Projected climate change and increasing wildfire size are anticipated to alter forest composition and carbon (C) dynamics in many western US forests, which could alter the carbon carrying capacity (CCC). The Sierra Nevada Mountains are heterogeneous with diverse forest types and species spanning a substantial latitudinal and elevation gradient. The effects of climate-wildfire interactions on different species are likely to vary across this diverse landscape. Understanding how community assembly and carbon stores will respond to projected climate is paramount for quantifying their future potential for regulating atmospheric CO2 concentration. In this study, we used a forest landscape model, LANDIS-II, to examine the effects of projected climate and large wildfires on long-term forest CCC and community assembly in the Sierra Nevada. We implemented simulations on three transects along the latitudinal gradient of the mountain range. We simulated forest response for 590 years using historical (baseline) conditions of climate and wildfire and projected climate from three models (GFDL, CCSM3 and CNRM) forced by the A2 emission scenario in combination with corresponding climate-specific large wildfire projections. We used projected climate and wildfire from 2090-2099 to drive simulations beyond 2099. We used total ecosystem carbon (TEC) as the measure of CCC.
Results/Conclusions
We found that compared to baseline conditions, CCC was reduced in all transects assuming climate and wildfire stabilize at conditions projected by late 21st Century. The CCC reduction varied from -19% to -73%, across climate model projections and transect locations. Net ecosystem carbon balance (NECB) declined across all transects and forests became a C source under the GFDL and CNRM climate projections. Projected climate and wildfire resulted in substantial forest decline, with the largest decrease in forested extent (75%) occurring in the southern Sierra transect. Projected climate and wildfire altered forest composition and individual species’ contributions to total biomass. In general, drought-tolerant pines accounted for a larger fraction of total biomass while less drought-tolerant species’ contribution to total biomass declined over the course of the simulation period. These results demonstrate the potential for large-scale vegetation type conversion and suggest a steep decline in the contribution of Sierran forests to the terrestrial carbon sink.