OREGON STATE UNIVERSITY

Thinning may help save old-growth pine forests

08/12/2003

BEND - A new study by forestry researchers at Oregon State University has found that old-growth ponderosa pine, even trees more than 250 years old, can increase their growth, improve their health and respond quickly to thinning that provides the trees with more water.

The research, just published in the professional journal Plant, Cell and Environment, may have important implications for the management of old-growth pine forests in the western U.S., scientists say.

Millions of acres of these forests are in very poor condition, suffering from a century of fire suppression that has led to vastly overcrowded conditions, inadequate water and nutrients, poor tree growth, catastrophic fire and huge epidemics of insects that prey on the weakened trees.

The new study, which used a technical analysis of carbon isotopes to gauge tree water stress and photosynthesis before and after thinning, showed that even trees which are hundreds of years old can increase their growth and presumably their ability to resist insect attack if they are given a chance.

"Some people believe that old-growth ponderosa pine forests are decadent, that they can't really respond to the aggressive thinning that would restore conditions similar to those we had before fire was excluded from these forests," said Stephen Fitzgerald, an Extension silviculture specialist with OSU and expert on the types of pine forests that dominate much of the drier portions of the American West.

"We found that simply isn't true," Fitzgerald said. "This research demonstrates we can restore the health and growth of these forests, even with trees that are hundreds of years old. Most likely, we will need to use a combination of mechanical thinning and controlled fire or other techniques that are appropriate to the site."

Such approaches have gained some interest and use in areas near the urban-wildland interface in parts of the West, the researchers said, but very little use to date in vast areas of interior old-growth forests that are hugely crowded and in very poor health. This sets the stage for uncharacteristically intense stand-replacement fires or insect attack, particularly by mountain pine beetles, that will eventually kill the weakened old-growth stands, Fitzgerald said.

Historically, fire moved through many dry forest regions of the West as often as every 10-15 years, the OSU scientists say. In areas suitable for ponderosa pine, this "thinning by fire" resulted in a park-like setting of 12-35 huge ponderosa pine trees per acre, with very little underbrush or other trees.

Trees 300-500 years old thrived - with some trees living up to 800 years - and these healthy ecosystems supported a broad range of other plants, grasses and wildlife species that were associated with these park-like old-growth conditions.

With fire exclusion during the past century, these same areas now often have 1,000 to 2,000 trees or saplings per acre, instead of 12-35, the researchers said. The heavy undergrowth creates a "ladder" that can turn natural ground fires into stand-replacement fires that can kill the large trees. And all the vegetation, starved by intense competition for water, light and nutrition, declines in health.

"Part of what's clear is that we cannot save these old-growth pine ecosystems simply by putting a line around them and leaving them alone," Fitzgerald said. "If we just walk away from millions of acres of dry pine forest we may lose it all to fire or insect attack. To save the old-growth ponderosa pine we still have, we'll have to act."

The new study builds on previous research conducted in the 1970's by OSU distinguished professor Richard Waring, which showed that lodgepole pine trees in the Cascade Range were more resistant to insect attack after thinning. In their research, the OSU scientists studied the historical effect of thinning on several ponderosa pine stands near Camp Sherman, Ore., which had 60-80 percent of the tree stocking removed and probably 90 percent or more of the total number of trees, leaving only the largest and healthiest ponderosa pines.

The scientists examined the ratio of different "isotopes" of carbon in annual tree rings, which provided them with an index of water stress and photosynthesis over the past 20 years, including before and after mechanical thinning.

"A tree grows by consuming carbon through the process of photosynthesis, during which they take in two types of carbon, the isotopes carbon 12 and carbon 13, through holes in the needles called stomates," said Nate McDowell, a former OSU graduate student and lead author on this study. "Trees take in differing amounts of these two types of carbon depending on the degree of water stress they are experiencing.

"By studying the ratio of these two isotopes in the annual tree rings, we can tell when and if water stress was alleviated by thinning, even before any increase in wood growth occurs," McDowell added. "It appears that trees may have a lag time of several years in their stem growth after water availability and subsequent photosynthesis have improved. This indicates that growth may be taking place elsewhere in the plant, such as in the roots or canopy."

The study showed that after aggressive thinning, which provides more water to the remaining trees, the old-growth trees responded immediately and dramatically. Their increased vigor and photosynthetic response will help make them more insect and disease resistant, and the change in the forest structure significantly reduced the risk of stand replacement fire. The trees continued a higher level of growth for up to 15 years following the thinning, the study showed.

The thinning regimens necessary to accomplish this depended on the site and its level of overcrowding, but often included removal of almost all small and some of the medium-sized trees, the researchers said, along with brush and other vegetation. Thinning can also help step down the fuels in these stands, the researchers said, so that fire can be re-introduced more safely.

"The type of thinning that was used in this study is being done in very few interior, old-growth forests across the West," Fitzgerald said. "It's becoming increasingly clear that these techniques can bring these forests back to health and growth, and that's something we have to consider if we're serious about preserving old-growth pine forests.

"Simply leaving them alone may be their doom."