Projected Climate-Fire Interactions Drive Forest to Shrubland Transition on an Arizona Sky Island

Abstract

Climate stressors on the forests of the American Southwest are shifting species distributions across spatial scales, lengthening potential fire seasons, and increasing the incidence of drought and insect-related die-off. A legacy of fire exclusion in forests once adapted to frequent surface fires is exacerbating these changes. Reducing stand densities and surface fuel loads has been proposed as a means of moderating fire behavior while reducing competition for water, but it is not established whether thinning treatments and restoration of surface fire regimes will be enough to offset the multiple manifestations of a changing climate. We examined the potential for prescribed fuel treatments and restoration of historical fire frequencies to mitigate the effects of climate on forest species distributions, composition, total biomass, and fire severity. We used an ecosystem process model to simulate the effects of projected climate, fire, and active management interactions along an ecological gradient of shrublands, woodlands, and forests on a mountain range in Arizona in the United States. We used historical climate conditions as a baseline to compare results from projected climate for the period 2005-2055 with and without fire and with no fuel treatments, a single-entry fuel treatment, and a second fuel treatment after 20 years. Simulated desert grassland and shrub communities remained compositionally stable and maintained or expanded their extents while woodland and forest communities lost basal area and total biomass and receded to the coolest and wettest aspects and drainages even without fire. Initial fuel treatments reduced the extent and relative mortality of high-severity patches for the first two decades, and secondary treatments at simulation year 20 extended these effects for the remaining 30 years of simulation. Immediate and future fuel treatments showed potential to mitigate the severity of fire effects under projected conditions and slow the transition from forest to shrubland in some vegetation types, however, a reduction in basal area and spatial extent of some forest species occurred regardless of management actions. Results are being used to inform local land managers and partners of potential landscape changes resulting from climate alone and from climate-fire interactions and to coordinate active management of fuels across ownerships.

Wildfire and topography drive woody plant diversity in a Sky Island mountain range in the Southwest USA

“…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, 2013). Although research supports the hypothesis that this vegetation‐type conversion will likely persist (Barton & Poulos, 2018; Guiterman et al., 2018; O’Connor et al., 2020), future episodic regeneration of these conifers under favorable conditions remains a possibility.…”

“…Projections call for continuing and even heightened drought (Ault et al., 2016; Wilder et al., 2013) and further intensification of fire in the Southwest (Abatzoglou & Williams, 2016; Kitzberger et al., 2017), putting the woodlands and forests of the Sky Islands at increased risk (Coop et al., 2020; O’Connor et al., 2020; Parks et al., 2019; Yanahan & Moore, 2019). 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.…”

“…Second, hotter, drier, more fiery conditions are likely to further narrow the window of conditions favorable to postfire regeneration of some species, even if seeds are available (Coop et al., 2020). These two impacts could consequently shift conditions outside the range to which much of the species pool is adapted (Johnstone et al, 2016; Guiterman et al., 2018; Coop et al., 2020; O’Connor et al., 2020; Miller & Safford, 2020), amplifying impacts on ecosystems, including the conversion of complex conifer–hardwood forests to simpler, more resilient nonconifer vegetation (Coop et al., 2020; Falk, 2013). 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; O’Connor et al., 2020; Yanahan & Moore, 2019).…”

Wildfire and topography drive woody plant diversity in a Sky Island mountain range in the Southwest USA

“…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.…”

Mixed-Severity Wildfire as a Driver of Vegetation Change in an Arizona Madrean Sky Island System, USA

“…The regeneration of obligate seeder tree species may also be threatened by increasing drought under climate change. Most projections indicate that western North America will experience increasing aridity, moisture-deficits, and larger and more frequent wildfires [104][105][106][107][108], a trajectory that would continue trends over the past three decades. More frequent and longer-duration droughts may hinder obligate seeder regeneration, even if seed sources are nearby [109,110].…”