Along environmental gradients, plants shift their ecological strategies, and therefore their functional traits. This environmentally-driven intraspecific trait variation is a potentially important but relatively little studied determinant of species distributions and interaction outcomes. Recently introduced exotic species may show relatively little intraspecific trait variation along environmental gradients because their genetic diversity is often low and they have had little time to adapt to local conditions in their new range. We sampled grassland communities in eight sites, across which annual precipitation ranges from 70 to 124 cm/year. We established vegetation plots at each site and measured functional traits (specific leaf area [SLA], height, leaf size) from individuals of each grass species occurring in each plot. In addition, we obtained seed mass and leaf N concentrations from published sources. We performed trait gradient analysis for each trait. This is an analytical approach that decomposes a species’ trait value into two components: one that describes its typical position along a gradient (beta), and one that describes how its trait value compares to species with which it co-occurs (alpha). We ask how native and exotic species differ in their mean traits, alpha and beta decompositions, and intraspecific trait variation along this precipitation gradient.
Results/Conclusions
Exotic grasses had higher SLA and seed mass than native grasses. Grass assemblages with high SLA tended to occur in dry, south-facing sites. As conditions shifted towards wetter and more north-facing, all but one species (the native grass Elymus multisetus) accordingly increased their SLA, revealing remarkably strong coordination of intraspecific trait shifts along this gradient. The trait gradient analysis for SLA revealed that native and exotic grasses do not segregate strongly along the gradient – that is, their beta components are indistinguishable. However, the alpha component clearly separated native and exotic species, with exotic species having higher SLA than the native species with which they co-occur. Results for height and leaf size were similar, revealing broad overlap of the two groups along the beta component, but clearer separation along the alpha component. However, there was no strong evidence that either group exhibits higher flexibility of traits along the environmental gradient. This suggests that site-to-site variation in trait states likely reflects phenotypic plasticity rather than local adaptation.