Cheatgrass alters flammability of native perennial grasses in laboratory combustion experiments

Abstract

Abstract Background The invasive annual grass cheatgrass (Bromus tectorum) increases fuel continuity, alters patterns of fire spread, and changes plant communities in sagebrush shrublands of the Great Basin (USA) and adjacent sagebrush steppe areas, but no studies have contrasted its flammability to native perennial grasses. Understanding cheatgrass flammability is crucial for predicting fire behavior, informing management decisions, and assessing fire potential of invaded areas. This study aimed to determine the flammability of cheatgrass compared to two native perennial grasses (Columbia needlegrass [Achnatherum nelsonii] and bluebunch wheatgrass [Pseudoroegneria spicata]) across a range of typical fire season fuel moistures. Results All three grass species had decreased flammability with increasing fuel moisture. Columbia needlegrass had on average 11% lower mass consumption than cheatgrass, and both perennial grasses had on average 13.5 s longer flaming durations and higher thermal doses (temperature over time) than cheatgrass. The addition of cheatgrass to the perennial grasses increased combined mass consumption, flaming duration, and thermal dose. For these three attributes, flammability increased with greater amounts of cheatgrass in the mixture, but flaming duration and thermal dose were not sensitive to cheatgrass fuel moisture. Maximum temperature and flame length of perennial grass combustion were similar with and without cheatgrass addition. Flammability of Columbia needlegrass when burned with cheatgrass was higher than expected based on the flammability of each respective species, suggesting that Columbia needlegrass may be susceptible to pre-heating from cheatgrass, causing increased mass consumption, flaming duration, and thermal dose. Conversely, flammability of bluebunch wheatgrass and cheatgrass together had both positive and negative interactive effects. Conclusions This study provides experimental evidence supporting previous qualitative observations of high cheatgrass flammability. Even at high fuel moisture, cheatgrass increased perennial grass flammability, suggesting that cheatgrass poses a significant fire threat to native grasses for an extended season than expected for the native grasses without cheatgrass. The study's findings inform invasive plant management and fire potential, and guide efforts to prevent or mitigate cheatgrass-induced wildfires.