Study finds crown fires can permanently kill off forest

Study finds crown fires can permanently kill off forest

12 February 2013

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USA — The ponderosa pine forests consumed in the most intense portions of the 462,000-acre Rodeo Chediski Fire may never come back, according to a long-term study of the slow, often faltering, effort of the forest to repair the damage from the massive blaze.

The researchers spent years comparing two, adjacent, 60-acre patches of forest — one consumed by a crown fire that killed almost every single tree and a second that suffered only moderate fire damage that ultimately killed only about 15 percent of the trees.

Although brush and grass returned quickly in even the severely burned sections, the ponderosa pines that had once dominated the area made almost no recovery even years later, concluded the Forest Service researchers at the Rocky Mountain Research Station.

Moreover, the study also found a dismaying post-script — even in the lightly burned area. Despite all the trees and downed wood consumed, the fire caused the delayed deaths of so many trees that the area wound up just as plagued by dead and downed wood as ever — and therefore remained at risk of another devastating fire in the future.

The Rodeo Chediski Fire was in 2002 the largest fire in recorded state history. It inflicted an estimated $175 million in damage, consumed 500 homes and forced 30,000 to flee their communities. The Wallow Fire, which consumed more acres, has since eclipsed it, but did less property damage.

The study paints a bleak picture of the future of the world’s largest ponderosa pine forest, which stretches from the Grand Canyon on into New Mexico and includes most of Rim Country. As a result of a century of grazing and fire suppression, millions of acres have such high tree densities that they remain vulnerable to high-intensity crown fires. The study suggests that such high-intensity fires can force long-term changes in the once fire-adapted ponderosa pine forests on which Rim Country’s tourist-oriented economy depends.

The researchers spent years exhaustively measuring the contrasting recovery of two stretches of forest, both on Stermer Ridge at the headwaters of the Little Colorado River. They estimated the surviving trees, the amount of wood on the ground, stream flows, soil absorption, the total mass of grass and shrubs and the number of elk, deer, squirrels, rabbits and other animals.

Watershed A suffered a high-intensity crown fire, which means the flames jumped from one treetop to the next — rather than burning along the ground. The fire killed about 55 percent of the trees immediately — and about 75 percent of the survivors within a year or two. The patch of ground ended up bereft of trees, with even the fire-adapted gambol oaks mostly dying off along with the ponderosas and junipers.

On the adjacent Watershed B, a road served as a firebreak that halted the crown fire next door. Instead, the fire there burned along the saplings, shrubs and downed wood on the ground. It consumed 5 percent of the trees immediately, with a total of about 15 percent dying from the effects of the fire in the next two years.

The study demonstrated the dramatic effects of such high-intensity fires.

For instance, the searing heat of the crown fire fused the soil in Watershed A, sharply reducing the rate at which the ground could absorb water — making it “hydrophobic.” Two-thirds of Watershed A had strongly hydrophobic soil and one third has moderately water repellent soil.

By contrast, in Watershed B only one-third of the soil was strongly and 15 percent moderately hydrophobic.

The combination of the loss of the trees and the changes in the soil produced dramatic changes for the next three years whenever it rained.

For instance, a storm on Watershed A produced a stream flow that was 2,230 times what would have flowed into that same stream with the same amount of rainfall before the storm, according to estimates. By contrast, the storm increased stream flows in Watershed B by about 50 percent compared to pre-fire levels.

In the fall of 2002, the severely burned area lost about 28 tons of topsoil per acre to erosion, compared to about 17 tons per acre in the moderately burned area. In the spring of 2004, the severely burned area lost 35 tons of topsoil per acre compared to about 20 tons in the moderately burned area. A stable area in a normal year will lose almost no topsoil to erosion.

The figures offer a sobering cautionary note for Rim Country, whose water future now depends on the Blue Ridge Reservoir, which sits in a small, wet, thickly forested watershed. Payson officials have urged the U.S. Forest Service to make thinning the watershed of the Blue Ridge Reservoir a high priority, for fear a crown fire could cause a dramatic increase in erosion — which would reduce the life of the deep, narrow reservoir.

The study found that plants, grass and shrubs returned to both areas quickly — with the severely burned area actually producing more grass and shrubs initially than the lightly burned area. That’s probably because in the lightly burned area most of the trees survived and continued to shade the ground and compete for water with the ground cover.

Elk actually used the severely burned area more than the lightly burned area initially, probably reflecting the initial, denser growth of grass. Mule deer returned quickly to both areas, but in smaller numbers.

On the other hand, many of the smaller animals like rabbits remained all but absent in the severely burned area — along with pine tree dependent species like Abert’s Squirrels.

Fewer birds also returned to the severely burned area, probably because they no longer had the diverse habitat offered by the tree canopy.

The biggest impact of the crown fire remained the virtual elimination of the ponderosa pines that had previously dominated the ecosystem.

“Most of the trees that survived the fire but were severely damaged by the burn died within two years of the burn and had started falling to the ground. Salvage cutting and fuel reduction treatments on Watershed A eliminated most of the remaining standing trees. Stocking of ponderosa pine seedlings after the wildfire was insufficient on either of the watersheds to sustain a ponderosa pine forest,” the researchers concluded.

The report included one other ironic — and discouraging note.

The crown fire did eliminate the possibility of another crown fire in the severely burned area, since it killed all the trees. However, the fire left behind enough brush and downed wood to sustain another ground fire.

Unfortunately, so did Watershed B. Because the fire didn’t completely consume most of the trees it damaged, tons of unburned dead wood remained on the ground. As a result, Watershed B could still sustain a serious fire. Moreover, the salvage logging actually left behind tons of scraps that pose a serious fire danger in the future. The findings suggest that it could take repeated low-intensity fires in a single area to fully restore the natural fire resistance of an intact ponderosa pine forest.

“Much of the hydrologic functioning of this watershed is returning slowly to its pre-fire level. The post-fire loadings of the flammable fuels were largely unchanged from their pre-fire estimates. Consequently, this watershed remains vulnerable to future wildfire events,” the researchers concluded.


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