Fire retardant dropped from aircraft has been found to kill fish, but a new study on Mount Jumbo suggests it also spreads some species of noxious weeds across Montana's native grasslands.
The study raises the possibility that the red slurry, while helping to slow a wildfire's advance, could ultimately worsen grassland fires by promoting the growth of cheatgrass, one of the most flammable invasive weeds in the West.
"By no means does this suggest that retardant should be eliminated as a method for fire control, only that there may be an environmental cost" to native grasslands, said Levi Besaw, a Salish Kootenai College student and the study's lead researcher.
Besaw and University of Montana researchers are studying the effects on annual and perennial plants from 13,000 gallons of fire retardant dropped on Missoula's Mount Jumbo during the Fourth of July fire in 2006.
According to preliminary results, the retardant's fertilizerlike nutrients significantly increased cheatgrass and tumbleweed mustard, both exotic annual species, at the expense of native perennial grasses on the mountainside.
The invaders benefit from the jolt of nitrogen and phosphorous in the slurry, which native and exotic perennials largely ignore because they are accustomed to nutrient-poor soils.
Cheatgrass and tumbleweed mustard didn't spread where the fire burned alone, but they exploded in areas that were burned and hit with retardant, the study found.
The two invaders have spread from 51 percent to 88 percent on Mount Jumbo since the retardant was dropped, although two perennial invaders, spotted knapweed and Dalmation toadflax, decreased.
The Fourth of July fire burned about 320 acres and fire retardant was dropped on about 12 acres of Mount Jumbo, where noxious weeds have become widespread over the past 20 years.
The two-year study, which is to be completed next spring, is being conducted by Besaw and Giles Thelan, a research specialist at UM's plant ecology laboratory.
The researchers have reseeded areas that were burned or doused with retardant to see which native plants grow back the best.
Nationwide, federal and state agencies drop an average of 15 million gallons of retardant annually and up to 40 million gallons in some years on wildland fires.
Fire retardant is about 85 percent water, but ammonia compounds constitute up to 90 percent of the retardant product. Thickeners, which give the mixture its molasseslike consistency, make up the rest. The slurry reduces the intensity and speed of wildfires by robbing them of oxygen and slowing fuel combustion.
Invasive weeds have spread across the United States in recent decades, crowding out native plants, worsening wildfires, reducing ecosystem diversity and damaging wildlife forage.
Cheatgrass is a major culprit in the West. It depletes soil water that native plants need and takes over burned land, creating volatile fuel for future wildfires.
Previous studies have shown that the ammonia compounds in fire retardant kill fish, algae and aquatic insects. A federal judge in Missoula on Jan. 11 threatened to block the U.S. Forest Service's use of all aerial fire retardants nationwide, except water, because the agency failed to conduct proper environmental studies.
But relatively little is known about the chemicals' impact on plants and animals on land.
Some studies have found that nitrogen and phosphorous in fire retardant act like fertilizer, but exotic weeds usually out-compete native plants for those nutrients, leading to a decline in the richness and diversity of indigenous vegetation.
Morgan Valliant, Missoula's conservation lands manager, said the study's preliminary results have major implications for managing native grasslands, which evolved with fire but not with the chemicals in fire retardant.
Valliant said aerial fire retardant will continue to be used on open spaces near cities to protect homes and people, but he said the study helps land managers understand how to restore prairie grasslands after wildfires are extinguished.
That means reseeding areas that were hit with fire retardant with native plants before exotic species can take hold, a task that costs about $25 an acre, he said.
Reporter John Cramer can be reached at 523-5259 or at email@example.com