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).