Abstract
The extent and frequency of fire has increased in many arid systems over the last century, with a large proportion of area in some regions undergoing transitions to novel conditions. Portions of the Mojave Desert in southwestern North America have undergone such transitions, most often from woody to herbaceous-dominated systems. These transitions have often been attributed to the proliferation of invasive annual grasses that promote more frequent fire, but recent evidence indicates that transitions can also occur independent of fire frequency if burn severity is high. In addition, high probability of ignition (i.e., potentially high fire frequency) and high burn severity may not always be geographically related. Therefore, our goals were to: (1) map potential burn severity, fire frequency, and probability of ignition across the Mojave; and, (2) evaluate spatial association among predicted burn severity, fire frequency and probability of ignition. We first mapped perimeters of 250 wildfires > 405 ha that occurred from 1972 to 2010, then extracted data on fire frequency (number of times burned from 1972 to 2010), burn severity (the difference Normalized Burn Ratio), and 15 predictor variables representing physiography, climate, ignition, and vegetation. Maximum entropy was used to predict probability of ignition and Random Forest models were used to predict dNBR and fire frequency. Areas with high burn severity and high ignition probability had opposite spatial trends; areas with high burn severity were predicted to predominantly be in the northwest part of the region whereas areas with high ignition probability were predicted to be in the northeast. The models indicate the existence of a number of spatially structured but temporally dynamic fire regimes throughout the Mojave Desert. Two prevalent and ecologically significant regimes include one with frequent fires of low to moderate severity and another with infrequent fire of high severity. Areas with high fire frequency are currently limited in extent (<1% total area). However, cover of invasive grasses can remain high decades after a burn of high or moderate severity, so grass-fire cycles could develop in areas where there may be expectations of infrequent fire as well as those with relatively high fire frequency.
Highlights
Fire in the North American deserts was historically characterized as a regime of relatively small, infrequent, and patchy burns (Wright and Bailey, 1982)
The distribution of area burned among the fires was highly skewed, with 83% occurring from fires between 412 and 5,000 ha (Figure 3)
Two studies using an ecologically sensible delineation of the Mojave have analyzed the annual number of wildfires and area burned, and both found no evidence of a trend in either variable (Brooks and Matchett, 2006; Klinger et al, unpublished)
Summary
Fire in the North American deserts was historically characterized as a regime of relatively small, infrequent, and patchy burns (Wright and Bailey, 1982). An interplay between elevation, latitude and topography results in significant climatic variation both among and within the four major North American deserts (Great Basin, Mojave, Sonoran, and Chihuahuan; MacMahon and Wagner, 1985; Hereford et al, 2006; Brooks and Chambers, 2011). While the studies reported an increase in numbers of fires and area burned in the western United States, inferring that those trends are representative of the deserts is problematic This becomes especially apparent when studies that were focused on a specific desert report patterns markedly different from the patterns reported in the large-scale studies (Brooks and Matchett, 2006; Syphard et al, 2017; Klinger et al unpublished)
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