Introduced annual grass increases regional fire activity across the arid western USA (1980–2009), Balch et al. 2013

Cheatgrass is an introduced annual grass that has spread everywhere throughout the western USA. It is among the first plants to emerge in the spring, after which it completes its life cycle, drying out in summer and thus creating a continuous, dry, fine fuel load across the landscape. This study examined the cheatgrass invasion’s effect on the fire regime of the Great Basin region of the western USA, finding that:

Fires were more likely to start in cheatgrass than in other vegetation types and that cheatgrass is associated with increased fire frequency, size, and duration [Balch 2013: 179-180].

Here, we have documented that cheatgrass-dominated areas, which currently cover ~40,000 km2, sustain increased fire probability compared with native vegetation types. As sites burn, more and more of them are likely to become cheatgrass grasslands thus increasing their future probability of burning. If future climate scenarios hold true, the combination of warmer temperatures and high water availability^[7] could yield larger fire events that are carried between forested or shrubland areas by invasive grasses, thus perpetuating a novel grass-fire cycle across the western United States and ultimately reducing cover of woody species [Balch 2013: 182].

In native shrub and grassland ecosystems of the arid western United States, high antecedent precipitation has been shown to be one of the strongest predictors of government-registered burned area (1977–2003), even more so than current-year temperature or drought conditions. The oscillation between wet years that enable substantial grass growth and dry years that desiccate those built-up fuels may create ideal conditions for high fire years, but this hypothesis remains untested for cheatgrass rangelands [Balch 2013: 174].

Fire-driven conversion of shrubland to grassland has been linked to a loss of carbon storage and available soil water [Balch 2013: 174].