Grass trait–abundance relationships and the role of the functional composition of the neighboring community

Abstract

Questions: Plant traits can predict a species‘ relative abundance and its influence on ecosystem processes. However, trait expression and the relative abundance of a species are also influenced by its abiotic and biotic environment. Here, we ask whether the relationship between plant traits and relative abundance is modified by abiotic (e.g., climate and topography) and biotic factors (e.g., community taxonomic and functional diversity) across Californian grasslands. Location: San Francisco Bay Area. Methods: We measured specific leaf area (SLA; leaf area/dry mass) and plant height of 19 grass species (family: Poaceae) across 117 plots. We also quantified the relative abundance of each species as well as several biotic attributes of the neighboring grass community including total plant cover, species richness and evenness, community-weighted mean (CWM) traits, and functional diversity. Using multiple linear regression, we assessed whether abundance could be predicted from traits and the interactions between traits and both biotic and abiotic factors. We fit similar models predicting traits from relative abundance. Results: Grass species had higher relative abundance in plots where they were taller and had higher SLA. They were also more abundant in communities with low functional richness (FRic) and high functional evenness (FEve), perhaps because of the low resource-use efficiency of their neighbors and a lack of dominant grasses. Neither abundance nor plant height were associated with abiotic variables, although SLA responded predictably to precipitation according to a bell-shaped curve. Grasses were taller where they were more abundant, but the impact of abundance on SLA depended on community FEve. Finally, we show strong evidence for community trait similarity, whereby an individual‘s trait expression was positively correlated with the traits of its grass neighbors. Conclusions: Taken together, these results imply that traits are predictive of abundance and vice versa, and these relationships depend on biotic interactions more than climate.