Intraspecific trait changes have large impacts on community functional composition but do not affect ecosystem function

O_LIPlant functional traits can provide a more mechanistic understanding of community responses to global change and effects on ecosystem functions. In particular, nitrogen enrichment shifts trait composition by promoting dominance of fast growing, acquisitive plants (with high specific leaf area [SLA] and low leaf dry matter content [LDMC]), and such fast species have higher aboveground biomass production. Changes in mean trait values can be due to a shift in species identity, a shift in species relative abundance and/or a shift in intraspecific trait values. However, we do not know the relative importance of these three shifts in determining responses to global change and effects on function. C_LIO_LIWe quantified the relative importance of composition, abundance and intraspecific shifts in driving variation in SLA and LDMC. We collected leaf samples in a large grassland experiment, which factorially manipulates functional composition (slow vs. fast species), plant species richness, nitrogen enrichment and foliar fungal pathogen removal. We fitted structural equation models to test the relative importance of abundance shifts, intraspecific shifts and sown trait composition in contributing to overall variation in community weighted mean traits and aboveground and belowground biomass production. C_LIO_LIWe found that intraspecific shifts were as important as abundance shifts in determining community weighted mean traits, and even had large effects relative to a wide initial gradient in trait composition. Intraspecific trait shifts resulted in convergence towards intermediate SLA, in diverse communities, although convergence was reduced by nitrogen addition and enhanced by pathogen removal. In contrast, large intraspecific shifts in LDMC were not influenced by the treatments. Belowground biomass was reduced by SLA and increased by LDMC, while aboveground biomass increased in communities dominated by high SLA species. However, despite large intraspecific trait shifts, intraspecific variation in these traits had no effect on above or belowground biomass production. C_LIO_LIOur results add to a growing body of literature showing large intraspecific trait variation and emphasise the importance of using field sampled data to determine community composition. However, they also show that intraspecific variation does not affect ecosystem functioning and therefore trait response-effect relationships may differ between vs. within species. C_LI