Science & Environment

Wildfires fanned by invasive grass species

Intense grass fire
Image caption A continuous cover of cheatgrass fuels an intense fire in the American West

New research indicates that a species of invasive grass is making wildfires in the western US larger, hotter and more frequent.

Scientists say that a variety called cheatgrass dries out and burns more rapidly than other vegetation.

They believe it has fuelled almost 80% of the largest fires in the American West over the last 10 years.

Researchers are looking at a range of solutions including using a fungus to attack the grass seed.

Originally transported to the US in soils on board ocean-going ships, the noxious, weedy grass continued its journey west in the 1800s with settlers and cattle ranchers.

Smoking grass

The species gets its name because it grows very early and very quickly and then dies, cheating other varieties out of valuable nutrients.

It is widely dispersed throughout the Great Basin of the American West, an area of 600,000 sq km that covers parts of Nevada and Utah, Colorado, Idaho, California and Oregon.

Image caption Satellite imagery shows that cheatgrass dominated areas burned more rapidly and more frequently

Scientists have long suspected that it played a key role in wildfires but this report is the most definitive evidence yet. Researchers used satellite imagery from Nasa to compare burnt areas with regions where cheatgrass dominates.

According to lead author Dr Jennifer Balch from Pennsylvania State University, the connection between the two was clear.

"We were able to pick out this species from space because it dries out earlier than native species," she told BBC News. "We looked at all the really big fires, all the ones over 100 sq km in size and cheatgrass influenced the majority of those, it's fuelling those really big fires."

Over the period from 2000 to 2009 Dr Balch and her colleagues say that cheatgrass influenced 39 of the 50 largest wildfires.

"Cheatgrass promotes fires that are easily ignited, and spread rapidly." she said. "They cover large areas and across this landscape that translates to more frequent fires."

Cheating by dying

The research suggests that cheatgrass and fires are in a mutually beneficial relationship.

Michael Kodas is an author and expert on wildfires in the western US. He agrees that cheatgrass is fire-dependent.

"I think the fires are believed to help the cheatgrass outcompete other species and expand its range faster, he told BBC News. "Basically the fires help speed the invasion."

Mr Kodas argues that the invasive species is extending the fire season across the Great Basin.

"Cheatgrass grows and dies earlier in the season," he said. "When native grasses and other plants are greening up and moist, it's already sprouted, spread its seed, and died. So after cheatgrass invades, wildfires can occur earlier in the season, when the native vegetation is still green and unlikely to burn."

Scientists are now working on ways of containing the threat and are investigating a range of control methods says Dr Balch.

Image caption The Constantia fire burning in California was exacerbated by cheatgrass

"Strategies can be as brute as mechanical removal or as intricate as introducing a fungus that attacks the seed. There are a lot of folks looking at ways to reduce or eliminate cheatgrass."

The researchers say that invasive grass species are increasing fires globally, from molasses grass in Hawaii to gamba grass in Australia.

The research is published in the journal, Global Change Biology.

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