Response variation across a strong rainfall gradient in two naturalized annual grass species.
Non-native annual grasses in California are a functional group based on taxonomic similarity, but they may be dissimilar in terms of evolutionary potential and capacity for plastic response. The ecological character of each species is conditioned by their evolutionary history, the genetic circumstances of their introduction, and their subsequent interactions with the selective pressures in their new ranges. Early population ecology studies revealed regional monomorphism and population genetic uniformity as well as strong patterns of local adaptation. Recent studies have both challenged and verified those conclusions, and have also contributed useful new information regarding the genetic makeup of these non-native grasses. We used two very abundant and widespread species to test the prediction that non-native annual grasses would show similar adaptive responses under the same environmental conditions. Using six populations of two naturalized annual grass species collected from identical locations across a strong south-to-north rainfall gradient, we manipulated soil quality and soil moisture to test response in growth, reproduction, and phenology. Avena barbata populations showed strong response conditioned by resource availability and with a correlated shift in flowering phenology. In contrast, Hordeum murinum populations showed very uniform responses to changes in resource availability independent of their position on the rainfall gradient and with no correlated phenological shifts. Several genetic and historical factors may contribute to idiosyncratic responses and general adaptive patterns in these and other non-native species, and a better understanding of these factors will help us understand their current distributions.