Disentangling drivers of annual grass invasion: Abiotic susceptibility vs. fire-induced conversion to cheatgrass dominance in the sagebrush biome
Alexandra Urza, Jeanne Chambers, David Board, Karen Short
Cheatgrass (Bromus tectorum), an invasive annual grass, is causing unprecedented ecological impacts in sagebrush ecosystems. Fire is known to favor cheatgrass over native plant species, and for this reason, preventing fire-related invasions has been the primary focus of cheatgrass mitigation efforts. Nonetheless, cheatgrass can become ecologically dominant even in the absence of fire, and recent research has suggested that the role of fire in driving ecosystem conversions to cheatgrass has been overemphasized. In this study we used a large database of plant species cover to characterize the role of abiotic conditions and fire on cheatgrass dominance - areas where more than 15 percent of the total plant cover was cheatgrass.
We found that most places on the landscape are more likely to convert to cheatgrass after they burn. However, the effect of burning varies widely. About 20 percent of the sagebrush biome, primarily in northwest Nevada, the Snake River Plain, and the Wasatch Front, is susceptible to cheatgrass dominance regardless of fire, and that area is expected to expand with climate change. In contrast, more than half of the sagebrush biome is currently resistant to cheatgrass dominance, which means that cheatgrass is unlikely to become dominant even after burning, although it might be present at low levels.
Less than one third of the sagebrush biome is predicted to require fire for conversion to cheatgrass dominance, yet that whole area is not likely to burn anytime soon. When we combined our models of cheatgrass susceptibility with predicted burn probability, we found that conversion to cheatgrass dominance in the absence of fire is expected to play a larger role in overall cheatgrass dominance risk than fire-induced conversion over the next 10 years. Shifting focus to non-fire-related invasions allows for more proactive management approaches, such as promoting native species to enhance resistance and reducing opportunities for cheatgrass seed dispersal.
https://www.fs.usda.gov/rm/pubs_journals/2024/rmrs_2024_urza_a001.pdf