Rain forests may slow their growth in warmer world. (Feel the Heat).

During a long-term research project in a Central American rain forest, mature trees grew more slowly in warm years than they did in cooler ones. This observation hints that tropical forests may become less efficient at removing planet-warming carbon dioxide from the atmosphere if global temperatures continue to rise.

From 1984 to 2000, scientists studied the old-growth forest at La Selva, Costa Rica. Annually, the team measured the diameter of all mature trees within a 2-square-kilometer area. They found that diameter growth varied significantly from year to year and was related to average daily temperature. The annual tree growth from 1984 to 1986, the coolest interval during the period, averaged 81 percent greater than the growth tallied during the record hot spell related to the El Nino that began late in 1997. The average daily temperature difference between the two periods was about 1.4[degrees]C.

Tree growth in the forest was also particularly slow during the El Nino year of 1987, says Deborah A. Clark, a biologist at University of Missouri-St. Louis. Clark and her colleagues report their results in an upcoming Proceedings of the National Academy of Sciences.

Looking at global carbon dioxide measurements during the same period, the researchers noticed that quantities of the gas attributable to land plants in tropical regions increased during warm years. That phenomenon could stem from typical plant-growth characteristics, the researchers say.

Plants use photosynthesis to convert sunlight, carbon dioxide, water, and nutrients into carbohydrates. When the plants tap into their stores of carbohydrates for chemical energy, however, they return carbon dioxide to the atmosphere--just as animals do--in the process called respiration. Although a plant's rate of photosynthesis begins to drop off above a temperature that's characteristic of its species, its rate of respiration continues to rise with increasing temperatures, says Clark.

Most of the observed global spikes in carbon dioxide during warm years probably stemmed from the increased respiration of tropical land plants, but some may have been produced by other sources, such as forest fires or agricultural burning, says Stephen C. Piper, a biogeochemist at Scripps Institution of Oceanography in La Jolla, Calif., and a coauthor of the team's report.

The growth rate of mature trees can be a useful indicator of the climate's effect on the rest of an ecosystem, says David S. Schimel of the National Center for Atmospheric Research in Boulder, Colo. The link that Clark's team discovered between slow growth rates in Costa Rican trees and increases in the atmospheric carbon dioxide traceable to tropical plants is "an innovative result that's hard to argue with" he says.