Invasion by non-native annual grasses: The importance of species biomass, composition, and time among California native grasses of the Central Valley

The Central Valley of California is noted for its dearth of remnant native grass populations and for low native grass seedling establishment within grasslands now dominated by non-native annual species. In contrast, remnant populations are common along the coast, and studies have shown an ability for seedlings and adults to compete with non-native annual grasses. The invasibility of well-established populations of native grasses in the Central Valley remains unclear. The objectives of this study were to compare the invasibility of native grasses differing in density and species composition and, given the species in this study, to assess the ability of mixes with greater species richness to resist invasion relative to their abilities in monoculture. In the Sacramento Valley of California, six species of native grasses were planted at three densities in monospecific and mixed-species plots. Percent cover of native perennial and non-native annual grasses was measured in years 2 and 3, and biomass was sampled in year 5. Native grass biomass and, to a lesser extent, species composition were important in explaining variation in non-native grass invasibility in the fifth year. Species-rich treatments did not experience less invasion than would be expected by the proportional invasibility of each species in monoculture. However, invasibility of plots consisting of slower growing, shorter statured species decreased over time, suggesting a successional benefit to diverse communities. This study demonstrates that established stands of native grasses in the Sacramento Valley can resist invasion by non-native annual grasses and that stand biomass is a particularly important factor in determining invasibility.