Forest pattern, fire, and climatic change in the Sierra Nevada

In the Sierra Nevada, distributions of forest tree species are largely cont rolled by the soil-moisture balance. Changes in temperature or precipitatio n as a result of increased greenhouse gas concentrations could lead to chan ges in species distributions. In addition, climatic change could increase t he frequency and severity of wildfires. We used a forest gap model develope d for Sierra Nevada forests to investigate the potential sensitivity of the se forests to climatic change, including a changing fire regime. Fuel moist ure influences the fire regime and couples fire to climate. Fires are also affected by fuel loads, which accumulate according to forest structure and composition. These model features were used to investigate the complex inte ractions between climate, fire, and forest dynamics. Eight hypothetical cli mate-change scenarios were simulated, including two general circulation mod el (GCM) predictions of a 2 x CO2 world. The response of forest structure, species composition, and the fire regime to these changes in the climate we re examined at four sites across an elevation gradient. Impacts on woody bi omass and species composition as a result of climatic change were site spec ific and depended on the environmental constraints of a site and the enviro nmental tolerances of the tree species simulated. Climatic change altered t he fire regime both directly and indirectly. Fire frequency responded direc tly to climate's influence on fuel moisture, whereas fire extent was affect ed by changes that occurred in either woody biomass or species composition. The influence of species composition on fuel-bed bulk density was particul arly important. Future fires in the Sierra Nevada could be both more freque nt and of greater spatial extent if GCM predictions prove true.