LAUSR

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 depends on variation in species composition across space (β-diversity), but empirical evidence is limited. Moreover, studies have not quantified the direct and indirect role the environment plays in co-structuring ecosystem functioning across spatial scales. We used 14 1.4-ha temperate-forest plots 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 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-m2 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.