Climate change is causing an increase in annual area burned in western US forests. Combined with hot and dry post-fire conditions, these changes may overcome the resilience mechanisms of fire-prone forests, resulting in ecosystem transitions or “fire-catalyzed change.” To quantify the potential for fire-catalyzed change, it is necessary to consider the combined likelihood of high-severity fire and the potential for climate to limit post-fire regeneration. Here we combined predictions of post-fire tree recruitment with existing projections of the likelihood of stand-replacing fire to quantify the vulnerability of ponderosa pine and Douglas-fir forests to fire-catalyzed change. We used a dataset of annually resolved establishment dates for 1847 ponderosa pine and Douglas-fir juveniles that regenerated following 26 fires in the US Intermountain West to model annual recruitment as a function of bioclimatic variables. We used this model to predict the probability of post-fire ponderosa pine and Douglas-fir recruitment for three decades (1980s, 2000s, and a +2°C warmer decade representing the future). We calculated the proportion of each intermountain ecoregion vulnerable to fire-catalyzed change by identifying where the probability of stand-replacing severity, if a fire were to occur, was high, and the probability of post-fire recruitment was low.
Results/Conclusions
Post-fire seedling recruitment had a nonlinear negative relationship with maximum temperature, vapor pressure deficit, and climatic water deficit. Across all intermountain ecoregions, 20% and 14% of the range of ponderosa pine and Douglas-fir, respectively, had climate conditions unsuitable for recruitment in the 1980s. These values increased to 31% and 19% in the 2000s, and 57% and 38% under the future climate scenario for ponderosa pine and Douglas-fir, respectively. Across all time periods, southern ecoregions had considerably lower climate suitability for post-fire recruitment than northern ecoregions. Vulnerability to fire-catalyzed change increased over time. Across the study region, 5.6% and 4.6% of the area was vulnerable to fire-catalyzed change in the 1980s, for ponderosa pine and Douglas-fir, respectively. These numbers increased to 14.1% (ponderosa pine) and 10.6% (Douglas-fir) under the future climate scenario. Our results suggest that over 10% of the range of ponderosa pine and Douglas-fir in the Intermountain West is vulnerable to fire-catalyzed change, given the likelihood of high-severity fire combined with the impacts of climate on post-fire tree regeneration. As fires continue to burn more area under climate change, post-fire recruitment failures due to warm and dry conditions will increasingly cause vegetation shifts in ponderosa pine and Douglas-fir forests.