The Giving Trees

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It turns out that forests hundreds of years old can continue to actively absorb carbon, holding great quantities in storage. Resprouting clear-cuts, on the other hand, often emit carbon for years, despite the rapid growth rate of young trees. This is because decomposer microbes in the forest soil, which release CO2 as they break down dead branches and roots, work more quickly after a stand is logged. On the dry eastern face of the Cascades, for example, where trees grow slowly, a replanted clear-cut gives off more CO2 than it absorbs for as much as 20 years. "That's a long time," Law observes, "during which microbes respiring in the soil, rather than trees photosynthesizing aboveground, dominate the carbon balance."

Can we develop a new model of forest economics that draws on this knowledge -- a model that makes sense to foresters as well as the policy makers and conservationists who are now taking the first steps toward developing a viable market in forest carbon? Depending on how we treat forests -- whether and for how long we allow them to grow -- they can be either major emitters of CO2 or highly efficient "sinks" that remove the greenhouse gas from the atmosphere. Because financial pressures drive deforestation, the hope is that putting a cash value on the carbon captured and stored by living trees will one day provide an alternative economic incentive to those who do the cutting.

From our windy perch atop the tower, Law and I look down on a 90-year-old stand of ponderosa pine quietly baking in the midday sun. These trees won't pack on much more girth in the next couple of decades, and in the eyes of a typical forester or timberland owner, they're more than ready for market. The conventional view is that this forest is also past its prime in its ability to sequester carbon dioxide.

Since the mid-1990s Law has monitored the movement of carbon in the ponderosa pine forests here along the Metolius River in the central Oregon Cascades, starting with a rare stand of ancient trees that contains pines as old as 250 years. She studies the forest ecosystem on every level, from the workings of a single leaf to sweeping landscape images produced by remote sensing satellites. She recently coauthored a study, published in the journal Biogeosciences, which tracks the exchange of carbon between land and air for the whole state of Oregon from 1980 to 2002. Earlier studies suggested that during the 1970s and early 1980s, publicly held Douglas fir forests in the West Cascades were being harvested so heavily that they emitted more carbon than they absorbed. After years of intense controversy over the loss of habitat for the northern spotted owl and other species that depend on old-growth trees, the federal Northwest Forest Plan curtailed most logging in the region's national forests, starting in the early 1990s. By the end of the decade the balance had tipped; on average, forests were offsetting up to 50 percent of the CO2 generated by Oregon's fossil fuel emissions each year.

Eddy flux measurement is one of Law's most crucial tools, enabling her to track the exchange of CO2 and water vapor between forest and air over large swaths of landscape, and at a level of detail that's never before been possible. The automated gas analyzers mounted on the eddy flux tower we're standing on measure CO2 concentrations 20 times per second. Meanwhile a sonic anemometer, a three-pronged device that resembles a robotic claw, tracks wind speed and direction. The combination of these two data sets reveals the shifting flow of carbon in and out of a forest, day or night, winter or summer. Law notes with pride that all the technology at this research site is powered by photovoltaic panels.

Other tools provide Law with additional insights into the flow of carbon through the intricate pathways of the forest. To photograph root growth, she slides a remote-controlled camera into a clear tube sunk belowground at a tree's base. Set on the forest floor are instrument-laden cylinders that hum to life every five minutes, lower themselves like miniature flying saucers, settle onto a patch of earth, and record the amount of carbon coming out of the soil.

Law's data show that this 90-year-old forest is, in fact, at the peak of its ability to absorb carbon. The uptake of carbon by ponderosa pines increases gradually, then reaches a plateau at some point between 50 years and 90 years. Once this plateau is reached, the trees and the soil will together continue to form a rich bank of stored carbon that cannot be equaled by any newly sprouted stand. During her work in California and the Pacific Northwest, she's found forests as old as 800 years that continue to absorb more carbon than they release.

Eddy flux technology has made it possible to set up a standardized way of tracking carbon in any ecosystem, anywhere in the world. More than 90 separate sites are now part of the AmeriFlux Network, studying jack pine and old-growth maple and birch in Michigan, loblolly and slash pine in North Carolina and Florida, and a Massachusetts hemlock forest, among others.

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