Biophysical Correlates with the Distribution of the Invasive Annual Red Brome (Bromus rubens) on a Mojave Desert Landscape

Abstract

Because of its ability to transform ecosystems by increasing the prevalence of fire, the invasive annual red brome is a priority exotic species for management in arid lands of the southwestern United States. By sampling red brome presence and 97 environmental (climatic, topographic, and soil) and native vegetation (e.g., perennial species richness) variables on 126 sites, we assessed biophysical correlates with red brome distribution on a 755,000-ha (1.9 million ac) Mojave Desert landscape. Brome occupied 55 of 126 (44%) 0.09-ha plots. The simplest models (i.e., those containing the fewest or most easily obtained variables) in multivariate (classification trees and nonparametric multiplicative regression) and univariate (χ2) models often portrayed red brome distribution as well, or nearly as well, as more complicated models containing more variables harder to obtain. The models varied, however, in their abilities for describing brome presence compared with absence. For example, a simple classification tree using only elevation, soil great group, parent material, and vegetation type improved estimates of brome presence for 55% of sites, absences for 87%, and overall for 73% of sites compared with a naïve model containing the observed frequency of brome in the data. Conversely, a more complicated model, including soil boron and sulfur, performed better for presences (96%) than for absences (73%; 83% overall). Results also showed variable support for two general postulates in invasive species science. Red brome distribution was not correlated with soil N, which is inconsistent with the supposition that nutrient-rich soils are more prone to invasion. Brome was correlated with native perennial species richness to support the postulate that exotic species abundance is correlated with species-rich habitats, but the correlation was weak (r = 0.38) and similar in strength to correlations with many other environmental variables. On this relatively low-elevation landscape, the areas currently most invaded by red brome include the higher elevations (> 777 m [2,549 ft]), limestone–sandstone soils, and burrobush and mixed perennial communities. Areas least inhabited by brome are the lowest elevations (< 491 m), gypsum soils, and creosotebush and saltbush communities.

Plant colonization and soil properties on newly exposed shoreline during drawdown of Lake Mead, Mojave Desert

“…Other exotic species displayed little pattern with surface age, apparently consistent with their abilities to occupy either moist or dry sites. For instance, this study's finding of Bromus rubens inhabiting recently submerged surfaces near the shoreline and never-submerged surfaces concurs with a landscape-scale distribution study of Bromus rubens within Lake Mead National Recreation Area, where the species occupied a variety of riparian and upland sites (Abella et al 2012).…”

Persistence and turnover in desert plant communities during a 37‐yr period of land use and climate change

“…), were not prevalent in the study area either based on a previous investigation (Abella et al. ). It is unclear why the study area is relatively free of nonnative plants, as the Colorado River on the eastern border and highways on the western and southern borders could serve as dispersal vectors, along with a main dirt road through the study area.…”

“…Our results were also inconsistent with the idea that nonnative plant invasion is accelerating at the highest elevations (Pauchard et al 2009). Nonnative annuals, more extensively distributed across the Mojave Desert than nonnative perennials (Abella et al 2015b), were not prevalent in the study area either based on a previous investigation (Abella et al 2012). It is unclear why the study area is relatively free of nonnative plants, as the Colorado River on the eastern border and highways on the western and southern borders could serve as dispersal vectors, along with a main dirt road through the study area.…”

“…On the other hand, the study area's granitic soils are not among the region's most susceptible to invasion (Abella et al. ), the high elevations remain isolated with few human impacts, and the National Park Service performs treatments for nonnative plants around the study area (Abella et al. ).…”

“…It is unclear why the study area is relatively free of nonnative plants, as the Colorado River on the eastern border and highways on the western and southern borders could serve as dispersal vectors, along with a main dirt road through the study area. On the other hand, the study area's granitic soils are not among the region's most susceptible to invasion (Abella et al 2012), the high elevations remain isolated with few human impacts, and the National Park Service performs treatments for nonnative plants around the study area (Abella et al 2009).…”