Burn it to ash and sterile soil.

Or thin it and let fire restore a healthy forest.

The choices remain stark, according to a mounting body of research on the effects of thinning the West’s dangerously overgrown, desperately unhealthy thickets.

The Wallow Fire remains a searing case in point — largely because the soil-scorching crown fire burned through numerous areas the White Mountain Stewardship Project had thinned

Those thinning projects dramatically reduced tree death in the years following the Wallow Fire, according to a massive study undertaken by researchers from the Ecological Restoration Institute and the School of Forestry at Northern Arizona University. They published their results in the journal of Forest Ecology and Management.

The research has vital implications for the entire western United States, where tree densities have increased to 10 or 20 times normal and megafires have come more and more often. Some 44 million homes in the U.S. have been built in the wildland-urban interface (WUI) — where forest meets subdivision. The number of damaging fires has soared in the past 20 years — with the annual federal wildfire budget now routinely topping $2 billion.

The almost 538,000-acre Wallow Fire remains the largest fire in recorded state history. The fire raced from treetop to treetop in the ponderosa pine thickets that have grown as a result of a century of grazing, logging and fire suppression.

A year after the Wallow Fire, researchers set up 18 matched sites for intensive study. The White Mountain Stewardship Project had treated half of those sites prior to the fire. The researchers compared the effect of the fire on those treated and untreated sites.

Here’s what they found:

— The treatments lowered tree densities by 80 percent and the total amount of biomass locked up in the trees by 57 percent.

— The fire killed 29 percent of the trees in the treated area compared to 80 percent in untreated areas. In many “high intensity” patches in untreated areas, the fire killed 99 percent of the trees.

— Fire intensity and therefore the effect on the soil proved much higher in the untreated areas. High intensity fires can turn the soil “hydrophobic” — which can result in floods that wash away topsoil and a long, slow recovery.

-- In treated areas, the fire killed just 6 percent of the large, fire-resistant ponderosa pines. In the untreated areas, 42 percent of those trees died.

— Treatment didn’t make much difference in the survival of tree species not well adapted to fire — like aspen and oaks.

— One year after the fire, the treated areas had 150 percent more grass and shrubs — which likely reflects the effect of the high temperatures on the seeds and soils in the untreated areas.

— Native plants constituted 84-89 percent of the recovered understory plant cover — about twice as much as in the untreated areas.

— Only about 3 percent of the treated areas generated “high severity” burns, which killed almost all the trees and altered the soil. However, high severity patches covered 34 percent of the untreated areas.

The researchers concluded, “the White Mountain Stewardship Project can be considered broadly successful in reducing fire severity.”

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