Western Wildfires – Invasive Grasses Promote Risk and Frequency
Biogeographer Bethany Bradley at UMass Amherst studies the relationship between invasive grasses and wildfire. She can discuss fires in the West and how invasive grasses increase their number and risk.
She and colleagues recently published results of the first national-scale analysis showing that across the United States, invasive grasses can double the number of fires. One species in particular, invasive cheatgrass, has a long, well-earned reputation as a firestarter in the American West, making wildfires worse and more common, Bradley says.
Further, it is now clear that this effect is pervasive, involving not just a single species. At least seven other non-native grasses can increase wildfire risk around the country, some doubling or even tripling the likelihood of fires in grass-invaded areas. For the 2019 analysis, Bradley and colleagues quantified impacts of invasive grasses on fire occurrence, size and frequency at a regional scale in the lower 48 states, the first pyro-geographic study to look at many species across the entire country.
They compiled a list of fire-starting suspects, relying on the Invasive Plant Atlas of the U.S., other scientific literature and a database called the Fire Effects Information System. They identified 12 invasive grass species for which there was enough data to map out “invaded” and “uninvaded” areas in pixels of 500 x 500 meters (roughly 62 acres).
They then used fire records to compare fire occurrence, size and frequency between invaded and uninvaded areas, from 2000–2015. The authors report that eight of the grasses, including cheatgrass (Bromus tectorum), significantly altered something about the regional fire regimes of ecosystems they invaded – increasing the frequency of fires, for example.
Where buffelgrass (Pennisetum ciliare) invaded, it increased fire frequency, and the presence of flammable invaders increased fire occurrence by between 27 and 230 percent. Bradley says, “This work shows that invasive species are one of the ‘big three’ ways that people are changing fire regimes – climate change more than doubles the likelihood of fire, human ignitions triple the fire season and now we can add invasive species fueling fires.”
People have helped invasive grasses take root in the United States both deliberately and accidentally by promoting their use for forage and as ornamentals, or transporting “hitchhiker seeds” in hay and seed mixes, she adds. Human-caused disturbances to native ecosystems also promote the spread of the grasses and enable the accumulation of enough fuel to carry fires. Fire management and invasive species management should be employed together, Bradley and colleagues suggest.
In the western United States, cheatgrass invasion and the fires that follow leads to 90% loss of above-ground biomass, as sagebrush (left) burns and cheatgrass monoculture (right) takes over.