Intraspecific trait variation, functional turnover and trait differences among native and exotic grasses along a precipitation gradient.
Questions: California's grasslands are heavily invaded by exotic species. Counteracting these invasions depends on understanding the functional differences between native and exotic species and how these shift along environmental gradients. Focusing on grasses, we ask: (a) how native and exotic trait means and intraspecific trait variation (ITV) differ; (b) how the functional composition of grass assemblages shifts with precipitation; and (c) how species shift their functional strategies as precipitation changes. Location: Marin County, California (USA). Methods: In spring 2017, we censused grassland vegetation plots across eight reserves in Marin County that are arrayed along a precipitation gradient. At each plot, we measured traits of grasses, including height, leaf area and specific leaf area (SLA). Results: We found modest differences in functional traits between native and exotic species. Exotic species had larger seeds and higher SLAs, indicating higher reproductive investments and a leaf strategy focused on obtaining rapid returns on investments with relatively short leaf life spans. Native and exotic species did not differ in the ITV for any of the measured traits. Variation in precipitation among sites drove strong turnover in the functional trait composition of grasses, as well as shifts within species. Wetter sites had shorter species with smaller leaf areas, smaller seeds and higher leaf N concentrations. There was also strong intraspecific SLA variation along the precipitation gradient; all species displayed lower SLA values as precipitation increased. Conclusions: Consistent with previous results, native species had more conservative leaf strategies. Grasses that display these conservative strategies tend to be more abundant in dry climates, leading to increasing exotic abundance in wetter climates. However, each species shifted towards more conservative strategies as precipitation increased, suggesting that the factors that drive species turnover and ITV differ.