Modeling the effects of projected climate on forest diversity and carbon dynamics in the Sierra Nevada, California
The Sierra Nevada spans substantial latitudinal and elevation gradients, and is occupied by a diversity of forest types. While forest types are relatively consistent along the latitudinal gradient, they sort along the elevation gradient. The predominant weather patterns that influence the mountain range vary with latitude. Generally, the northern Sierra receives more precipitation, while the southern sierra tends to be drier. Understanding how forest growth and succession will respond to changing climatic conditions is paramount for improving our understanding of climate-driven changes in forest type distribution and the carbon cycle, both of which are necessary for informing climate change mitigation and adaptation efforts. We used a landscape forest succession model, LANDIS-II, to quantify the effects of projected changes in climate on forest growth in the Sierra Nevada. We parameterized the initial communities within the study area using US Forest Service Forest Inventory and Analysis data. We simulated forest response over one hundred years (2000-2099) using historical climate (PRISM climate dataset) and three downscaled general circulation model (GCM) projections (GFDL, CNRM and CCSM3), driven by the A2 emission scenario. We sought to determine the forest response to different climate projections and compare forest trajectories across the mountain range.
We found that climate exerts diverse effects on forest biomass across the latitudinal gradient and simulation period. By assuming no disturbance, the A2 climate projections generally resulted in reduced above ground biomass (AGB) compared to historical climate across the latitudinal gradient. In northern areas, the late-century mean difference in AGB from projections with historical climate was -23.8, -3.2 and 9.5 Mg ha-1 and in the southern portions it was -17.0, -6.9 and -5.9 Mg ha-1 under GFDL, CNRM and CCSM3, respectively. The AGB in the northern part of the study region was consistently higher than that of the southern region throughout the simulation period. Similar patterns occurred for aboveground net primary productivity (ANPP), indicating that drought stress might be exacerbated in the southern portion of the Sierra as climate continues to warm. Growth varied by species across the study region. In the southern region, drought intolerant species (e.g. firs) had a negative growth response, while drought tolerant species (e.g. pines) had a neutral or positive growth response with warming climate. These results suggest that managing for more drought tolerant species may help buffer climate-driven declines in ANPP in the drier southern Sierra.