The Giving Trees

(Page 3 of 5)

On the other side of the country, in Petersham, Massachusetts, atmospheric chemist Steven Wofsy of Harvard University, another member of the AmeriFlux Network, studies a site that even more dramatically defies the theory that trees lose their ability to soak up carbon with age. Wofsy began the world's first long-term, large-scale eddy flux study at Harvard Forest in 1989. Since it was flattened in a 1938 hurricane, the stand has been increasingly dominated by red oak, a large tree with dense wood that absorbs impressive amounts of carbon.

"All the ecological models said that temperate forests stop their net carbon uptake at about 50 years," Wofsy says. "Eddy flux data has clearly shown that this is not true." At the start of the study, when the trees were a half century old, the researchers found that Harvard Forest was absorbing about 0.8 tons of carbon per acre every year. After 15 years the rate of carbon uptake -- expected to decline with age -- had instead doubled. It's too soon to tell how long that trend will continue in Harvard's forest of oak and maple; the Northeast has none of the intact older forests that Law is able to study out West.

Working with CarboEurope and other networks collecting eddy flux data, AmeriFlux researchers have been able to piece together a global picture of the interactions between the greenhouse gas and natural landscapes. While biological differences between one kind of forest and another may mean that the rate of continued carbon uptake will vary, an important general principle holds true. "Across forest types globally," Law says, "we find that the amount of carbon stored is high in older forests, and that live carbon [the carbon in living wood] continues to accumulate for centuries." AmeriFlux's findings are now publicly available online, and climate modelers are beginning to use the data to forecast the ways forest growth-or forest loss-could affect climate. Such models are used in simulations by the Intergovernmental Panel on Climate Change, whose authoritative reports shape climate policies worldwide.

But these findings are news to the foresters I know. All of them remember, from college textbooks, a graph of tree growth that shows young trees bulking up rapidly over the first few decades of their lives, reaching a peak at 60 years to 70 years. After that, growth rates drop off. This pattern, which indicates that the most profitable point at which to harvest timber comes before the trees reach a century of growth, is deeply ingrained forestry wisdom. Since individual trees grow by taking in CO2 during photosynthesis, most foresters believe that the same pattern that maximizes marketable timber also applies to overall carbon absorption by the forest as a whole. But the intertwined workings of trees, the microbes thriving in the soil, and the creatures who feed and shelter among them are much more complex than that.

"Young trees ‘eat' atmospheric carbon like teenagers devour pizza," wrote forester William Wade Keye in a recent opinion piece for the Sacramento Bee. "Mature trees store carbon, but does old growth capture more atmospheric CO2 than younger timber stands? No, it doesn't. Old forests have many ecological values, but they're essentially geriatric wards when it comes to their net growth."

Keye's argument ignores the importance of the large amounts of carbon held in the living wood and fertile soil of old forests. When such stands are cut, about a third of the carbon is captured in marketable timber; the rest is rapidly released into the atmosphere. Like most foresters, Keye appears unaware of recent studies by Law, Wofsy, and their colleagues. Eddy flux measurement, supplemented by careful accounting of the carbon absorbed and released from leaves, the live roots burgeoning beneath the soil, and the rotting detritus of the forest floor, reflects the life of forests in far greater detail than traditional forestry analyses, which are based on measuring only those trees that are large enough to produce marketable timber.

Today, market forces are intensifying the timber industry's impact on climate. "To compete with much cheaper supplies of fiber and wood from overseas, U.S. landowners have been harvesting more and more aggressively," says Laurie Wayburn, president of the Pacific Forest Trust, a nonprofit group based in San Francisco that focuses on conservation of privately held forests. "Pressure for timberland to deliver faster profits has transformed this industry from a long-term investment strategy for many owners to one that has to deliver an 18 percent to 20 percent return. The only way to accomplish that is through selling off land for development."

These trends can erase the ability of commercial forests to act as carbon sinks. In the fertile woods of the Oregon Coast Range, it was once common practice for landowners to wait until stands were 50 to 80 years old before logging. Now they are cutting timber as young as 30 or 40 years old. Law has found that as a result of the accelerating pace of harvest, the region no longer has any net value as a carbon sink.

"If people got paid for all the biomass in a forest rather than just the part that can be made into two-by-fours, that would be different," says Bill Stewart, a forestry specialist at the University of California at Berkeley. "If carbon comes to have real cash value, that will change the way everyone does business."

Continued...