Simulating vegetation shifts and carbon cycling in Yosemite National Park

The vegetation in Yosemite National Park changed during the 20th century and may change in the 21st century in response to climate change. Vegetation surveys made during the 1930s and the 1990s provide benchmark records separated by 60 years. This study uses the MC1 dynamic global vegetation model to forecast and hindcast spatially explicit potential vegetation patterns in Yosemite National Park for the 20th and 21st centuries, and uses the vegetation surveys to calibrate the model and assess model performance. The model hindcast for the 20th century was for minor vegetation changes, the most significant being a shift from montane conifer forest to mixed forest in 5% of the Park area. However the vegetation surveys record an increase of montane conifer forest amounting to 15% of the Park area, most coming from subalpine forest. The fact that the 1997 survey is more closely matched by simulation results from earlier in the 20th century than by the simulation results for the late 20th century leads to an interpretation of the “hindcast” as being temporally ahead of the observed vegetation, and of the observed vegetation change as reflecting a transition from earlier climates. In contrast to the minimal change simulated for the 20th century, the simulations for the future show large changes in potential vegetation in the mid and late 21st century, brought about by rising temperatures and by a large increase in wildfire, especially near the end of the century. Results of these simulations at 800 m resolution are compared with results from earlier studies at coarser spatial resolution. The biogeography algorithm used in this study is described in detail. A metric for comparing two time series of annual vegetation maps is developed.