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Modern forests suffer from century-old logging legacy

December 05, 2005

By the early 20th century, loggers had harvested more than 90 percent of the forests covering the upper Great Lakes region. The legacy of that destruction continues to have a substantial impact on the environment, researchers say.

Nearly 70 years after this major disturbance, experimental forested plots in the current study have not returned to a point where they store as much carbon as the original stands. And researchers aren't sure just how long it might take to return to that point.

Forests serve as storage areas for carbon in the form of carbon dioxide, a key atmospheric pollutant that contributes to global climate change.

Although many of these harvested areas have regrown, poor forest management practices at the turn of the 20th century have reduced by half the amount of carbon that modern forests can store, said Christopher Gough, the study's lead author and a postdoctoral researcher in evolution, ecology and organismal biology at Ohio State University.

"It's remarkable that there is still this huge reduction in forest productivity," Gough said.

The more carbon that a forest can store, the more productive that forest is thought to be.

Scientists estimate that forests in North America today store about 10 to 12 percent of the total amount of carbon emitted by sources such as industry and automobiles in the United States and Canada .

"We're living with the consequences of bad management practices from a hundred years ago," said Peter Curtis, a study co-author and a professor of evolution, ecology and organismal biology at Ohio State . "This legacy is actually reducing the potential carbon storage capabilities of today's forests."

Gough, Curtis and their colleagues presented the findings December 8 in San Francisco at the meeting of the American Geophysical Union.

The researchers measured the amount of carbon stored in several forested study plots that were harvested and burned at some point during the past 100 years. These areas were part of a biological research station in northern lower Michigan. Study plots were left to regrow after experimental clear-cut harvesting and burning anywhere from 6 to 68 years ago. This experimental disturbance imposed by researchers was in addition to the widespread logging and fire destruction of the early 20th century.

Several adjacent plots that had not been experimentally harvested and burned were used as a control.

The measurements showed that the maximum annual amount of carbon stored in the harvested and burned study plots was half of the amount stored in the control forest.

A century ago, loggers were likely to clear an entire area of trees but take only the choice trunks. That left behind an abundance of smaller trees, along with branches and leaves. The debris dried out and in effect became kindling; in many cases these clear-cut harvests were followed by uncontrolled fires caused by lightning or other means.

"This kind of slash followed by burning is similar to the current patterns of disturbance in many developing countries," Curtis said.

Although controlled burning is sometimes used as a forest management practice today, the uncontrolled fires of a hundred years ago were devastating.

"The slash - the branches and leaves and smaller trees that loggers leave behind - contains nutrients that are eventually recycled back into the forest system," Gough said. "But a lot of these nutrients went up in smoke with the fires, and the aftermath continues to have a negative impact on a forest's ability to store carbon decades later."

Carbon remains in leaves, tree trunks, branches and roots, and in the debris that cover a forest floor. Carbon is also stored in soil -- microorganisms break down dead leaves and branches into minute particles that eventually become part of the earth. When a forest is clear-cut harvested and burned, much of the carbon it contains is released into the atmosphere.

"We now know that the amount of carbon that an acre of forest can store depends on how severely it was disturbed in the first place," said Gough.

Gough and Curtis worked with researchers from the University of Michigan's Biological Station in Pellston, Mich.; Middlebury College in Middlebury, Vt.; and Colorado State University, in Fort Collins.

Ohio State University