Climate change and grassland restoration in California: lessons from six years of rainfall manipulation in a north coast grassland.
Native perennial bunchgrasses have undergone steep declines across much of California but persist in sizable populations along the northern coast. The longer rainy season and less severe summer drought in this region are thought to facilitate bunchgrass persistence in the face of extensive invasion by exotic annual species. Changes in the seasonality and intensity of precipitation that accompany global climate change could critically influence efforts to conserve and restore these plants in California grasslands. We established a large-scale manipulation of rainfall in a protected Mendocino County grassland to investigate how predicted shifts in precipitation affect the performance of three native perennial bunchgrass species in exotic-dominated stands. We added seeds, plugs, and mature tussocks of Danthonia californica, Elymus glaucus, and Elymus multisetus into replicate plots of exotic annual grassland and subjected the plots to one of three experimental precipitation regimes: increased winter rainfall, increased spring rainfall, and ambient rainfall. Responses to rainfall addition varied widely by age class and species and depended heavily on seasonal timing of the increase. Establishment from seed was rare for all three species and showed little response to water addition, likely due to concomitant changes in the surrounding communities. Production of exotic annual grasses rose markedly following repeated extensions of the rainy season, and while established bunchgrasses benefited despite this change, new plants could not establish into thickening stands of exotic vegetation. In contrast, survival was high for transplanted plugs and tussocks of all three species across all three rainfall treatments, suggesting that plugs and tussocks can survive a wide range of climatic conditions and high local densities of exotic annual grasses. Restoration approaches focused on these life stages may be most robust to changing climate. Transplanted individuals can provide a continual source of propagules to surrounding areas that then recruit during years in which conditions in the physical and biological environment are amenable to seedling establishment.