Tropical Forests Fuel Warming?

Precarious position? A new paper argues that tropical trees may put out more CO2 than they soak up.

A provocative study of long-term tropical tree growth warns that global warming could happen faster than anticipated. The researchers found that in hot years, trees in a Costa Rican rainforest grow less, and the tropics worldwide release lots of carbon dioxide (CO2). If tropical forests globally are as sensitive to warming, the authors suggest, they could soon become a major source of the greenhouse gas instead of a sink. That, in turn, could send global temperatures soaring much faster than anyone has estimated.

Trees soak up CO2 through photosynthesis, but they also continuously respire the gas. A decade ago, most climate scientists believed that these processes balanced out. Some recent studies have suggested that tropical forests are now absorbing carbon, perhaps because extra CO2 from human activities acts as a fertilizer.

Not so, according to the new work. Ecologists Deborah and David Clark of the University of Missouri collected tree growth data from 1984 to 2000 for six species of trees at La Selva biological station in Costa Rica. Looking across their data, they saw a pattern of decline in tree growth with warmer night temperatures, which, they hypothesize, cause trees to release more CO2 through respiration.

Meanwhile, collaborators Dave Keeling and Stephen Piper of the Scripps Institution of Oceanography in La Jolla, California, charted annual variations in atmospheric CO2 stored or released by tropical plants on land. These CO2 fluctuations matched up with annual tree growth at La Selva. Together, the authors argue, the data suggest that less growth, combined with fires, deforestation, and other processes, is making tropical forests a source of CO2 in warmer years. During the El Niño of 1997 to 1998, they say, tropical forests pumped out a whopping 6.7 petagrams of carbon a year, an amount equal to CO2 from industrial sources. The scientists report their findings online today in the Proceedings of the National Academy of Sciences.

The link between slower tree growth and warmer temperatures "is fascinating" and "extremely valuable in showing how sensitive systems might be" to global warming, says ecologist David Schimel of the National Center for Atmospheric Research in Boulder, Colorado. But others say it's a stretch to link a single forest to atmospheric CO2 levels across the tropics. For now, says Chris Field of the Carnegie Institution of Washington at Stanford: "It's a very intriguing and potentially very important data set."

Related sites
La Selva biological station
Background on the carbon cycle
The International Center for Tropical Ecology, with links to the Clarks' Web sites

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