Abstract
In many grasslands, species with specific traits occupy unique temporal positions within communities. Such intra-annual segregation is predicted to be greatest in systems with high variability (i.e., seasonality) in precipitation and temperature because fluctuating environmental conditions select for species that capitalize on resources at different times. However, because most studies on intra-annual community dynamics have been conducted at individual sites, relationships between seasonality and intra-annual turnover at global scales have not yet been identified. Furthermore, the same characteristics that promote species-specific responses to seasonal fluctuations in environmental conditions may also drive species-specific responses to global change drivers such as eutrophication. Research provides evidence that eutrophication alters inter-annual plant community dynamics yet understanding of how it alters intra-annual dynamics remains limited.
We used early-season and late-season compositional data collected from 10 grassland sites around the world to ask how seasonality in precipitation and temperature as well as nutrient enrichment shape intra-annual turnover in plant communities. In addition, we assessed whether changes in the abundances of specific functional groups including annual forbs, perennial forbs, C3 and C4 graminoids, and legumes underpin compositional differences between early- and late-season communities and treatments. We found that temperature seasonality and intra-annual turnover were positively related but observed no relationship between precipitation seasonality and intra-annual turnover. These results conflict with theoretical predictions from earlier studies which suggest precipitation seasonality drives intra-annual turnover in plant communities. In addition, nutrient enrichment increased intra-annual turnover; this pattern was underpinned by increased replacement of species between early- and late-season communities in fertilized conditions. This finding mirrors patterns observed at inter-annual scales and suggests fertilization can alter compositional dynamics via similar mechanisms at varied temporal scales. Finally, fertilization reduced the abundance of C4 graminoids and legumes and eliminated intra-annual differences in these groups. In contrast, fertilization resulted in intra-annual differences in C3 graminoids which were not observed in control conditions, and increased abundance of C3 graminoids and annual forbs overall. Our study provides novel insight into how seasonality and nutrient enrichment shape intra-annual grassland dynamics and highlights how discerning these patterns is essential to our understanding of biodiversity in these systems.
Competing Interest Statement
The authors have declared no competing interest.