UC Davis

The 7-million-acre Sacramento River Headwaters Region, and its surrounding 3-million- acre buffer zone, has an extraordinarily important role for biodiversity as well as water supply in California. Delineated by three mountain ranges (the northern Sierra, southern Cascade and Klamath-Trinity) it has a widely varied topography, range of soil types and geology that support an extraordinarily wide variety of plants and habitats. The region contains over 80% of California’s natural habitat types and hosts over 60% of its vertebrate species, 62 of which are imperiled. One of 33 globally recognised biodiversity “hot spots”, it is also the source of the vast majority of California’s utilized water, supplying the largest reservoirs which are the backbone of the state water system. Many species and habitats are already under assault in California due to habitat loss, cover type and land use conversion and habitat degradation. Climate change is clearly exacerbating this trend. This region has more intact habitats than most in California, and presents more opportunity to stem habitat loss and reverse degradation, due to its relatively lower human settlement footprint and population. Meta-analyses show that this region has remained cooler and wetter than the rest of the state over the past 100-125 years, providing a reliable context for consistent habitat function. With such a major role in supporting California’s biodiversity, the focus of this project was to evaluate how persistent this habitat service might be in the future 100 years as climate change intensifies. We utilized the two more plausible Representative Concentration Pathway (RCP) climate change scenarios, the RPC 4.5 and the RPC 8.5, and modelled these under both of the two leading global climate change models: the MIROC—which projects a warmer and drier future-- and the CRNM—which projects a wetter and warmer future—downscaled to California. Our project identified where the models agree that conditions supporting refugia—and connecting corridors/safe passage to them-- will be 2040, 2070 and 2100. Specifically, we identified where there was a consensus outcome of both models identifying where temperature and precipitation regimes would continue to support the natural habitats—and the amazing array of biodiversity they contain-currently existing in the region. These are critical areas for conservation and restoration, as they will likely continue to support current habitats even under climate change. Additionally, we identified where exposure to precipitation and temperature changes was relatively more moderate, and thus these areas have a strong potential for being able to be resilient under climate change, especially if these areas had been managed to have more natural resilience. These also have high value for conservation and restoration, as they may serve as buffers and risk mitigation allowing more opportunity for species to survive. Finally, we identified those areas where the projected changes in precipitation and temperature will cause environmental stress so great as to likely no longer support the existing habitats, and thus the biodiversity they support. These areas may have high value for conservation for a variety of reasons, but the habitats they support currently are not projected to persist.1Pacific Forest Trust, Final Report WC-1835JG