Invasive grasses increase fire occurrence and frequency across US ecoregions, Fusco et al. 2019

It has long been suspected that the increasing abundance of invasive grass species may contribute to wildfires in the United States by adding abundant new fuels to ecosystems, increasing the range of conditions that lead to fire ignition, and enabling the development of larger, hotter fires. The new fire regimes (patterns of fire duration, intensity, and spread) that emerge can in turn destabilize wildlife and lead to local extinctions while expanding favorable habitat for the invasive species, for many of these grasses recover quickly after fires, providing renewed fuel and potentially increasing the frequency of fires.

The authors of this paper provide a comprehensive analysis on the impact of 12 non-native grasses on the occurrence (whether a fire occurred in a particular place), frequency (how many times a place burned), and size of wildfires. The research was conducted across 29 US ecoregions, including deserts, temperate forests, wetlands, woodlands, river valleys, shrublands, and coastal plains. Data were collected and combined from fire records and records of invasive grasses, and results from “invaded” regions and nearby “uninvaded” regions were compared. The authors also considered human activities and ecological factors related to fire.

One of the most notorious impacts of nonnative, invasive grasses is the alteration of fire regimes. Yet, most evidence of these impacts comes from local-scale studies, making it unclear whether they have broader implications for national and regional fire management. Our analysis of 12 invasive grasses documents regional-scale alteration of fire regimes for 8 species, which are already increasing fire occurrence by up to 230% and fire frequency by up to 150%. These impacts were demonstrated across US ecoregions and vegetation types, suggesting that many ecosystems are vulnerable to a novel grass-fire cycle. Managing existing grass invasions and preventing future introductions presents a key opportunity to remediate the ecological and economic consequences of invasive species and fire [Fusco 2019: 23594]

The results of this analysis showed that 8 of the 12 invasive grass species examined were associated with significantly higher fire occurrence and fire frequency, and that fire occurrence more than doubled for two of these species. Three of the species did not impact fire occurrence, and a decrease in fires was associated with one species (a wetlands grass species). The impact on fire size was variable, with two species associated with larger fires, three species associated with smaller fires.

Individually, climate change is expected to increase the potential for fire occurrence by 150% by the end of the century based on projected changes in temperature and precipitation. Here we show that 8 invasive grass species are already associated with increased rates of fire occurrence by 27 to 230%, and 6 invasive grass species are associated with increased mean fire frequency by 24 to 150%, compounding current and future fire risk across the United States. [Fusco et al. 2019: 23595]

The authors suggest that fire and invasive species managers work together to create integrated management plans; otherwise, the convergence of human activities, climate change, and invasive species will continue to promote wildfires across the United States.