Nutrient enrichment alters seasonal β-diversity in global grasslands

Intra-annual (i.e. seasonal) temporal niche partitioning is essential to the maintenance of biodiversity in many plant communities. However, understanding of how climate and global change drivers such as eutrophication influence seasonal niche partitioning in plant assemblages remains limited. We used early-season and late-season compositional data collected from 10 grassland sites around the world to explore relationships between climate variability and intra-annual species segregation (i.e. seasonal β-diversity) and to assess how nutrient enrichment alters seasonal β-diversity in plant communities. We then assessed whether changes in seasonal β-diversity in response to nutrient enrichment are underpinned by species turnover or nestedness and determined how specific functional groups (i.e. annual forbs, perennial forbs, C3 and C4 graminoids and legumes) respond to eutrophication within and across early and late sampling dates. We found a positive relationship between intra-annual temperature variability and seasonal β-diversity but observed no relationship between intra-annual precipitation variability and seasonal β-diversity. Nutrient enrichment increased seasonal β-diversity and increased turnover of species between early- and late-season communities. Nutrient enrichment reduced the abundance of C4 graminoids and legumes within and across sampling timepoints and eliminated intra-annual differences in these groups. In contrast, nutrient enrichment resulted in seasonal differences in C3 graminoids, which were not observed in control conditions and increased abundance of C3 graminoids and annual forbs within and across early and late sampling dates. Synthesis: Our understanding of how grasslands respond to various components of global change is primarily based on studies that document community changes at inter-annual scales. Using early-season and late-season compositional data from 10 grassland sites around the world, we show that nutrient enrichment increases seasonal β-diversity and alters intra-annual dynamics of specific functional groups in unique ways.