Repeated fire and extended drought influence forest resilience in Arizona Sky Islands

Background: Wildfire size and severity have been increasing in the southwestern US since the mid-1980s as a direct result of anthropogenic climate change and land management practices. Large, high severity wildfire, including reburns, combined with two decades of drought, may result in ecosystem reorganization, as some areas dominated by conifer forest are no longer able to recover, and are replaced or outcompeted by oaks or shrubs. We apply a unifying resilience framework distinguishing post-fire processes of persistence, recovery, and reorganization of mixed conifer and pine-oak forests in two southern Arizona Sky Island mountain ranges. The Santa Catalina and Rincon Mountains experienced multiple wildfires in the early 2000s, followed by a large reburn in the Santa Catalina Mountains in 2020. We evaluated conifer radial tree growth as a measure of individual tree persistence, seedling recruitment as a measure of conifer and oak population recovery, and differences in species importance values in burned and unburned areas, to detect potential ecosystem reorganization, including changes in dominant plant trait groups. Results: In our tree growth analysis, we found resilience to both drought and wildfire in three dominant conifer species. Ponderosa pines (Pinus ponderosa) and southwestern white pines (Pinus strobiformis) that survived both high and low severity fire showed non-significant positive growth trajectories following wildfire exposure in 2003. Douglas-fir (Pseudotsuga menziesii) growth was more climate-dependent and adversely affected by fire exposure. In areas that burned only once, post-fire conifer recruitment was found in the majority of burned plots that were sampled 18–19 years after fire, although density varied greatly. We found evidence of incipient community reorganization reflecting plant traits (e.g., resprouting species) especially in mixed-conifer communities. Composition analysis indicated some loss of conifer overstory dominance in areas burned at high severity in higher elevations; in general, these were replaced mainly by aspen (Populus tremuloides). Conclusions: The study landscapes showed evidence of all three post-fire processes of persistence, recovery, and reorganization in the 18–19 years after the initial fire events. The complex mosaic of fire severity and topography promoted landscape-scale recovery, while some high severity areas show evidence of incipient or transient reorganization. Early (1 year) post-fire vegetation responses following the 2020 reburn were highly variable and dependent on sequences of fire severity and plant traits; these constitute initial conditions for future trajectories of change in southwestern forests under the influence of changing climate and fire regimes.