairie vole burrows and its effects on grassland plant diversity at the patch (1 m²) and metacommunity (>5 ha) scales. We expected vole burrows to increase patch-scale plant species diversity by locally reducing competition for resources or creating niche opportunities that increase the presence of fugitive species. At the metacommunity scale, we expected burrows to increase resource heterogeneity and have a community composition distinct from the matrix. We measured resource variables and plant community composition in 30 paired plots representing disturbed burrows and undisturbed matrix patches in a cool-season grassland. Vole disturbance affected the mean values of nine resource variables measured and contributed more to resource heterogeneity in the metacommunity than matrix plots. Disturbance increased local plant species richness, metacommunity evenness, and the presence and abundance of fugitive species. To learn more about the contribution of burrow and matrix habitats to metacommunity diversity, we compared community similarity among burrow and matrix plots. Using Sorenson’s similarity index, which considers only presence–absence data, we found no difference in community similarity among burrows and matrix plots. Using a proportional similarity index, which considers both presence–absence and relative abundance data, we found low community similarity among burrows. Burrows appeared to shift the identity of dominant species away from the species dominant in the matrix. They also allowed subordinate species to persist in higher abundances. The patterns we observed are consistent with several diversity-maintaining mechanisms, including a successional mosaic and alternative successional trajectories. We also found evidence that prairie voles may be ecosystem engineers.     

Microtopographic heterogeneity constrains alpine plant diversity, Glacier National Park, MT

Theoretical and empirical evidence exists for a positive relationship between environmental heterogeneity and species diversity. Alpine plant communities can exhibit exceptional diversity at a fine scale, which niche theory would suggest is the result of fine scale spatial heterogeneity of the environment. To test if species diversity of alpine plants is driven by environmental heterogeneity, we sampled vascular plant species composition, microtopography, and ground cover within 1?m² plots with and without solifluction forms in Glacier National Park, MT. We analyzed the relationship between microtopographic heterogeneity and species richness at the plot and sub-plot scale with linear and quantile regression, respectively. Species richness does not differ between the plots varying in cover type. Species richness is negatively related to the fractal dimension (D) of the ground surface and non-vegetated ground cover within 1?m² plots. At a finer scale, the standard deviation of elevation and slope appear to impose a limit on species richness such that more variable sub-plots have lower species richness. Contrary to our expectations, microtopographic heterogeneity does not promote the diversity of alpine plants. The negative relationship between topographic heterogeneity and species richness is contrary to the theoretical prediction that environmental heterogeneity generally results in greater species diversity. It is possible that microtopographic variability represents a measure of soil disturbance, which would be expected to have a negative effect on species diversity in alpine tundra due to its low productivity.     

Are two strategies better than one? Manipulation of seed density and soil community in an experimental prairie restoration

Restoration practitioners have a variety of practices to choose from when designing a restoration, and different strategies may address different goals. Knowledge of how to best use multiple strategies could improve restoration outcomes. Here, we examine two commonly suggested strategies in a single tallgrass prairie restoration experiment: increased forb sowing density and prairie soil inoculation. We designed a study with two different forb seeding densities. Within these densities, we transplanted seedlings into 1‐m² plots that had been grown in either a whole prairie soil inoculum or sterilized prairie soil. After 4 years, we found positive effects of both high forb sowing density and inoculation treatments on the ratio of seeded to nonseeded plant cover in these plots, and negative effects of both treatments on nonseeded plant diversity. No effects of either treatment were seen on seeded plant diversity. Each strategy also affected the plant community in different ways: high forb sowing density increased seeded forb richness and decreased native nonseeded plant cover, while inoculation decreased non‐native cover, and tended to increase average successional stage of the community. These effects on restoration outcome were typically independent of each other, with the result that plots with both manipulations had the most positive effects on restoration outcomes. We thus advocate the combined use of these restoration strategies, and further studies which focus on both seeding and soil community manipulation in restoration.     

