Climate-driven limits to future carbon storage in California’s wildland ecosystems

Abstract:

Increased ecosystem carbon storage is a key component of climate mitigation strategies. The State of California has set a goal of carbon neutrality by 2045, on a pathway that involves reversing the current trend of land carbon loss and increasing total ecosystem carbon stocks by roughly 4.4% by 2100. In this study we use a variety of statistical forecasting methods, including random forests and climate analogues, to explore the climate-driven challenges and uncertainties associated with future carbon storage in California’s forests and shrublands. We find that seasonal patterns of temperature and precipitation are strong controllers of the spatial distribution of aboveground live carbon. With a space-for-time substitution, we find that RCP8.5 projections of temperature and precipitation will likely drive decreases of approximately 15% in aboveground live carbon, with coastal and low/mid-elevation mountain areas being most vulnerable. Declines are only 5% under wet scenarios but up to 22% under dry scenarios. In general, even under RCP4.5, the increased temperature systematically causes biomass declines, and the spread of projected precipitation across 32 CMIP5 models introduces substantial uncertainty in the magnitude of that decline. However, the largest source of uncertainty is the capacity of different tree species to migrate to areas of future habitability, quantified through random forest niche models of 20 different species. As a whole, this study explores the uncertainties associated with future wildland biomass distributions, highlights the spatial patterns of vulnerability, and identifies areas where targeted management could have the largest impact on facilitating carbon sequestration.