Differences between introduced and native grassland species revealed by responses to prolonged drought

Introduced species are a typical feature of modern plant communities and experience environmental challenges alongside native species, though introduced species are often hypothesized to be functionally distinct. Changes in the environment may reveal distinct responses to environmental change for native and introduced components of plant communities. We tested for differences in native and introduced plant community composition in California grasslands collected during a period of severe drought. Sampling was conducted across serpentine soils, which are resilient against colonization by introduced species and maintain diverse native species assemblages, and across heavily invaded non-serpentine soils. On native dominated serpentine soils, there was no relationship between climatic variables and community composition over the survey period. On heavily invaded non-serpentine soils, compositional differences were significantly associated with precipitation. In particular, the native component of serpentine soils was only weakly correlated with current year precipitation and the native component of non-serpentine soils did not differ from null communities. In contrast, introduced species retained strong associations with precipitation on both soil types. We identified three introduced Eurasian grasses, Bromus hordeaceus, Elymus caput-medusae, and Avena fatua, as the greatest contributors to compositional change in non-serpentine communities. These analyses suggest that the majority of compositional change during this period of severe drought manifested within introduced species. Our results are consistent with a hypothesis that communities adapted to low-stress, high-competition plant conditions will experience substantial changes under drought, while communities adapted to high-stress, low-competition conditions exhibit greater resilience.