Spatial and environmental determinants of plant species diversity in a temperate desert

Aims Deserts are one of the ecosystems most sensitive to global climate change. However, there are few studies examining how changing abiotic and biotic factors under climate change will affect plant species diversity in the temperate deserts of Asia. This study aimed to: (i) characterize species distributions and diversity patterns in an Asian temperate desert; and (ii) to quantify the effects of spatial and environment variables on plant species diversity.Methods We surveyed 61 sites to examine the relationship between plant species diversity and several spatial/environmental variables in the Gurbantunggut Desert. Spatial and environmental variables were used to predict plant species diversity in separate multiple regression and ordination models. Variation in species responses to spatial and environmental conditions was partitioned by combining these variables in a redundancy analysis (RDA) and by creating multivariate regression trees (MRT).Important findings We found 92 plant species across the 61 sites. Elevation and geographic location were the dominant environmental factors underlying variation in site species richness. A RDA indicated that 93% of the variance in the species–environment relationships was explained by altitude, latitude, longitude, precipitation and slope position. Precipitation and topographic heterogeneity, through their effects on water availability, were more important than soil chemistry in determining the distribution of species. MRT analyses categorized communities into four groups based on latitude, soil pH and elevation, explaining 42.3% of the standardized species variance. Soil pH strongly influenced community composition within homogeneous geographic areas. Our findings suggest that precipitation and topographic heterogeneity, rather than edaphic heterogeneity, are more closely correlated to the number of species and their distributions in the temperate desert.     

Multispecies coexistence of trees in tropical forests: spatial signals of topographic niche differentiation increase with environmental heterogeneity

Neutral and niche theories give contrasting explanations for the maintenance of tropical tree species diversity. Both have some empirical support, but methods to disentangle their effects have not yet been developed. We applied a statistical measure of spatial structure to data from 14 large tropical forest plots to test a prediction of niche theory that is incompatible with neutral theory: that species in heterogeneous environments should separate out in space according to their niche preferences. We chose plots across a range of topographic heterogeneity, and tested whether pairwise spatial associations among species were more variable in more heterogeneous sites. We found strong support for this prediction, based on a strong positive relationship between variance in the spatial structure of species pairs and topographic heterogeneity across sites. We interpret this pattern as evidence of pervasive niche differentiation, which increases in importance with increasing environmental heterogeneity.     

Plant species diversity and environmental heterogeneity: spatial scale and competing hypotheses

Questions: What is the observed relationship between plant species diversity and spatial environmental heterogeneity? Does the relationship scale predictably with sample plot size? What are the relative contributions to diversity patterns of variables linked to productivity or available energy compared to those corresponding to spatial heterogeneity? Methods: Observational and experimental studies that quantified relationships between plant species richness and within‐sample spatial environmental heterogeneity were reviewed. Effect size in experimental studies was quantified as the standardized mean difference between control (homogeneous) and heterogeneous treatments. For observational studies, effect sizes in individual studies were examined graphically across a gradient of plot size (focal scale). Relative contributions of variables representing spatial heterogeneity were compared to those representing available energy using a response ratio. Results: Forty‐one observational and 11 experimental studies quantified plant species diversity and spatial environmental heterogeneity. Observational studies reported positive species diversity‐spatial heterogeneity correlations at all points across a plot size gradient from ～1.0 × 10^?1 to ～1.0 × 10¹¹ m², although many studies reported spatial heterogeneity variables with no significant relationships to species diversity. The cross‐study effect size in experimental studies was not significantly different from zero. Available energy variables explained consistently more of the variance in species richness than spatial heterogeneity variables, especially at the smallest and largest plot sizes. Main conclusions: Species diversity was not related to spatial heterogeneity in a way predictable by plot size. Positive heterogeneity‐diversity relationships were common, confirming the importance of niche differentiation in species diversity patterns, but future studies examining a range of spatial scales in the same system are required to determine the role of dispersal and available energy in these patterns.     

