TOP-DOWN CLIMATIC PROCESSES MODULATE BIODIVERSITY-FUNCTIONING RELATIONSHIPS ACROSS NORTH AMERICAN FORESTS

Abstract

Experimental evidence indicates that more diverse communities sustain higher levels of ecosystem functioning. Generally, bottom-up processes accounting for community selection and complementarity effects on biodiversity are used to explain biodiversity effects on ecosystem functioning. However, top-down macroecological processes can also influence the distribution of ecosystem functions across large environmental gradients and, importantly, biodiversity patterns. Here, we tested whether past climate instability, current climate, and climate-driven species abundance explain tree richness gradients, and therefore, determine the relationship between species richness and ecosystem functioning using 137,808 forest inventory plots across continental extents including forests of Canada, the US, and Mexico. Consistent with previous research, we found a positive monotonic relationship between species richness and biomass (measured as level stand basal area) across North American forests. However, structural equation models revealed that the indirect effect of past climate instability, current climate conditions, and climate-driven individuals abundance underlie species richness patterns and the positive covariation between species richness and biomass. The importance of top-down climatic processes varied from boreal to tropical regions, yet their significant effects were pervasive across all forest types, indicating that climate strongly modulates the strength of the relationships between species richness and basal area. Our results help to understand the assembly processes driving the ubiquitous positive monotonic relationship between species richness and basal area across continental extents. This knowledge, obtained from adopting a macroecological perspective, helps understanding biodiversity-functioning relationships, which are usually explained by mechanisms at the community level. Significance statementMore biodiverse communities sustain higher levels of ecosystem functions (BEF hypothesis). The role of biotic interactions among trees within ecological communities determining BEF has been widely studied, but it is not well known how past climatic variability, current climate, and species abundance determine BEF. Using 137,808 forest inventory plots across North American forests located in Canada, the US, and Mexico, we document a positive relationship between tree species richness and stand basal area. Detailed analysis revealed that the positive BEF relationship is underlined by a strong and indirect effect of past and current climate. Our results highlight the importance of top-down climate processes operating at macroecological scales to understand biodiversity-functioning relationships.