Invasive Grass May Be Contributing To Extreme Wildfires
December 6, 2012

Invasive Grass May Be Contributing To Extreme Wildfires

April Flowers for - Your Universe Online

An international team of scientists comprised of members from Penn State, UMass Amherst, UC Santa Barbara (UCSB) and University College London has revealed that an invasive grass species may be one reason that fires are bigger and more frequent in certain regions of the western U.S.

Using satellite imagery, the team identified cheatgrass — a plant species accidentally introduced by western settlers during the 1800s — in a disproportionately high number of fires located in the Great Basin. The Great Basin is an arid area that covers over 370,000 square miles of Nevada, Utah, Colorado, Idaho, California and Oregon.

"Over the past decade, cheatgrass fueled the majority of the largest fires, influencing 39 of the largest 50 fires," Jennifer Balch, assistant professor, Penn State's Department of Geography and Earth and Environmental Systems Institute, said in a statement. "That's much higher than what it should be when you consider how much of the Great Basin that cheatgrass covers."

Cheatgrass grasslands dominate only about 6 percent of the Great Basin. However, the average size of the fires in these grasslands was significantly larger than the average fire in most regions dominated by other vegetation, such as pinyon-juniper areas, montane shrubland and agricultural land. The findings of this study were published online in the journal Global Change Biology.

The team found that cheatgrass may play a role in increasing the frequency as well as the average size of wild fires in the western U.S.

"From 2000 to 2009, cheatgrass burned twice as much as any other vegetation," said Balch.

Compared to other species like sagebrush, which has a 196-year fire return interval (the time between fires in a region), the land dominated by cheatgrass has a shorter fire-return interval of 78 years.

"What's happening is that cheatgrass is creating a novel grass-fire cycle that makes future fires more likely," said Balch, who started this work at the National Center for Ecological Analysis and Synthesis (NCEAS). "Fire promotes cheatgrass and cheatgrass promotes fires."

One reason the plant is involved in larger and more frequent blazes may be the ability of cheatgrass to rapidly spread and fill in the ground between other plant species.

Cheatgrass-influenced fires create a difficult management challenge, according to Balch. The fires threaten many areas, including agricultural lands, residential areas, and habitats for threatened species such as the greater sage grouse.

Scientists have recognized cheatgrass-driven fires for decades; however, remote sensing technology has allowed the new study to take a regional approach to assessing the overall problem. The team compared regional land cover maps that included cover of cheatgrass to burned areas detected by NASA's Moderate Resolution Imaging Spectroradiometer (MODIS) from 2000 to 2009. The team combined data sets, matched fire dates and perimeters with land cover data to reach their conclusions.

The team demonstrated that over the past 10 years, cheatgrass fueled most (close to 80 percent) of the largest fires across the western U.S.

"Historically, the way remote sensing worked, you could only tell the difference between broad land cover classes such as trees versus wetlands, for instance," Bethany Bradley, a biogeographer at the University of Massachusetts Amherst, said in a statement. "It is very difficult to capture those details at the species level."

"One of the tricky things about fires in the West is high year-to-year variability. Grass fires tend to occur the year after a wet year, because there is plenty of dry, standing biomass ready to be burned. So trying to figure out this fire relationship you need to have a time series with enough fire years to produce a strong signal. With a full decade of data, we were able to capture the relationship."

She adds, "Cheatgrass leaves a continuous cover, which is why it may burn more frequently. The native vegetation in such dry landscapes is often shrubby and separated by bare soil, which can stop a fire, but cheatgrass forms a continuous fuel."

The researchers were able to use the satellite imagery to better pinpoint the growth of different species by noticing what conditions favor the growth of certain species. For example, while other species grow during dry times, cheatgrass grows during wet periods.

"What you end up seeing is that most years when it is dry, the cheatgrass doesn't grow much," said Bradley. "But when there are wet seasons that occur due to the El Nino cycle, cheatgrass cover is very dense and continuous."

This becomes a major concern when you realize that cheatgrass dominates more than 24,000 square miles. The changes in vegetation, according to the team, can be detected in the satellite images.

Management agencies in the West will use this new information to create better management plans.

"Managers have until now been trying to model fire risk under future climate and development conditions without any information on cheatgrass's influence. So now they have one more tool to introduce more information and accuracy into their models," says Bradley.

"Using remote sensing we can relate climate conditions to fire response and ecosystem phenology over time, and potentially predict how those ecosystems might be affected by consistently warming temperatures in the future," Bradley says.

"Being able to detect cheatgrass and burns really enabled us to ask the important question: 'How does an invasive plant change fire activity across the entire Great Basin?'" Balch said.

The team hopes to use this regional approach in further investigations to learn more about how different landscape types such as shrub, forest and wetland ecosystems respond to climate on a yearly basis.

The National Science Foundation (NSF) and the NCEAS supported this research.