Across the U.S., longer fire seasons and more intense burns are becoming the norm. For decades we have heard from Smokey the Bear, “Only YOU can prevent forest fires.” To an extent this is true. Humans cause 85% of wildfires and thanks to decades of extinguishing both man-made and natural fires, increased tree death due to drought and pest infestations, and climate change the fire season is longer each year. But these commonly attributed causes for wildfires overlooked another culprit, invasive grasses.
What’s so special about invasive grasses?
Invasive grasses are weeds in the ecosystem, usually introduced to an ecosystem by human activities and often displace native species. But they also promote fire by changing their local environment. Here is how it works: when a population of nonnative grasses invades an ecosystem, it increases the fuel for the fire, which elevates the chance of larger and longer burning fires, and opens even more space for invasive grasses to grow. This “grass-fire-cycle” is well-documented across fire-prone areas of the U.S. It’s like throwing a pebble into a pond and watching the ripples fan out; every ripple becomes larger, creating even more ripples until the pond has lost its still surface.
Living in Southern California, I have grown accustomed to the hills turning brown every summer filled with the dried-out corpses of invasive grasses. A small spark could set the whole hill ablaze and clear the way for more habitat for the next cohort. A group of researchers from the University of Massachusetts, Amherst recognized this pattern and set out to determine how the invasion of individual grass species can change the fire frequency across the landscape.
The authors identified 12 nonnative grass species that are known to increase the number of fires locally. Using information about where the grasses are found and information about wildfires from 2000-2015, the authors compared wildfire occurrence between areas with and without grass invasion. Astoundingly, eight of the 12 locations with invasive grass species revealed a greater number of fires compared to adjacent un-invaded landscapes across 29 different ecosystem types. The most notable invasive grass associated with more fires was Schismus barbatus, a Mediterranean species invasive to California. The researchers found 5% of the sites without S. barbatus were burned compared to 16.5% of the S. barabtus invaded sites. This provides evidence that the grass-fire interactions that have previously only been studied on small scales within local regions, have impacts spanning the entire range of the invasive grass.
Why is grass invasion increasing fire frequency?
Invasive grasses usually grow close together creating a continuous line of fuel. Without natural gaps in the vegetation acting as fire breaks, large swaths of grassland burn easily. Additionally, many of these grasses dry out early in the growing season, making them flammable in a range of climate conditions, even in seasons when the native species are still growing and active. Invasive grass abundance is also directly related to human activities. Invasive grasses were originally introduced by people unintentionally through transportation of farm seed and livestock from other countries and dominate in disturbed environments such as roadsides and the edges of the urban-wildland interface. These connections increase both the spread of the invasive species and the risk to human lives and structures. Smokey the Bear was right, humans are largely the cause of massive, destructive wildfires. But can we stop what we started?
According to the researchers, climate change is predicted to increase the annual fire occurrence by 150%, but the mere presence of these invasive grasses can increase fire occurrence by up to 230%. Fire ecology is complex, involving components like climate, vegetation, and human influence on the landscapes. Humans brought the invasive species here where they continue to thrive as the climate warms, fueling more wildfires. Solutions to curbing wildfires are not straightforward.
What can we do to break the cycle?
If left alone, invasive species will continue to spread, promoting more extreme wildfires, harming the economy, and putting human lives at risk. There are people actively managing invasive species populations by using animals to eat the grass before it makes seed or manually removing the grass from the ground by hand or with machines. There are also people actively managing wildfires, often using prescribed burns outside of the hot and dry season to reduce the amount of flammable material. Based on our expanding knowledge on the grass-fire-cycle, if these groups work together, we may be able to mitigate invasive grass expansion and wildfires. In many places like California, these invasive grasses are highly abundant and likely here to stay, so the best thing we can do is limit the spread by removing seeds from our shoes before hiking in a new area, staying on the trails, and reducing human-caused sparks as we continue to work on understanding the complex interactions of climate, fire, and invasive species.
Every spring when the California hills turn green, I feel a little pang of guilt for marveling at the sight. I know the greenness will not last and the grasses will soon become fuel for fires that could threaten my home. But for now, I will look at the green hills and think about a future where native species reign and fires are a natural phenomenon rather than existential threat.
Fusco E.J., J.T. Finn, J.K., Balch, R.C. Nagy, and B.A. Bradley. 2019. Invasive grasses increase fire occurrence and frequency across US ecoregions. PNAS: 1908253116. https://doi.org/10.1073/pnas.1908253116