Forest resilience to climate change and disturbances are of escalating concern globally, because of the potential for forest loss or major shifts in forest composition. We used a large, multi-regional dataset documenting post-fire tree regeneration in the U.S. Rocky Mountains to ask if and how climate change over the last several decades has impacted the rate and density of post-fire tree regeneration. We analyzed data from 1485 sites; these included field measurements on >62,000 tree seedlings across 52 wildfires that burned between 1988-2011.
Results/Conclusions
Greater moisture stress, largely associated with climatic warming, resulted in less tree regeneration, particularly in low-elevation, dry forests, indicating reduced resilience. Recruitment failures were more common and tree seedling densities were lower in more recent wildfires, especially for fires that burned after 1999. At sites that burned before 2000 and that possessed post-fire tree seedlings, 30% had seedling densities lower than needed to return to pre-fire tree densities, whereas this increased to 54% of sites that burned in fires occurring after 2000. Results are consistent even after taking into account the effects of time-since-fire on post-fire regeneration. Our results highlight post-fire climate (i.e., 3-year average post-fire), 30-year climate water deficit (i.e., average annual water deficit from 1985-2015), and seed availability as dominant controls of post-fire tree regeneration for wildfires before 2000, whereas only the 30-year climate trends were predictive of successful regeneration for fires after 2000. Recent wildfires are promoting shifts in forest cover where trees fail to regenerate post-fire, implying the initiation of a major climate-induced reduction in low elevation and low latitude forest extent and tree density with important consequences for carbon storage, water regulation and supply, and other ecosystem services.