Grass, Humans, and Heat: An Ecoregional Characterization of Annual Grass Invasion in the Mojave Desert

Exotic annual grasses, such as those from the genus Bromus, are invading and altering unique ecosystems in the desert Southwest through fire feedbacks and landcover change. Management of these exotic grasses is critical for the conservation of desert animal and plant species. Previous studies of invasive grass populations in the region have primarily been conducted on a plot scale, an approach important for identifying physiological traits and local effects but is unable to capture the full spatial or temporal extent of invasion. Using invasive grass occurrence data derived from Landsat imagery and collected over a 12-year period, we set out to characterize biophysical and human related correlates to hotspots of exotic annual grass invasion and to identify the most vulnerable subregions within the Mojave Desert. We employed a Getis-Ord Gi* Hotspot analysis on the aggregated exotic grass occurrence data to identify areas of high persistence and density. We then developed a model to identify biophysical variables predictive of hotspots as well as subregions vulnerable to invasion. We found that much of the Mojave Desert ecoregion had detectable populations for at least one year out of twelve but that the most problematic hotspot areas coincided with regions of moderate to low summer temperatures and high winter rainfall. The ecotypes most vulnerable to invasion are Mojave basins, low ranges and footslopes, and Mojave woodland and shrubland. These results support current research where the timing and amount of precipitation controls invasive grass distribution in the Mojave Desert and provides hints regarding the upper limits of heat tolerance for invasive annual grasses. These results also help to supplement current knowledge about invasions and create maps of areas for potential concern for wildfire and wildlife managers.