Old-growth trees still soaking up CO2, study shows


CORVALLIS - New research is finding that old-growth coniferous forests of the Pacific Northwest are still vigorously active, may have more ability to "store" carbon than had been appreciated in the past, and are not the idle, decaying ecosystems they have sometimes been portrayed to be.

In pioneering studies done with the huge "canopy crane" that hovers over an old-growth stand northeast of Portland, Ore., researchers from Oregon State University are also discovering that light is the driving force in these processes and that the real action is way up high where the sun shines the brightest.

Studies such as this, the scientists said, may have important implications for forest management policies that seek to use forests as a carbon "sink," lowering the level of atmospheric carbon dioxide and ultimately helping to mitigate the greenhouse effect.

"It appears these older forests are more active and may be stronger carbon sinks than we thought," said Bill Winner, an OSU professor of botany and plant pathology. "There's a huge amount of carbon tied up in old-growth ecosystems and, even at a very old age, they are still capable of absorbing a lot of carbon dioxide."

In preliminary results, Winner and OSU colleagues Sean Thomas and Mark Harmon have found:

- In all seasons, the physiological activity level of conifer needles is higher at the brightly-lit tops of trees than at the bottom or in younger saplings that receive more shade.

- The photosynthetic rate of trees does not decline in summer due to drought and water stress, as had been presumed.

- The biggest constraint on photosynthesis is the lower light levels during the region's eternally-overcast winter days, which can cause up to a 60 percent drop in photosynthesis in some tree species.

- In a system like that studied, Harmon found that about 70 percent of the carbon storage is in live vegetation, 15 percent is in the litter and logs on the forest floor, and 15 percent in the mineral soil.

"The use of this crane has allowed us to make meaningful samples high in the forest canopy for extended periods," Winner said. "We've never really had that capability before, and that's helping to answer some long-standing questions about old-growth ecological processes."

These trees and other vegetation generate energy from sunlight, water and carbon dioxide, in the process "storing" some of the carbon in the form of wood and foliage, and releasing oxygen. The woody tissue, soil and decaying matter on the forest floor also "exhale" some carbon dioxide. The key environmental impact, researchers say, comes from the balance of carbon released versus that retained.

For this type of forest, it's now clear that the heaviest rates of photosynthesis and carbon storage happen when the daylight is longest and the light is brightest. Preliminary calculations suggest that during summer months this site "stored" from 2.7 to 14 grams of carbon dioxide per square meter, per day.

According to Harmon, when an old-growth forest is clearcut, it changes from a carbon sink to a carbon source - meaning the same land now gives off more carbon dioxide than it takes in - for a period of at least 30-40 years.

Studies such as these may help provide better ways to determine exactly how much carbon is being released or retained by such forests, Winner said, and provide the basis for better forest management and policy decisions that wish to take carbon cycles and greenhouse concerns into account.

Some efforts are underway in the U.S. and elsewhere to develop systems of carbon "credit trading" where the emitter of certain greenhouse gases may pay other entities, such as the owners of a forest, for activities that result in carbon accumulation and help mitigate the greenhouse effect.

"Looking at an old growth forest, you can imagine that half the dry biomass is the element carbon, and that carbon in the trees originated as carbon dioxide gas in the atmosphere," Winner said. "It was acquired by the trees via the process of photosynthesis. Now, for the very first time in the canopy of old growth trees, we're exploring the connection between carbon in forests and the atmosphere."

Understanding those linkages will be essential for managing forest resources and knowing how forests may affect atmospheric carbon dioxide concentrations and ultimately the Earth's climate, Winner said.

Research on this project at the Wind River Canopy Crane in Washington state is being supported by the Westgec Program of the U.S. Department of Energy, Winner said, which seeks to better understand global change processes.