Contemporary Fire Regimes Provide a Critical Perspective on Restoration Needs in the Mexico-United States Borderlands

Abstract

The relationship between people and wildfire has always been paradoxical: fire is an essential ecological process and management tool, but can also be detrimental to life and property. Consequently, fire regimes have been modified throughout history through both intentional burning to promote benefits and active suppression to reduce risks. Reintroducing fire and its benefits back into the Sky Island mountains of the United States-Mexico borderlands has the potential to reduce adverse effects of altered fire regimes and build resilient ecosystems and human communities. To help guide regional fire restoration, we describe the frequency and severity of recent fires over a 32-year period (1985-2017) across a vast binational region in the United States-Mexico borderlands and assess variation in fire frequency and severity across climate gradients and in relation to vegetation and land tenure classes. We synthesize relevant literature on historical fire regimes within 9 major vegetation types and assess how observed contemporary fire characteristics vary from expectations based on historical patterns. Less than 28% of the study area burned during the observation period, excluding vegetation types in warmer climates that are not adapted to fire (eg, Desertscrub and Thornscrub). Average severity of recent fires was low despite some extreme outliers in cooler, wetter environments. Midway along regional temperature and precipitation gradients, approximately 64% of Pine-Oak Forests burned at least once, with fire frequencies that mainly corresponded to historical expectations on private lands in Mexico but less so on communal lands, suggesting the influence of land management. Fire frequency was higher than historical expectations in extremely cool and wet environments that support forest types such as Spruce-Fir, indicating threats to these systems possibly attributable to drought and other factors. In contrast, fires were absent or infrequent across large areas of Woodlands (~73% unburned) and Grasslands (~88% unburned) due possibly to overgrazing, which reduces abundance and continuity of fine fuels needed to carry fire. Our findings provide a new depiction of fire regimes in the Sky Islands that can help inform fire management, restoration, and regional conservation planning, fostered by local and traditional knowledge and collaboration among landowners and managers.

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

“…As a result of intensive livestock grazing and then active fire suppression (Leopold, 1924; Swetnam et al., 2001), the incidence of wildfires declined drastically in the Chiricahua Mountains after the 1880s, leading to a pervasive accumulation of live and dead fuels (Baisan & Morino, 2000; Taylor et al., 2021). Similarly altered fire regimes have been well documented for many of the Sky Islands in Arizona (O’Connor et al., 2014; Swetnam et al., 2001), whereas anthropogenic shifts in fire regimes have been less pronounced on the Mexico side of the US–Mexico border (Meunier et al., 2014; Villarreal et al., 2019, 2020).…”

“…Madrean pine–oak forests, which occur primarily in canyons and at intermediate elevations, supported a frequent, low‐severity, surface fire regime, with a mean fire return interval of about 2 to 15 years (Baisan & Morino, 2000; Barton et al., 2001; Kaib et al., 1996; Swetnam & Baisan, 1996; Swetnam et al., 1989, 2001). In contrast, piñon woodland and juniper woodland, which occur primarily on drier, lower elevation sites, were characterized by mixed‐severity fire regimes, including infrequent, stand‐replacing fires (Baisan & Morino, 2000; Taylor et al., 2021; Villarreal et al., 2020).…”

Wildfire Risk and Hazardous Fuel Reduction Treatments Along the US-Mexico Border: A Review of the Science (1986-2019)

“…While future warming is likely to extend the fire season length and increase fuel flammability, 4 previous research in the US-Mexico border region has revealed the importance of antecedent wet conditions in promoting fine fuel production necessary to increase wildfire activity. 72 Villarreal et al 97 report in this special collection that recent fires deviate from historical fire regimes for most ecosystems along the border, especially at extreme ends of bioclimatic gradients. These results point out the need to experimentally impose climate extremes through rainfall or temperature manipulation 98 in conjunction with fire to determine important interactions that affect ecosystem condition.…”

“…Similarly, the patchwork of land-uses and ownership along the border creates incentive to further explore how these factors influence wildfire activity and invites new research that crosses jurisdictional boundaries, including across the international border. Indeed, Villarreal et al 2,97 emphasize the need for continued collaboration and shared data sets from both sides of the border to adequately learn from historical fire regimes and understand the potential of future changes. Connectivity of fuels and related fire hazards across the US-Mexico border makes collaborative resource management increasingly important to reduce the risk of transboundary wildfire transmission and to improve ecosystem health.…”

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

“…Fire frequency and burn severity patterns are changing rapidly throughout southwestern North America in response to increasing aridity from climate change and more than a century of fuel accumulation under fire suppression [27][28][29]. Burn severity is increasing within pine-oak forests where live and surface fuel loads are high due to fire suppression [28][29][30][31], and within this vegetation type, high-severity, wildfire-driven transitions to shrublands are a well-documented phenomenon [32][33][34][35][36]. Recent high-severity fires in pine-oak forests at many sites have promoted the recovery of resprouters but less post-fire regeneration by obligate seeder taxa [32,34,35,37,38].…”