Modeling vegetation distribution and ecosystem productivity under climate change scenarios for the Sierra Nevada, California

Background/Question/Methods

Forest ecosystems play an important role in maintaining climate stability as a vital carbon sink at regional and global scales.  Changing climatic conditions are projected to alter the distribution, composition and productivity of forest ecosystems globally.  Dynamic Global Vegetation Models (DGVMs) have been used to characterize vegetation dynamics to better understand the relationship between carbon cycling in the biosphere and atmospheric CO₂ concentrations and climate change.  DGVMs typically use coarsely gridded spatial climate data at global scales that are not accurate or useful for assessing regional or local effects of projected changes in climate.  This study used downscaled climate projections (800 m) in the Lund-Potsdam-Jena DGVM to assess future vegetation dynamics and productivity (Gross Primary Production (GPP)) under changing climate and atmospheric CO₂ concentrations in the Sierra Nevada, California.  The model used three different greenhouse emission scenarios: (1) constant atmospheric CO₂ concentration of 370 ppm; (2) Representative Concentration Pathways (RCP) 4.5, a stabilization scenario; and (3) RCP 8.5, a high emission scenario. 

Results/Conclusions

The largest changes in vegetation dynamics occurred in the Temperate Needleleaved Evergreen (TNE), the Temperate Broadleaved Evergreen (TBE) and the Boreal Needleleaved Evergreen (BNE) plant functional types. Although there was not much change in the spatial extent of TNE between the constant CO₂ scenario and the RCP scenarios, the mean percent fractional cover of TNE decreased for each scenario. This decrease primarily occurred elevations above 1800 m.  For all three scenarios, TBE increased in spatial extent and percent fractional coverage. TBE moved upslope and eastward as atmospheric CO₂ increased.  BNE, which now primarily occurs in the higher elevations and on the eastern side of the Sierra Nevada, showed a small decline in spatial extent but a large decline in percent fractional cover.   For GPP, there was a projected 12.2% increase for the constant CO₂ scenario, a 43.4% increase for the RCP 4.5 scenario and a 103.6% increase for the RCP 8.5 scenario due primarily to increases in the spatial extent and increased density of the more productive broadleaved TBE plant functional types.    Although actual observations of changing vegetation dynamics in the Sierra Nevada and other regions have been varied, DGVMs can provide valuable insight for decision makers on how to manage forests under future climate scenarios that impact forest patterns and processes.