Increased burning in a warming climate reduces carbon uptake in the Greater Yellowstone Ecosystem despite productivity gains

Abstract

Abstract The effects of changing climate and disturbance on mountain forest carbon (C) stocks vary with tree species distributions and over elevational gradients. Warming can not only increase C uptake by stimulating productivity at high elevations but also enhance C release by increasing respiration and the frequency, intensity and size of wildfires. To understand the consequences of climate change for temperate mountain forests, we simulated interactions among climate, wildfire, tree species and their combined effects on regional C stocks in forests of the Greater Yellowstone Ecosystem, USA (GYE) with the LANDIS‐II landscape change model. Simulations used historical climate and future potential climate represented by downscaled projections from five general circulation models (GCMs) that bracket the range of variability under the representative concentration pathway (RCP) 8.5 emissions scenario. Total ecosystem C increased by 67% through 2100 in simulations with historical climate, and by 38%–69% with GCM climate. Differences in C uptake among GCMs resulted primarily from variation in area burned, not productivity. Warming increased productivity by extending the growing season, especially near upper tree line, but did not offset biomass losses to fire. By 2100, simulated area burned increased by 27%–215% under GCM climate, with the largest increases after 2050. With warming >3°C in mean annual temperature, the increased frequency of large fires reduced live C stocks by 4%–36% relative to the control, historical climate scenario. However, relative losses in total C were delayed under GCMs with large increases in summer precipitation and buffered by C retained in soils and the wood of fire‐killed trees. Increasing fire size limited seed dispersal, and reductions in soil moisture limited seedling establishment; both effects will likely constrain long‐term forest regeneration and C uptake. Synthesis. Forests in the GYE can maintain a C sink through the mid‐century in a warming climate but continued warming may cause the loss of forest area, live above‐ground biomass and, ultimately, ecosystem C. Future changes in C stocks in similar forests throughout western North America will depend on regional thresholds for extensive wildfire and forest regeneration and therefore, changes may occur earlier in drier regions.