Scale-specific correlations between habitat heterogeneity and soil fauna diversity along a landscape structure gradient

Habitat heterogeneity contributes to the maintenance of diversity, but the extent that landscape-scale rather than local-scale heterogeneity influences the diversity of soil invertebrates—species with small range sizes—is less clear. Using a Scottish habitat heterogeneity gradient we correlated Collembola and lumbricid worm species richness and abundance with different elements (forest cover, habitat richness and patchiness) and qualities (plant species richness, soil variables) of habitat heterogeneity, at landscape (1 km²) and local (up to 200 m²) scales. Soil fauna assemblages showed considerable turnover in species composition along this habitat heterogeneity gradient. Soil fauna species richness and turnover was greatest in landscapes that were a mosaic of habitats. Soil fauna diversity was hump-shaped along a gradient of forest cover, peaking where there was a mixture of forest and open habitats in the landscape. Landscape-scale habitat richness was positively correlated with lumbricid diversity, while Collembola and lumbricid abundances were negatively and positively related to landscape spatial patchiness. Furthermore, soil fauna diversity was positively correlated with plant diversity, which in turn peaked in the sites that were a mosaic of forest and open habitat patches. There was less evidence that local-scale habitat variables (habitat richness, tree cover, plant species richness, litter cover, soil pH, depth of organic horizon) affected soil fauna diversity: Collembola diversity was independent of all these measures, while lumbricid diversity positively and negatively correlated with vascular plant species richness and tree canopy density. Landscape-scale habitat heterogeneity affects soil diversity regardless of taxon, while the influence of habitat heterogeneity at local scales is dependent on taxon identity, and hence ecological traits, e.g. body size. Landscape-scale habitat heterogeneity by providing different niches and refuges, together with passive dispersal and population patch dynamics, positively contributes to soil faunal diversity. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Plant diversity increases spatio‐temporal niche complementarity in plant‐pollinator interactions

Ongoing biodiversity decline impairs ecosystem processes, including pollination. Flower visitation, an important indicator of pollination services, is influenced by plant species richness. However, the spatio‐temporal responses of different pollinator groups to plant species richness have not yet been analyzed experimentally. Here, we used an experimental plant species richness gradient to analyze plant–pollinator interactions with an unprecedented spatio‐temporal resolution. We observed four pollinator functional groups (honeybees, bumblebees, solitary bees, and hoverflies) in experimental plots at three different vegetation strata between sunrise and sunset. Visits were modified by plant species richness interacting with time and space. Furthermore, the complementarity of pollinator functional groups in space and time was stronger in species‐rich mixtures. We conclude that high plant diversity should ensure stable pollination services, mediated via spatio‐temporal niche complementarity in flower visitation.     

Effects of foundation species genotypic diversity on subordinate species richness in an assembling community

Foundation (dominant or matrix) species play a key role in structuring plant communities, influencing processes from population to ecosystem scales. However, the effects of genotypic diversity of foundation species on these processes have not been thoroughly assessed in the context of assembling plant communities. We modified the classical filter model of community assembly to include genotypic diversity as part of the biotic filter. We hypothesized that the proportion of fit genotypes (i.e. competitively superior and dominant) affects niche space availability for subordinate species to establish with consequence for species diversity. To test this hypothesis, we used an individual‐based simulation model where a foundation species of varying genotypic diversity (number of genotypes and variability among genotypes) competes for space with subordinate species on a spatially heterogeneous lattice. Our model addresses a real and practical problem in restoration ecology: choosing the level of genetic diversity of re‐introduced foundation and subordinate species. Genotypic diversity of foundation species significantly affected equilibrium community diversity, measured as species richness, either positively or negatively, depending upon environmental heterogeneity. Increases in genotypic diversity gave the foundation species a wider niche breadth. Under conditions of high environmental heterogeneity, this wider niche breadth decreased niche space for other species, lowering species richness with increased genotypic diversity until the genotypes of the foundation species saturated the landscape. With a low level of environmental heterogeneity, increasing genotypic diversity caused the foundation species niche breadth to be overdispersed, resulting in a weak positive relationship with species richness. Under these conditions, some genotypes are maladapted to the environment lowering fitness of the foundation species. These effects of genotypic diversity were secondary to the larger effects of overall foundation species fitness and environmental heterogeneity. The novel aspect of incorporating genotype diversity in combination with environmental heterogeneity in community assembly models include predictions of either positive or negative relationships between species diversity and genotypic diversity depending on environmental heterogeneity, and the conditions under which these factors are potentially relevant. Mechanistically, differential niche availability is imposed by the foundation species.     

