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

Enhanced ecosystem carbon storage is a key component of many climate
mitigation pathways. The State of California has set an ambitious goal of
carbon neutrality by 2045, relying in part on enhanced carbon
sequestration in natural and working lands. We used statistical modeling,
including random forests and climate analogues, to explore the
climate-driven challenges and uncertainties associated with the goal of
long-term carbon sequestration in forests and shrublands. We found that
seasonal patterns of temperature and precipitation are strong controllers
of the spatial distribution of aboveground live carbon. RCP8.5 projections
of temperature and precipitation were estimated to drive decreases of 16.1
± 7.5% in aboveground live carbon by the end of the century, with coastal
areas of central and northern California and low/mid-elevation mountain
areas being most vulnerable. With RCP4.5 projections, declines were less
severe, with 8.8 ± 5.3% carbon loss. In either scenario, the increased
temperature systematically caused biomass declines, and the spread of
projected precipitation across 32 CMIP5 models introduced substantial
uncertainty in the magnitude of that decline. Projected changes in the
environmental niche for the 20 most biomass-dominant tree species revealed
widespread replacement of conifers by oak species in low elevations of
central and northern California, with corresponding decline in carbon
storage depending on expected migration rates. The spatial patterns of
vulnerability we identify may allow policymakers to assess where carbon
sequestration in aboveground biomass is an appropriate part of a climate
mitigation portfolio, and where future climate-driven carbon losses may be
a liability.