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North America Taking Up Some of South American Slack

Disappearing forests in South America partially offset by fast growing North America trees.

Anyone interested in the environment and global warming is likely to be aware of the problem of South American rainforests being cleared and put into agricultural production. Less well known is that some of that loss is being offset by forests in North America.

Reforestation after timber harvesting operations, or other land uses that removed native forests, has long been a priority with conservationists in North America. Protecting the natural beauty of trees, wildlife habitat and ensuring a a sustainable forestry harvest have topped the reasons for these efforts. Now new research indicates that the reforestation of former farmland over the last century has played an important role in reducing the accumulation of greenhouse gasses in the atmosphere.

Scientists at Princeton University report that changes in land use have been critical in allowing North American forests to regrow and soak up large amounts of carbon dioxide. Previous studies had suggested that other factors, such as the fertilizing effects of carbon dioxide, were spurring forests to absorb more carbon dioxide.

"Changes in the way we manage our land have had a real impact on the global environment," said the paper's lead author, John Caspersen. The finding makes it clear, however, that this benefit will not continue indefinitely, because the regrowth of forests will slow as they mature.

Scientists have been trying for more than a decade to track the fate of carbon dioxide pumped into the atmosphere as a result of burning fossil fuels. Early studies showed that despite six billion tons of the gas emitted each year, only three or four billion tons accumulate in the atmosphere. Landmark studies from Princeton and elsewhere showed that trees and other land plants, which absorb carbon dioxide during photosynthesis, were taking up a large part of the "missing" carbon. Then, in 1996, a Princeton-led group reported that much of this absorption was happening in the United States and neighboring countries - a phenomenon called the "North American carbon sink."

Still, it was not clear what was causing North America to absorb so much carbon. Some evidence suggested that carbon dioxide itself would stimulate plant growth, thus causing more carbon dioxide uptake. Increased nitrogen pollution and global warming also could stimulate plant growth. Studies published in recent years have estimated that these "enhancement" effects account for 25 percent to 75 percent of the forest carbon sink.

The new study puts that figure at only 2 percent, with the rest coming from the recovery of forests on land that had been cleared for agriculture in the 1800s. In a collaboration between scientists at Princeton, the University of New Hampshire and the U.S. Forest Service, the researchers performed a careful analysis of inventory data in five states, comparing recent growth rates to historical growth rates. The analysis showed that forests have been growing at nearly the same rate for most of this century, ruling out major enhancement effects.

"Identifying the cause of the extra carbon uptake in forests allows us to better predict the future of the sink," said Caspersen, who is in the Department of Ecology and Evolutionary Biology. The result is also important for scientists developing computer models of ecosystems and climate. Many of these models only take into account physiological processes, such as the supply of nutrients and carbon dioxide, whereas the dominant factor governing carbon uptake in North American forests is historical changes in land use.

"It will be difficult to gain an accurate understanding of how our climate is changing without coming to terms with the effects of land-use changes," concluded Caspersen.