5 CONCLUSION
Over the past several decades, temperatures have risen (Gonzalez et al. 2018), moisture availability has declined (Cook et al., 2015), and forest fire activity has increased throughout western North America (Westerling, 2016; Singleton et al., 2019; Coop et al., 2020). These changes have depressed post-fire tree regeneration (Stevens-Rumman & Morgan, 2019; van Mantgem et al., 2019), compromised forest resilience (Hessburg et al., 2019; Coop et al., 2020), and converted previously conifer-dominated stands to non-conifer vegetation (Barton & Poulos, 2018; Coop et al., 2020). The impact of the Horseshoe Two Megafire on species diversity in Chiricahua National Park was relatively benign, comparatively. From a broader conservation and management perspective, however, these quantitative metrics may be misleading. Although the number of species across all plots increased, two ecologically important species–Pinus arizonica and Pseudotsuga menziesii – disappeared from the plots (but not the entire park), a result of fire-induced mortality and lack of regeneration from seed after the fire (Barton & Poulos, 2018; Taylor et al., in press). Conifer species such as these are foundational elements of the functional and structural complexity of these forests. Despite their replacement by even more shrub species, the loss of these two conifers undoubtedly cascades to other trophic levels and ecosystem processes. The population of the endemic subspecies of the Mexican fox squirrel (Sciurus nayaritensis chiricahuae ), for example, nests preferentially in sites with large conifers that have experienced low-severity fire (Doumas & Koprowski, 2012).
Projections call for continuing and even heightened drought (Wilder et al. 2013, Ault et al. 2016) and further intensification of fire in the Southwest (Abatzoglou et al., 2016; Kitzberger et al. 2017), putting the woodlands and forests of the Sky Islands at increased risk (Parks et al., 2019; Yanahan & Moore, 2019; Coop et al., 2020; O’Connor et al., 2020). Although the impact of the Horseshoe Two Fire on species diversity was modest in this context, the pattern documented in our study might not replay in future fires for at least two reasons. First, the projected meteorological conditions may increase both the prevalence and frequency of higher severity fire to the extent that species regenerating from seed will not have sufficient time to complete their life cycles between fire events (the so-called “interval squeeze” problem; Enright et al., 2016). Second, hotter, drier, more fiery conditions are likely to further narrow the window of conditions favorable to post-fire regeneration of some species, even if seeds are available (Coop et al., 2020). These two impacts would thus shift conditions outside the range to which much of the species pool is adapted (Johnstone et al., 2016; Coop et al., 2020; Miller & Safford, 2020), amplifying impacts on ecosystems, including the conversion of complex conifer-hardwood forests to simpler, more resilient non-conifer vegetation (Falk, 2013; Coop et al., 2020). Such environmental changes might, moreover, reduce species diversity at both local and landscape scales, as species pools become constrained by repeated fires and less favorable regeneration conditions (Falk, 2013; Yanahan & Moore, 2019; O’Conner et al., 2020).
Restoring the woodlands and forests described in this paper to pre-Anthropocene historic norms may no longer be realistic, given the degree of current and projected ecological transformation (see Falk et al., 2019; Coop et al., 2020; McWethy et al., 2020). One management response to this reality is to embrace wholesale ecosystem transition to a vegetation mosaic that is less structurally and functionally complex but more resilient in the face of a hotter, drier environment with more fire activity. Alternatively, efforts could be made to direct future biotic change with the goal of conserving important elements of current ecosystems that engender species and functional diversity across all taxa. Interventions such as strategic forest thinning, prescribed fire, protection of refugia for sensitive species, and restoration planting have shown promise in achieving these goals (Strom & Fulé, 2007; Laushman et al., 2020; Villareal et al., 2020), but could be greatly enhanced by more comprehensive scientific and social-ecological underpinnings (Falk et al., 2019; McWethy et al., 2019; Coop et al., 2020).