Short-interval severe fire erodes the resilience of subalpine lodgepole pine forests

Significance Increased burning in subalpine and boreal forests dominated by obligate seeders and historically characterized by infrequent, stand-replacing fires has raised the specter of novel fire regimes in which young forests reburn before having recovered from previous fire. Empirical study of forest responses to such changing fire regimes is challenging; trees are long lived, the timing and location of fires are unpredictable, and forest responses unfold slowly. Short-interval stand-replacing fires in lodgepole pine forests of Greater Yellowstone led to substantial losses of biological legacies and reduced tree regeneration, which together delayed simulated recovery of aboveground carbon for >150 years. Results suggest profound changes in forest structure and function if short-interval fires become more common in a warmer world with more fire. Subalpine forests in the northern Rocky Mountains have been resilient to stand-replacing fires that historically burned at 100- to 300-year intervals. Fire intervals are projected to decline drastically as climate warms, and forests that reburn before recovering from previous fire may lose their ability to rebound. We studied recent fires in Greater Yellowstone (Wyoming, United States) and asked whether short-interval (<30 years) stand-replacing fires can erode lodgepole pine (Pinus contorta var. latifolia) forest resilience via increased burn severity, reduced early postfire tree regeneration, reduced carbon stocks, and slower carbon recovery. During 2016, fires reburned young lodgepole pine forests that regenerated after wildfires in 1988 and 2000. During 2017, we sampled 0.25-ha plots in stand-replacing reburns (n = 18) and nearby young forests that did not reburn (n = 9). We also simulated stand development with and without reburns to assess carbon recovery trajectories. Nearly all prefire biomass was combusted (“crown fire plus”) in some reburns in which prefire trees were dense and small (≤4-cm basal diameter). Postfire tree seedling density was reduced sixfold relative to the previous (long-interval) fire, and high-density stands (>40,000 stems ha−1) were converted to sparse stands (<1,000 stems ha−1). In reburns, coarse wood biomass and aboveground carbon stocks were reduced by 65 and 62%, respectively, relative to areas that did not reburn. Increased carbon loss plus sparse tree regeneration delayed simulated carbon recovery by >150 years. Forests did not transition to nonforest, but extreme burn severity and reduced tree recovery foreshadow an erosion of forest resilience.