2.1 Study Area
Located in southeastern Arizona (32°00′20” N, 109°21′24” W), Chiricahua National Monument (CHIR) encompasses 4850 ha in the Chiricahua Mountains (Figure 1), which are part of the Sky Islands, an archipelago-like northern extension of the Mexican Sierra Madre Occidental (DeBano et al., 1995). Elevations in CHIR range from 1,562 to 2,228 m a.s.l. Soils are shallow and derived mainly from volcanic rhyolites and monzonites deposited in the early- to mid-Miocene, although pre-Tertiary rock is prominent at lower elevations (Drewes & Williams, 1973). The terrain of CHIR varies from level desert grassland to highly-dissected, rocky uplands with steep-walled canyons and incised towers.
The climate is semiarid (annual precipitation\(\overset{\overline{}}{X}\ \)= 490.2 mm), typically with a dry season from April-June (\(\overset{\overline{}}{X}\) = 42.7 mm) and a rainy season from July-September (\(\overset{\overline{}}{X}\)=251.5 mm), driven by the North American Monsoon System (Adams & Comrie, 1997). Near the CHIR visitor center at 1650 m a.s.l., mean minimum and maximum temperatures are \(\overset{\overline{}}{X}\) = −1.2° C and 13.4° C, respectively, for January and \(\overset{\overline{}}{X}\) = 15.5 and 31.8° C, respectively, for July. From low to high elevation, temperature decreases and moisture availability increases because of the combined effects of reduced evaporative demand and orographic lifting of moisture-laden air (Shreve, 1915; Whittaker et al., 1968; Whittaker & Niering, 1975; Barton, 1994; Vivoni et al., 2007).
Before Euro-American settlement in the 1870s, the land that is now Chiricahua National Monument experienced primarily surface fires in Madrean pine-oak forests, with a mean fire return interval of about 2 to 15 years, and a mixed fire severity regime in drier woodlands and chaparral, with a return interval of 20 to 100 years (Swetnam et al., 1989; Kaib et al., 1996; Swetnam & Baisan, 1996; Baisan & Morino, 2000; Barton et al., 2001; Swetnam et al., 2001). Fire was uncommon from the 1890s through the 1980s, initially in response to intensive livestock grazing and then active fire suppression (Leopold, 1924; Swetnam, et al., 2001). During the extremely dry year of 2011 (Williams et al., 2014), the Horseshoe Two Fire burned >90,000-ha, about 75% of the mountain range, 12.4% at high and 29.7% at moderate severity (Arechederra-Romero, 2012). This pattern of altered fire regime has been well-documented for many of the Sky Islands on the Arizona side (Swetnam et al., 2001; O’Connor et al., 2014), whereas anthropogenic shifts in recent fire regimes have been much less pronounced on the Mexico side of the US-Mexico border (Meunier et al., 2014 Villareal et al., 2019, 2020).
The Sky Islands of the USA-Mexico borderlands support high levels of biodiversity and endemism, a product of the mixing of continental biomes and an isolated location midway between tropical and temperate regions (Whittaker & Niering, 1975; Gehlbach, 1981; DeBano et al., 1995; Poulos et al., 2007). Major plant communities in CHIR include semi-desert grassland, interior chaparral, piñon-oak-juniper woodland, Madrean pine-oak woodland, canyon gallery forest, and mixed conifer forest (Roseberry and Dole, 1939; Reeves, 1976; Barton, 1994; Bennett et al., 1996 Poulos et al., 2007). Plant nomenclature in this study followed the USDA Plants Database (USDA 2021), with aid from Bennett et al. (1996).