Small-scale heterogeneity shapes grassland diversity in low-to-intermediate resource environments

Questions

Soil resource heterogeneity influences the outcome of plant–plant interactions and, consequently, species co-existence and diversity patterns. The magnitude and direction of heterogeneity effects vary widely, and the processes underlying such variations are not fully understood. In this study, we explored how and under what resource conditions small-scale heterogeneity modulates grassland plant diversity.

Location

Oderhänge Mallnow, Potsdam, Brandenburg, Germany.

Methods

We expanded the individual-based plant community model (IBC-grass) to incorporate dynamic below-ground resource maps, simulating spatial heterogeneity of resource availability. Empirical centimeter-scale data of soil C/N ratio were integrated into the model, accounting for both configurational and compositional heterogeneity. We then analyzed the interplay between small-scale heterogeneity and resource availability on the interaction and co-existence of plant species and overall diversity.

Results

Our results showed significant differences between the low- and high-resource scenarios, with both configurational and compositional heterogeneity having a positive effect on species richness and Simpson's diversity, but only under low-resource conditions. As compositional heterogeneity in the fine-scale C/N ratio increased, we observed a positive shift in Simpson's diversity and species richness, with the highest effects at the highest level of variability tested. We observed little to no effect in nutrient-rich scenarios, and a shift to negative effects at the intermediate resource level. The study demonstrates that site-specific resource levels underpin how fine-scale heterogeneity influences plant diversity and species co-existence, and partly explains the divergent effects recorded in different empirical studies.

Conclusions

This study provides mechanistic insights into the complex relationship between resource heterogeneity and diversity patterns. It highlights the context-dependent effects of small-scale heterogeneity, which can be positive under low-resource, neutral under high-resource, and negative under intermediate-resource conditions. These findings provide a foundation for future investigations into small-scale heterogeneity–diversity relationships, contributing to a deeper understanding of the processes that promote species co-existence in plant communities.     

Effects of spatial heterogeneity on butterfly species richness in Rocky Mountain National Park, CO, USA

We investigated butterfly responses to plot-level characteristics (plant species richness, vegetation height, and range in NDVI [normalized difference vegetation index]) and spatial heterogeneity in topography and landscape patterns (composition and configuration) at multiple spatial scales. Stratified random sampling was used to collect data on butterfly species richness from seventy-six 20 × 50 m plots. The plant species richness and average vegetation height data were collected from 76 modified-Whittaker plots overlaid on 76 butterfly plots. Spatial heterogeneity around sample plots was quantified by measuring topographic variables and landscape metrics at eight spatial extents (radii of 300, 600 to 2,400 m). The number of butterfly species recorded was strongly positively correlated with plant species richness, proportion of shrubland and mean patch size of shrubland. Patterns in butterfly species richness were negatively correlated with other variables including mean patch size, average vegetation height, elevation, and range in NDVI. The best predictive model selected using Akaike’s Information Criterion corrected for small sample size (AIC_c), explained 62% of the variation in butterfly species richness at the 2,100 m spatial extent. Average vegetation height and mean patch size were among the best predictors of butterfly species richness. The models that included plot-level information and topographic variables explained relatively less variation in butterfly species richness, and were improved significantly after including landscape metrics. Our results suggest that spatial heterogeneity greatly influences patterns in butterfly species richness, and that it should be explicitly considered in conservation and management actions.     

Coexistence through spatio-temporal heterogeneity and species sorting in grassland plant communities

The effect of spatial heterogeneity on species coexistence relies on the degree of niche heterogeneity in the habitat and the ability of species to exploit the available niche opportunities. We studied species coexistence in a perennial grassland, and tested whether small-scale disturbances create environmental heterogeneity that affects coexistence and whether the functional diversity of species in the species pool affects the ability of community composition to reflect heterogeneity through species sorting. We manipulated the spatio-temporal heterogeneity of disturbance and the functional diversity of species added as seed and measured their impact on the spatial turnover of species composition. Disturbance increased environmental heterogeneity and spatial turnover, and the effect of heterogeneity on turnover was greatest in the presence of a functionally diverse species pool, showing the importance of trait variation among species for exploiting environmental heterogeneity, and suggesting that coexistence occurred due to species sorting among heterogeneous niches.     

