Conifer Resilience Following Wildfire and Drought in Southeastern Arizona Sky Islands

Abstract

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. This increase in 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 conversion, where 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 (SCM) following wildfires in 2002, 2003 and 2020. For our tree growth analysis, we found a striking resilience to both drought and wildfire in three dominant conifers. Pines that burned at high and low severity in particular showed non-significant positive growth trajectories following wildfire exposure in 2003. Douglas-fir growth was more climate-dependent and less fire-dependent than Pinus 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 found 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. Community composition analysis for areas that burned in the most recent fire showed wide variability and may be seen as a starting point for future trajectories of change in Southwestern forests under the influence of changing climate and fire regimes.