The reforestation of former farmland over the last century has played an important role in reducing the accumulation of greenhouse gasses in the atmosphere, according to Princeton scientists.
The scientists, publishing in the Nov. 10 issue of Science, reported 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. The results could have important implications for policymakers wrestling with the question of how to reduce the accumulation of carbon dioxide in the atmosphere.
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. And understanding the how the sink will evolve and decline could be important in making plans to reduce greenhouse emissions.
Negotiators from around the world are due to meet in The Hague this month in a push to resolve details of the Kyoto Protocol, which seeks to limit the production of carbon dioxide and other greenhouse gasses. Among the chief issues, according to the United Nations, is deciding on "rules for obtaining credit for improving 'sinks' (by planting new trees to absorb carbon dioxide from the atmosphere, for example, thus offsetting emissions)."
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.
Materials provided by Princeton University. Note: Content may be edited for style and length.
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