Fire has shaped western ponderosa pine (Pinus ponderosa) forest landscapes for millennia. Yet, contemporary land management coupled with warming and drought has led to shifts in forest structure and severe wildfires. A growing body of evidence suggests that under altered fire regimes and climate change, ponderosa pine forests may be vulnerable to fire-driven conversion to a different forest type or non-forest vegetation. However, the extent and direction of recent fire-induced changes in southwestern US ponderosa pine forests have not been subject to region-wide evaluation. Here, our objective was to assess recent fire effects in ponderosa pine forests using long-term repeated samples of stand composition and structure from the US Forest Service’s Forest Inventory and Analysis (FIA) program and satellite-derived burn severity (predicted Composite Burn Index; CBI and difference normalized burn ratio; dNBR). We compiled and analyzed FIA plots dominated by ponderosa pine and associated species within the southwestern states of Arizona and New Mexico to quantify regional trends for ponderosa pine (e.g., forest losses or gains), link changes to wildfire severity, and characterize vegetation changes. Among our 685 plots, we found 26% of plots burned at least once from 1996-2017. Plots that burned within the study period exhibited a 46% loss of ponderosa pine trees and plots that did not burn decreased by 11%. Small ponderosa pine trees (12.7-24.5 cm diameter) exhibited the greatest declines in the number of trees and basal area compared to trees greater than 24.5 cm. Overall regeneration rates decreased over time, and approximately 11% of plots lost all ponderosa pine. Satellite-derived burn severity (predicted CBI) was a strong predictor of tree mortality and more than half of burned plots burned at moderate-high severity levels. Post-fire vegetation was influenced by fire severity and we observed transitions in species composition, with resprouting species (Quercus gambelii) establishing post-fire more than any other new species. This study contributes to an emerging ecological understanding of forest vulnerability to changing disturbance regimes. Methods employed herein offer scalable opportunities to quantify changes across forest biomes using long-term monitoring data. As importantly, our findings inform regional and local land management efforts to sustain these valued forest types in an era of change. Our results point toward two key themes for land management: restoring low-severity fire regimes and retaining large trees to ensure the long-term persistence of ponderosa pine forests in the southwest US.
