Repeated Fire and Extended Drought Influence Forest Resilience in Arizona Sky Islands

Abstract Background Wildfire size, severity, and frequency have been increasing in the southwestern US since the mid-1980s as a direct result of anthropogenic climate change and land management practices. Significantly, high severity burn area in Arizona and New Mexico has been increasing at a rate of about 1,000 ha per year since 1985. More frequent, higher severity wildfire, combined with two decades of drought, threatens the persistence, regeneration, and resilience of conifer trees in the dry pine forests of Southern Arizona’s sky islands. Failure of conifers to recover may result in ecosystem reorganization as forested areas are replaced by oak or shrub woodlands. Here we report on radial tree growth, conifer regeneration, and community composition in the Santa Catalina Mountains following a series of wildfires in 2002, 2003 and 2020. Results In our tree growth analysis, we found a striking resilience to both drought and wildfire in three dominant conifers. 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 less fire-dependent than Pine growth. For areas that burned only in the earlier fires, conifer regeneration over the 17–18-year interval was found in the majority of burned plots, although density varied greatly. Community composition analysis in these areas indicated some loss of conifer overstory dominance in areas burned at high severity; in general, these were replaced mainly by Aspen (Populus tremuloides) in higher elevation stands, not deciduous or evergreen oaks. Conclusions Early post fire vegetation responses following the 2020 fire event were highly variable, in contrast to longer-term successional processes seen 17–18 years after the initial fire events. This wide variability one-year post-fire may be seen as a starting point for future trajectories of change in Southwestern forests under the influence of changing climate and fire regimes.