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Despite the importance of biodiversity-ecosystem functioning (BEF) relationships in ecology and conservation, relatively little is known about how BEF relationships change across spatial scales. Theory predicts that change in BEF relationships with increasing spatial scale will depend on variation in species composition across space (β-diversity), but empirical evidence for this is limited. Moreover, studies have not quantified the direct and indirect role the environment plays in costructuring ecosystem functioning across spatial scales. We used 14 temperate-forest plots 1.4 ha in size containing 18,323 trees to quantify scale-dependence between aboveground tree biomass and three components of tree-species diversity-α-diversity (average local diversity), γ-diversity (total diversity), and β-diversity. Using structural-equation models, we quantified the direct effects of each diversity component and the environment (soil nutrients and topography), as well as indirect effects of the environment, on tree biomass across 11 spatial extents ranging from 400 to 14,400 m2 . Our results show that the relationship between β-diversity and tree biomass strengthened with increasing spatial extent. Moreover, β-diversity appeared to be a stronger predictor of biomass than α-diversity and γ-diversity at intermediate to large spatial extents. The environment had strong direct and indirect effects on biomass, but, in contrast to diversity, these effects did not strengthen with increasing spatial extent. This study provides some of the first empirical evidence that β-diversity underpins the scaling of BEF relationships in naturally complex ecosystems.

Despite decades of research on the species-pool concept and the recent explosion of interest in trait-based frameworks in ecology and biogeography, surprisingly little is known about how spatial and temporal changes in species-pool functional diversity (SPFD) influence biodiversity and the processes underlying community assembly. Current trait-based frameworks focus primarily on community assembly from a static regional species pool, without considering how spatial or temporal variation in SPFD alters the relative importance of deterministic and stochastic assembly processes. Likewise, species-pool concepts primarily focus on how the number of species in the species pool influences local biodiversity. However, species pools with similar richness can vary substantially in functional-trait diversity, which can strongly influence community assembly and biodiversity responses to environmental change. Here, we integrate recent advances in community ecology, trait-based ecology, and biogeography to provide a more comprehensive framework that explicitly considers how variation in SPFD, among regions and within regions through time, influences the relative importance of community assembly processes and patterns of biodiversity. First, we provide a brief overview of the primary ecological and evolutionary processes that create differences in SPFD among regions and within regions through time. We then illustrate how SPFD may influence fundamental processes of local community assembly (dispersal, ecological drift, niche selection). Higher SPFD may increase the relative importance of deterministic community assembly when greater functional diversity in the species pool increases niche selection across environmental gradients. In contrast, lower SPFD may increase the relative importance of stochastic community assembly when high functional redundancy in the species pool increases the influence of dispersal history or ecological drift. Next, we outline experimental and observational approaches for testing the influence of SPFD on assembly processes and biodiversity. Finally, we highlight applications of this framework for restoration and conservation. This species-pool functional diversity framework has the potential to advance our understanding of how local- and regional-scale processes jointly influence patterns of biodiversity across biogeographic regions, changes in biodiversity within regions over time, and restoration outcomes and conservation efforts in ecosystems altered by environmental change.

Relationships between biodiversity and ecosystem functioning depend on the processes structuring community assembly. However, predicting biodiversity‐ecosystem functioning (BEF) relationships based on community assembly remains challenging because assembly outcomes are often contingent on history and the consequences of history for ecosystem functions are poorly understood. In a grassland restoration experiment, we isolated the role of history for the relationships between plant biodiversity and multiple ecosystem functions by initiating assembly in three different years, while controlling for all other aspects of community assembly. We found that two aspects of assembly history—establishment year and succession—altered species and trait community trajectories, which in turn altered net primary productivity, decomposition rates, and floral resources. Moreover, history altered BEF relationships (which ranged from positive to negative), both within and across functions, by modifying the causal pathways linking species identity, traits, diversity, and ecosystem functions. Our results show that the interplay of deterministic succession and environmental stochasticity during establishment mediate historical contingencies that cause variation in biodiversity and ecosystem functions, even under otherwise identical assembly conditions. An explicit attention to history is needed to understand why biodiversity‐ecosystem function relationships vary in natural ecosystems: a critical question at the intersection of fundamental theory and applications to environmental change biology and ecosystem restoration. [ABSTRACT FROM AUTHOR]

Ecological restoration is notoriously unpredictable because similar actions can result in different outcomes. Outcomes can also differ for species and functional components of communities depending on how restoration actions and abiotic conditions alter community assembly trajectories. Quantifying variation in community trajectories across restorations for both species and traits is rare, but can help to resolve underlying assembly processes and refine strategies to maximize restoration success.We quantified the importance of soil resources, seed mix richness, and prescribed fire for variation in plant species and functional trait trajectories over 6 years across 20 restored tallgrass prairies in the mid‐western United States. We predicted stronger convergence for traits than species, with species and trait compositions converging more across restorations on resource‐poor soils and with frequent fires due to stronger abiotic filtering. In contrast, we predicted species and trait compositions would converge more slowly or diverge across restorations with resource‐rich soils, with less frequent fires, and seeded with more species due to weaker filtering and more stochasticity.Communities generally converged over time; however, the rate of convergence was determined by soil resources, not restoration actions. Restorations converged more across resource‐rich sites. In contrast to patterns of species convergence, variation in trait composition remained stable over time regardless of soil resources or restoration actions.The unexpected pattern of species, but not trait, convergence during community assembly appears to result from increasing dominance of a native C4 bunchgrass, Andropogon gerardii that coincides with proportional declines of other C4 grasses that share similar traits.Synthesis and applications. Restoration outcomes may be more predictable than typically considered. Trait compositions were stable and variation in species compositions decreased over time owing to site conditions, where resource‐rich soils produced more consistent outcomes. Our study shows monitoring multiple facets of biodiversity across restorations can reveal why outcomes vary and inform broad‐scale restoration planning. [ABSTRACT FROM AUTHOR]