Forest management may mitigate global warming

A study published today, Nov. 23, in the journal Science suggests that forest management may be used to restrain the increase of atmospheric carbon dioxide, a greenhouse gas.

Many groups have proposed forest management as a simple way to offset global warming. More trees, they argue, will remove from the atmosphere more carbon dioxide, a gas plants use to grow and reproduce. But, numerous climatic and ecological factors confound this apparently simple solution, report researchers from the University of Wisconsin–Madison and Harvard University.

During the last century, the world’s average temperatures have risen by one degree and sea levels have risen by more than six inches as a result of increased amounts of both natural and human-produced greenhouse gases such as carbon dioxide in the atmosphere. Many scientists believe high levels of carbon dioxide, which is released when fossil fuels and wood products burn, could permanently alter the environment.

To investigate forest management as a method for controlling global warming, researchers conducted a decade-long study of carbon exchange between the atmosphere and Harvard Forest, a 60-year-old forest stand dominated by northern red oaks. Specifically, they measured how much carbon the trees and soils stored and how much they released.

In the short term, carbon exchange depended primarily on physical and climatic factors such as time of day or season. “At night, the trees respire, so more carbon dioxide is released from the forest,” says Carol Barford, one of the researchers who now works at the Center for Sustainability and the Global Environment. “During the day, the trees photosynthesize, which requires the net uptake of carbon.” Seasonal patterns also produced fluctuations. The date the fall and spring seasons began, the amount of snow covering the ground or the amount of rain during the summer all seem to affect carbon exchange, she says. “Discrete weather features make a big difference in carbon balance from year to year.” But Barford adds, “Over a decade, all those variations wash out.”

The researchers’ results suggest that, in the long term, ecological factors — not climatic ones — change carbon balance. The types of tree species in the forest, their growth rate and the age of the forest can all alter carbon uptake. These factors, Barford notes, can be influenced by forest management. At maturity, for example, trees store less carbon and remove less carbon dioxide from the atmosphere. The number of dead trees also affects carbon balance. When a tree decays, Barford explains, it releases some of its carbon back into the air.

Barford and the Harvard team used two methods to determine carbon dioxide levels. A tower-based method, known as eddy covariance, measured the net storage and net release of carbon into the air. Ground-based biometry, a longer-standing method, measured changes in the trees’ diameter, a direct indication of carbon storage. Both measurements produced similar results — an important finding, Barford says, that brings validation to the newer method.

These results suggest that forest management can help mitigate global warming by controlling carbon exchange — but numerous factors should be considered. “What to do about forest management is a complex issue,” Barford stresses. “Our results do not lead clearly to any one management recommendation.”