Multidimensional structure of grass functional traits among species and assemblages

Abstract Questions Research efforts have sought to understand trait–trait relationships among species and trait–environment relationships. However, connections between these two approaches are rare, despite the fact that species‐level trait–trait correlations constrain the possible trait–environment correlations. We ask how functional traits of grasses are related to each other and to environmental variation. Location Global, with particular focus on the continental United States. Methods We compiled distribution data for grasses with three spatial grains – TDWG Level 3 ‘botanical countries’, US counties and vegetation plots within the US. We combined these data with trait data compiled from published sources for 14 traits describing physical and chemical features of the leaves, seeds, roots and entire plant. Trait–trait relationships were explored using correlations and PCA, and trait–environment relationships using regression. Finally, we implemented a null model to predict trait–trait correlations at the assemblage level from those at the species level. Results The functional trait composition of grass species varied strongly along environmental gradients. At the species level, there were two main clusters of related traits – one describing general plant size (including height, seed mass, leaf size and rooting depth), and one describing the leaf economics spectrum (including specific leaf area, N mass and P mass ). Most trait–trait correlations at the assemblage level did not differ significantly from that predicted from the species level, suggesting that the former are strongly constrained by the latter. Trait–trait and trait–environment relationships in grasses were broadly similar to those observed for other groups, with some exceptions related to the particular growth form, physiology and ecology of grass species. Conclusions The unique evolutionary history and ecological role of grasses has led to some unusual trait–climate relationships in the group. Co‐variation among traits at the species level is an important template upon which environmental filters act to determine assemblage trait composition.