SISTERS, Ore. » A patch of ponderosa pines here in the Deschutes National Forest has been carefully pruned over the last few years to demonstrate the U.S. Forest Service's priorities in the changing West: improving forest health and protecting against devastating wildfire while still supporting the timber economy.

Yet occasionally, when tour groups come through, someone will ask what role the trees might play as the nation addresses global warming. After all, forests soak up carbon dioxide as they grow.

“;We've always said that's outside the scope of this project,”; said Michael Keown, the environmental coordinator for the Sisters Ranger District, which includes more than 300,000 acres in the Deschutes forest in central Oregon. “;But those days have come and gone.”;

The giant evergreens of the West have long been proclaimed essential, whether the cause was saving salmon and spotted owls or small towns and their sawmills. Now, with evidence showing that American forests store 15 percent or more of the carbon gases produced in the nation, expectations are growing for them to do even more.

Over the next 50 years or so, experts say, some forests could be cultivated to grow bigger, more resilient trees, potentially increasing their carbon storage by 50 percent and providing an important “;bridge”; to a time when the nation will theoretically have shifted away from greenhouse-gas producing fossil fuels.

But even as some private forests are already being marketed as “;carbon sinks,”; or storehouses, government agencies and academics are struggling to better understand and measure how carbon is stored and released. After decades of controversy surrounding the management of forests, debate persists over how they can best be used to fight global warming while also being protected from their threats, including more and bigger wildfires.

“;While healthy, functioning forests may serve as a means to sequester carbon, under current practices, many of our Western forests are at risk of turning from a carbon sink to a carbon source,”; Tom Tidwell, the head of the Forest Service, told a Senate subcommittee on Nov. 18 in a hearing on forest management and climate change.

“;Projections indicate that while these forests continue to sequester more carbon in the short term,”; Tidwell said, “;in 30 to 50 years, disturbances such as fire and insects and disease could dramatically change the role of forests, thereby emitting more carbon than currently sequestering.”;

The challenges and benefits range by region. Studies show that the potential carbon capacity of the predominantly fir forests on the wet west side of the Cascade Range in the Pacific Northwest is at least three times as high as that of the drier regions over the mountains and to the southwest.

Many drier forests, including here east of the Cascades, have grown unnaturally dense after logging and efforts to save them from wildfires. Experts say measures taken to stop fires can end up causing more devastating ones by allowing the growth of small trees and underbrush, “;ladder fuels”; that ignite bigger trees.

On federal lands, the Forest Service has recently emphasized removing ladder fuels, including in the demonstration project here in the Metolius Basin.

“;The suite of things we're doing benefits the carbon sequestration,”; said Brian Tandy, who helps oversee forest growth in the Deschutes. “;We weren't doing it to address some of that specifically, but the way we're moving is sort of in line with that.”;

Still, after years of fights over logging practices, including lawsuits to reduce clear cutting on federal land, distrust of the Forest Service's motives remains. Tandy made a point of saying that one reason he does what he does is to help meet “;society's needs for wood products.”;