Plant community impact on productivity: trait diversity or key(stone) species effects?

Outside controlled experimental plots, the impact of community attributes
on primary productivity has rarely been compared to that of individual
species. Here, we identified plant species of high importance for
productivity (key species) in >29,000 diverse grassland communities
in the European Alps, and compared their effects with those of
community-level measures of functional composition (weighted means,
variances, skewness, and kurtosis). After accounting for the environment,
the five most important key species jointly explained more deviance of
productivity than any measure of functional composition alone. Key species
were generally tall with high specific leaf areas. By dividing the
observations according to distinct habitats, the explanatory power of key
species and functional composition increased and key-species plant types
and functional composition-productivity relationships varied
systematically, presumably because of changing interactions and trade-offs
between traits. Our results advocate for a careful consideration of
species’ individual effects on ecosystem functioning in complement to
community-level measures.