California Institute for Energy and Environment (CIEE)

The objective of this study was to dynamically simulate the response of vegetation distribution, carbon, and fire to three scenarios of future climate change for California using the MAPSSCENTURY (MC1) dynamic general vegetation model. Under all three scenarios, Alpine/Subalpine Forest cover declined with increased growing season length and warmth, and increases in the productivity of evergreen hardwoods with increased temperature led to the displacement of Evergreen Conifer Forest by Mixed Evergreen Forest. The simulated responses to changes in precipitation were complex, involving not only the effect on vegetation productivity, but also changes in tree-grass competition mediated by fire. Grassland expanded, largely at the expense of Woodland and Shrubland, even under the relatively cool and moist PCM-A2 climate scenario where increased woody plant production was offset by increased wildfire. Increases in net primary productivity (NPP) under the PCM-A2 climate scenario contributed to a simulated carbon sink of about 321 teragrams (353.8 million tons) for California by the end of the century. Declines in net primary productivity (NPP) under the two warmer and drier GFDL climate scenarios, most evident under the GFDL-A2 scenario, contributed to a net loss of carbon ranging from about 76 to 129 Tg (83.8 to 142.2 million tons) by the end of the century. Total annual area burned in California increased under all three scenarios, ranging from 9%–15% above the historical norm by the end of the century. Regional variation in the simulated changes in area burned was largely a product of changes in vegetation productivity and shifts in the relative dominance of woody plants and grasses. Annual biomass consumption by fire by the end of the century was about 18% greater than the historical norm under the more productive PCM-A2 scenario. Under the warmer and drier GFDL scenarios, simulated biomass consumption was also greater than normal for the first few decades of the century as droughtstressed woodlands and shrublands burned and were converted to grassland. After this transitional period, lower than normal NPP produced less fuel, and biomass consumed was at, or below, the historical norm by the end of the century under the GFDL scenarios. Considerable uncertainty exists with respect to regional-scale impacts of global warming on the natural ecosystem of California. Much of this uncertainty resides in the differences among different GCM climate scenarios and assumed trajectories of future greenhouse gas emissions as illustrated in this study. In addition, ecosystem models and their response to projected climate change can always be improved through careful testing and enhancement of model processes. The direct effects of increasing CO2 on ecosystem productivity and water use, and assumptions regarding fire suppression and the availability of ignition sources, were identified as sources of uncertainty to be addressed through further model testing and development.