Climate patterns shape forest wildfire recovery

Scenes of fire-scorched hillsides and charred tree trunks are becoming more common every year as climate change causes wildfires to intensify. It’s less clear how the warming climate affects forests’ ability to recover after fires. Models that explore how climate variability shapes that recovery are limited in spatial and temporal scope. Caitlin Littlefield (University of Montana) and colleagues now confirm that severe droughts are making it increasingly difficult for ponderosa pine trees in the western US to recover after a fire and that the most severely stricken regions alternate between the Northern Rockies and Southwest US every four years. The finding highlights the need for forest restoration and replanting within a critical window of time following a fire.

Littlefield and her colleagues first examined anomalies in climatic water deficits across the western states from 1980 to 2019. They found that a north–south climatic dipole rendered one region’s growing season anomalously dry on average every four years and the other’s anomalously wet. Furthermore, the average values for both the dry and wet periods steadily became warmer and drier over time. Then, the researchers examined records of when ponderosa seedlings established after fires across the Northern Rocky Mountains and the US Southwest. In both regions, the seedlings’ likelihood of survival was significantly worse during the anomalously dry period than the anomalously wet one. That likelihood gradually decreased as both regions warmed.

Additionally, by building on three decades of US Forest Service data, Littlefield calculated the likelihood that seedling trees were present during postfire periods. Her models led to the conclusion that drought conditions not only hindered short-term forest recovery after a fire, but they also had long-term implications for conifer presence. For the most extreme drought anomalies, young pine trees were one-fourth to one-third as likely to be present several decades after a fire as were trees that began their recovery in a wet postfire year.

The results indicate that variations in large-scale climate dynamics, combined with a trend in warming, hinder tree seedling regeneration. Even if average temperature and moisture conditions stay within values tenable for seedlings to grow, strong anomalies could continue to force ecosystems away from conditions that are suitable for postfire recovery. (C. E. Littlefield et al., Proc. Natl. Acad. Sci. USA, 2020, doi:10.1073/pnas.2007434117 .)