Study of Tripod fire reaffirms the value of thinning forests

WINTHROP — A three-year U.S. Forest Service study confirmed what fire managers already knew: Wildfires are not nearly as deadly for trees in areas previously thinned and burned.

A study of the Tripod Complex Fire — which in 2006 burned 175,000 acres from Winthrop to Conconully to Loomis — showed the fire killed more than 85 percent of the trees in parts of the forest that had not been treated. But only 43 percent of the trees died in areas where thinning and prescribed fires were used to reduce fuels. And for larger trees with an eight-inch or larger diameter, only one-quarter were killed by the fire.

The study was published in the August issue of the Canadian Journal of Forest Research.

“It is a common sense thing, but the strange thing is, we don’t have a lot of empirical data to validate this kind of management,” said Dave Peterson, research biologist for the Forest Service’s Pacific Northwest Research Station. He co-authored the study with Susan Prichard, a University of Washington research scientist.

Thinning produced only slightly better results than no treatment at all. About 81 percent of trees died in an areas where thinning only was used to reduce fuels, the study found.

Peterson said looking at the Tripod fire, the difference is profound. “It’s pretty black over much of that landscape,” he said. But the areas previously burned are “like green postage stamps.”

He said no areas were studied where prescribed burning was used without thinning because the Okanogan-Wenatchee National Forest virtually always thins before burning.

Peterson said fire managers have long known that dead fuels on the ground add energy to a wildfire, and quickly carry it across dead stands and into the tops of living trees.

But few studies have been done to look at the effectiveness of thinning and controlled burning.

The Tripod fire proved to be the perfect testing area, because the fire was so large, and because the Okanogan-Wenatchee National Forest had excellent records about fire management efforts throughout the burned area, he said.

“We know exactly what they did, and how long ago they did it,” he said.

Peterson said the areas studied were between 8 and 100 acres in size, and had the same basic slope, elevation, and composition of species. “They were all within the mixed conifer, dry forest area, with ponderosa pine and Douglas fir being the dominant species,” he said.

The study can help show that even small controlled burns can be used to protect particular resources, like buildings, a lake or an old growth forest.

“Rather than just plopping them randomly, we might use them to reduce the spread of a large fire,” he said.

He said other studies have shown the benefits of fire. This study supports continuing the use of thinning and burning to reduce the severity of wildfires in the dry, ponderosa pine forests.