Nitrate leaching as a function of plant community richness and composition,and the scaling of soil nutrients,in a restored temperate grassland

Two, two-factor experiments manipulated species and functional form plant richness and the spatial scaling of either nitrogen (N) or phosphorous (P) in restored tallgrass prairie in North Dakota, USA. Nitrate (NO₃ ⁻) leaching was measured in these plots and analyzed for its response to the treatment factors and measured plant community parameters. Nitrate extracted from anion exchange resin was regressed against the first principal component of species and functional form richness, the spatial scaling of N or P, the measured biomass of the functional forms used and the plot values for plant parameters based on weighted averages by species biomass. The treatments applied in the N and P experiments were 1, 2, 5, 10, or 20 plant species taxa, and the application of fertilizer in a random fractal pattern with either fine-scale or coarse-scale heterogeneity. Nitrate leaching decreased with plant diversity and increased by a factor of two going from fine-scale to coarse-scale N. It was also related to a number of plant functional parameters, and was positively correlated with the biomass of late successional C₃ grasses (Koeleria cristata (Lam.) Beauv., Poa pratensis L., Stipa comata Trin. & Rupr., and Stipa viridula Trin.), which are known from previous studies to have negative mycorrhizal responsiveness and are characterized by high root lateral spread per unit of root biomass. Our results show that while plant diversity has a highly significant influence on plant community uptake of NO₃ ⁻, this effect is mediated by the scaling of soil N and the functional traits of the species comprising the plant assemblage.     

Mycorrhizal fungal identity and richness determine the diversity and productivity of a tallgrass prairie system

We investigated the effects of arbuscular mycorrhizal fungal (AMF) species richness and composition on plant community productivity and diversity, and whether AMF mediate plant species coexistence by promoting niche differentiation in phosphorus use. Our experiment manipulated AMF species richness and identity across a range of P conditions in tallgrass prairie mesocosms. We showed that increasing AMF richness promoted plant diversity and productivity, but that this AMF richness effect was small relative to the effects of individual AMF species. We found little support for AMF-facilitated complementarity in P use. Rather, the AMF richness effect appeared to be caused by the inclusion of particular diversity- and productivity-promoting AMF (a sampling effect). Furthermore, the identity of the diversity-promoting fungi changed with P environment, as did the relationship between the diversity-promoting and productivity-promoting benefits of AMF. Our results suggest that plant diversity and productivity are more responsive to AMF identity than to AMF diversity per se, and that AMF identity and P environment can interact in complex ways to alter community-level properties.     

Soil heterogeneity and community composition jointly influence grassland biomass

Question: Does the spatial pattern of nutrient supply modify community biomass responses to changes in both species composition and richness? Location: Duke University Phytotron (Durham, North Carolina, USA). Methods: We conducted a microcosm experiment to evaluate individual plant and whole community responses to species richness, species composition and soil nutrient heterogeneity. The experiment consisted of seven levels of species composition (all possible combinations of Lolium perenne, Poa pratensis and Plantago lanceolata) crossed with three levels of soil nutrient distribution (homogeneous, heterogeneous‐up, and heterogeneous‐down, where up and down indicates the location of a nutrient patch in either the upper or the lower half of the soil column, respectively). Results: Communities containing Plantago and Lolium responded to nutrient heterogeneity by increasing above‐ and below‐ground biomass. Nutrient heterogeneity also increased size inequalities among individuals of these species. Significant species composition X nutrient heterogeneity interactions on community biomass and individual size inequality were observed when nutrient patches were located in the upper 10 cm of the soil columns. However, root proliferation in nutrient patches was equivalent regardless of the vertical placement of the patch. Conclusions: Our results suggest that nutrient heterogeneity may interact with plant species composition to determine community biomass, and that small‐scale vertical differences in the location of nutrient patches affect individual and community responses to this heterogeneity.