It's been known for a while that global warming speeds the decomposition of organic matter in soil, speeding up carbon dioxide release.
But new research shows that it can also have a beneficial effect, by allowing trees to store more carbon.
Jerry Melillo of the Marine Biological Laboratory (MBL) carried out a seven-year study at Harvard Forest in central Massachusetts. A quarter-acre section of the forest was artificially warmed by about nine degrees Farenheit, to simulate the amount of climate warming that might be observed by the end of the century.
The study confirmed that a warmer climate causes more rapid decomposition and increased the amount of carbon dioxide being released. However, it also showed, for the first time in a field experiment, that warmer temperatures stimulate trees to store more carbon as woody tissue, partially offsetting this loss.
The effect is caused by more nitrogen being made available to the trees with warmer soil.
"Tree growth in many of the forests in the United States is limited by the lack of nitrogen," says Melillo.
"We found that warming causes nitrogen compounds locked up in soil organic matter to be released as inorganic forms of nitrogen such as ammonium, a common form of nitrogen found in garden fertilizer. When trees take up this inorganic nitrogen, they grow faster and store more carbon."
We still can't rest easy, says Melillo. While the increased tree growth can be expected to show up in many temperate and boreal forests in North America, Europe and Eurasia, it may not occur in the tropics. There' he warns, tree growth is often limited by factors other than nitrogen.
"The carbon balance of forest ecosystems in a changing climate will also depend on other factors that will change over the century, such as water availability, the effects of increased temperature on both plant photosynthesis and aboveground plant respiration, and the atmospheric concentration of carbon dioxide," he warns.