Cheatgrass (Bromus tectorum) distribution in the intermountain Western United States and its relationship to fire frequency, seasonality, and ignitions

• Published in: Biological invasions Vol. 20; no. 6; pp. 1493 - 1506
• Main Authors: Bradley, Bethany A., Curtis, Caroline A., Fusco, Emily J., Abatzoglou, John T., Balch, Jennifer K., Dadashi, Sepideh, Tuanmu, Mao-Ning
• Format: Journal Article
• Language: English
• Published: Cham Springer International Publishing 01.06.2018; Springer Nature B.V
• Subjects: Abundance; Anthropogenic factors; Biomedical and Life Sciences; Bromus tectorum; burning; Developmental Biology; ecological invasion; Ecology; ecosystems; environmental impact; fire frequency; fire hazard; Fire hazards; Freshwater & Marine Ecology; Hazards; Human influences; Intermountain West region; invasive species; Life Sciences; Mathematical models; Original Paper; Phenology; Plant Sciences; risk; Risk reduction; satellites; Seasonal variations; surveys; variance; Western United States; Western states; Intermountain West region; Western United States; Fire regime alteration; Invasive grass; Invasive plant; Grass-fire cycle; Moderate Resolution Imaging Spectroradiometer (MODIS) burned area product
• DOI: 10.1007/s10530-017-1641-8
• ISSN: 1387-3547

Cheatgrass ( Bromus tectorum ) is an invasive grass pervasive across the Intermountain Western US and linked to major increases in fire frequency. Despite widespread ecological impacts associated with cheatgrass, we lack a spatially extensive model of cheatgrass invasion in the Intermountain West. Here, we leverage satellite phenology predictors and thousands of field surveys of cheatgrass abundance to create regional models of cheatgrass distribution and percent cover. We compare cheatgrass presence to fire probability, fire seasonality and ignition source. Regional models of percent cover had low predictive power (34% of variance explained), but distribution models based on a threshold of 15% cover to differentiate high abundance from low abundance had an overall accuracy of 74%. Cheatgrass achieves ≥ 15% cover over 210,000 km 2 (31%) of the Intermountain West. These lands were twice as likely to burn as those with low abundance, and four times more likely to burn multiple times between 2000 and 2015. Fire probability increased rapidly at low cheatgrass cover (1–5%) but remained similar at higher cover, suggesting that even small amounts of cheatgrass in an ecosystem can increase fire risk. Abundant cheatgrass was also associated with a 10 days earlier fire seasonality and interacted strongly with anthropogenic ignitions. Fire in cheatgrass was particularly associated with human activity, suggesting that increased awareness of fire danger in invaded areas could reduce risk. This study suggests that cheatgrass is much more spatially extensive and abundant than previously documented and that invasion greatly increases fire frequency, even at low percent cover.