Temporal niche segregation, or the temporal separation of resource use by species, promotes coexistence in many ecological communities. Evidence suggests that seasonal fluctuations in precipitation and temperature (i.e., seasonality) may be particularly important drivers of seasonal dynamics in herbaceous plant communities in which species with distinct resource-use strategies often dominate in different seasons. However, in these systems, nutrient enrichment may promote the dominance of species with specific resource-use strategies for longer periods of time and disrupt within-season community dynamics. Given the importance of temporal niche segregation to species coexistence in grasslands, elucidating how seasonality and fertilization affect seasonal community composition is essential to our understanding of how biodiversity is maintained in these valuable systems.
We used early-season and late-season compositional data collected from 10 grassland sites around the world that are part of the Nutrient Network, a globally replicated nutrient-enrichment experiment to: (1) assess the relationships between precipitation and temperature seasonality and seasonal beta-diversity, (2) examined the effects of fertilization on seasonal beta-diversity, and (3) assess whether species from specific functional groups (i.e., graminoid, forb, legume) or with specific characteristics (e.g., C3 vs. C4) drive changes in seasonal beta-diversity among seasons and fertilization treatments.
Using deviations from a null model of beta-diversity (i.e., Bray-Curtis dissimilarity) as our response, we found a significant negative relationship between precipitation seasonality and seasonal beta-diversity across study sites. This result is contrary to other studies and may arise from differences in results when observed beta-diversity vs. null models of beta-diversity are used to assess community dissimilarity. Nutrient enrichment had a small positive effect on seasonal temporal beta-diversity. Compositional differences were driven by unique shifts of species from specific functional groups or with specific characteristics among seasons and treatments. For example, nutrient enrichment resulted in higher cover of annuals and C3 grasses but lower cover of C4 grasses both early and late in the season. However, annual cover varied between early and late sampling dates in fertilized plots but not control plots, whereas C4 grass cover varied between early and late sampling dates in control plots but not fertilized plots. Our results demonstrate that nutrient enrichment can disrupt within-season plant community dynamics by altering the overall dominance of specific species across and within growing seasons. These findings advance our understanding of how biodiversity is maintained in grassland systems which are threatened by global change drivers such as nutrient enrichment.