Alien grasses are making wildfires in the US up to twice as large and three times as frequent, the first nationwide study of their impact has found.
One species, cheatgrass, is now widespread in California and was involved in last year’s Thomas Fire, the largest recorded in the state until eclipsed by the ongoing Mendocino fire raging since early this week.
Like cheatgrass — already documented as a fire hazard at a local level — many of the alien grasses are finer and therefore easier than native species to ignite, and occupy ground within and between patches of native grass. Other species, such as silkreed, grow more than 3 metres high and so can spread a ground fire up into trees.
“The invasive grasses can provide horizontal and vertical continuity, which means they can act as an intermediate fuel to carry fire across a distance, or into a canopy,” says Emily Fusco of the University of Massachusetts in Amherst, who presented the preliminary results of her study on 9 August in New Orleans at the annual meeting of the Ecological Society of America.
Fusco analysed the impact of nine of the most widespread alien grasses in areas they’ve invaded across the US. Fusco combined this data with ground and satellite fire records, comparing fire frequency and size in infested habitats versus comparable but uninvaded habitats.
On average, fires in infested areas were twice as large and up to three times more frequent. All species except one made fires more frequent, and all but two made fires larger.
Silkreed, introduced from Asia as an ornamental to Florida in the 1900s, poses particular hazards, because it carries fire into trees. “Our preliminary results suggest it is doubling fire occurrence and quadrupling average fire size where it has invaded,” Fusco says.
In fire-ravaged California, cheatgrass is extremely widespread throughout. In the north, the site of the recent Carr fire, an alien species called medusahead is common.
“Knowing how these invasive grasses impact fire is important for future fire management, because they’re a novel fuel source,” says Fusco. “We also know that many invasive grasses will continue to spread, and may become fire hazards in new ecosystems in future,” she says.
“If we can detect new infestations early, and manage them at low densities, we might stand a chance of getting rid of them before they become a problem, and possibly try restoring invaded ecosystems back to a more native state,” she says.
Power line worries
At the same meeting, Jon Keeley of the US Geological Survey in Three Rivers, California, warned that ruptured power lines are also an increasing fire hazard.
In a study on ignition sources in California going back to 1905, he found that in the past 40 years, direct human-caused ignition through arson, equipment or vehicle fires has dropped 10-fold compared with the preceding decades, but ignition from power-lines has either remained static or increased.
“In the northern part of the state, cases involving power lines have doubled in the past couple of decades,” he says. They were to blame, for example, in the massive fires last year that killed at least 15 people.
Keeley said that power lines can be ruptured by the powerful winds that frequently sweep across northern California at 80 kilometres per hour in bone dry conditions. To address the threat, power providers in San Diego county have installed around 200 weather stations along power lines to measure wind speeds and shut off power above potentially hazardous threshholds. The problem is that this might deprive communities fighting fires of power for pumping water.