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).