Abstract
Climate stressors on 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, USA. 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 twenty 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, from climate-fire interactions, and to coordinate active management of fuels across ownerships.
Highlights
Projected warming temperatures and increased moisture variability are likely to cause changes to the frequency and severity of disturbances in many forested ecosystems (Bentz et al, 2010; Abatzoglou and Kolden, 2013; Harris et al, 2016; Riley et al, 2019)
We developed a database of species parameters for the 16 most common tree, shrub, and grass components in 10 ecological response units (ERUs) representing Chihuahuan desert scrub, semi-desert grassland, Madrean encinal woodland, Madrean pine–oak woodland, pinyon–juniper grassland, ponderosa woodland, mixed-conifer forest, aspen woodland, FIGURE 1 | Structure of nested tree, plot, stand, and site layers that make up the FireBGCv2 simulation landscape (Keane et al, 2011)
Inclusion of fire in simulations of historical climate resulted in a dynamic equilibrium between firerelated losses and new growth at ∼3.5 kg/m2
Summary
Projected warming temperatures and increased moisture variability are likely to cause changes to the frequency and severity of disturbances in many forested ecosystems (Bentz et al, 2010; Abatzoglou and Kolden, 2013; Harris et al, 2016; Riley et al, 2019). Under the influence of changing climate, species ranges are projected to shift across scales from landscapes to entire species ranges (Chen et al, 2011; Notaro et al, 2012). For sessile species, such as plants, these shifts at any scale are the net demographic result of mortality and recruitment failure at the trailing edge of the distribution (extinction debt) and successful recruitment along the leading edge (immigration or colonization credit) (Jackson and Sax, 2010; Evans et al, 2016; Talluto et al, 2017). Species range shifts from climate pressure alone can occur abruptly from transient climate episodes, such as heat waves (Allen et al, 2015; Ruthrof et al, 2018; Law et al, 2019), but broader changes in species distributions are anticipated to occur over multiple decades, even under the accelerated velocity of anthropogenic climate change (Adams et al, 2009; Burrows et al, 2014)
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