Functional traits predict species responses to environmental variation in a California grassland annual plant community

1. Turnover in species composition and the dominant functional strategies in plant communities across environmental gradients is a common pattern across biomes, and is often assumed to reflect shifts in trait optima. However, the extent to which community-wide trait turnover patterns reflect changes in how plant traits affect the vital rates that ultimately determine fitness remain unclear. 2. We tested whether shifts in the community-weighted means of four key functional traits across an environmental gradient in a southern California grassland reflect variation in how these traits affect species' germination and fecundity across the landscape. 3. We asked whether models that included trait-environment interactions help explain variation in two key vital rates (germination rates and fecundity), as well as an integrative measure of fitness incorporating both vital rates (the product of germination rate and fecundity). To do so, we planted seeds of 17 annual plant species at 16 sites in cleared patches with no competitors, and quantified the lifetime seed production of 1360 individuals. We also measured community composition and a variety of abiotic variables across the same sites. This allowed us to evaluate whether observed shifts in community-weighted mean traits matched the direction of any trait-environment interactions detected in the plant performance experiment. 4. We found that commonly measured plant functional traits do help explain variation in species responses to the environment-for example, high-SLA species had a demographic advantage (higher germination rates and fecundity) in sites with high soil Ca:Mg levels, while low-SLA species had an advantage in low Ca:Mg soils. We also found that shifts in community-weighted mean traits often reflect the direction of these trait-environment interactions, though not all trait-environment relationships at the community level reflect changes in optimal trait values across these gradients. 5. Synthesis. Our results show how shifts in trait-fitness relationships can give rise to turnover in plant phenotypes across environmental gradients, a fundamental pattern in ecology. We highlight the value of plant functional traits in predicting species responses to environmental variation, and emphasise the need for more widespread study of trait-performance relationships to improve predictions of community responses to global change.