Sierra fires have become larger and more damaging in the last 20 years

The study, published online this month in the journal Ecosystems, found that between 1984 and 2006, the proportion of burned areas where no trees survived increased, on average, to nearly 30%, from 17%.

Climate is playing some role, the study said. But it blamed a bigger factor: Federal efforts to quench most blazes quickly have thwarted the Sierra Nevada's natural cycle of frequent, house-cleaning fires and left forests packed with fuel.

"This just blind effort to continue to put everything out is probably backfiring on us," said Hugh Safford, a U.S. Forest Service ecologist and one of the study's authors. "We've created our own nightmare."

Blazes in mid- and low-elevation forests have grown more severe in large part because there is more to burn. A jump in average annual precipitation across the range since 1908 has promoted forest growth, while a rise in temperature is diminishing the mountain snowpack and lengthening the fire season.

The study, based on satellite imagery of the Sierra and southern Cascade ranges, also found that the average size of severely burned forest patches caused by individual fires has roughly doubled in recent decades.

"It may simply be that most low- and middle-elevation forest lands in the study region are ready and primed to burn," the researchers wrote.

When fires burn more fiercely, they can cause more ecological damage, destroying habitat for wildlife species, increasing erosion and releasing more carbon into the atmosphere.

Safford said there are places where wildfires have to be aggressively fought, such as Southern California. But in more remote areas far from development, Safford said, it may be better not to rush to douse the flames.

"We have to think a little more strategically about what we're doing out there," he said.

Federal land managers have the freedom to monitor, rather than fight, lightning-caused blazes that don't threaten people or property. But only about 30,000 of the more than 1.3 million acres that burned in California were managed that way.

The Forest Service is making more use of another approach that lets firefighters focus their efforts on the most worrisome parts of a wildfire rather than battling all of it.

That strategy was followed last summer in some of the Northern California fires that burned in Big Sur.

Along with Safford, Forest Service remote sensing specialist Jay Miller, M. Crimmins of the University of Arizona and A.E. Thode of Northern Arizona University conducted the research.

Boxall is a Times staff writer

bettina.boxall@latimes.com