Data collected on forests in Panama and Malaysia has revealed that global warming could reduce the growth of trees in tropical rainforests by 50 percent, besides severely affecting their ability to remove carbon dioxide from the air.
According to Ken Feeley of Harvard University's Arnold Arboretum in Boston, the study shows that rising average temperatures have reduced growth rates by up to 50 percent in the two rainforests, which have both experienced climate warming above the world average over the past few decades.
Feeley, who presented his research at an annual meeting of the Ecological Society of America in San Jose, California, warned that if other rainforests follow suit as world temperatures rise, important carbon stores such as the pristine old-growth forests of the Amazon, could conceivably stop storing as much carbon.
The amount of carbon that a forest stores depends on the balance between the rate at which it draws carbon dioxide from the atmosphere through photosynthesis and the rate at which it gives carbon dioxide back through respiration.
In carbon sinks, which are mostly found at high latitudes, photosynthesis outstrips respiration and the amount of carbon stored increases. In general, tropical forests are today thought to act as stable stores of carbon, with their photosynthetic input and their respiratory output more or less in balance.
Some scientists and environmentalists have suggested that, given the way carbon dioxide spurs plant growth, tropical forests could in time come to act as a sink, offsetting some of the man-made carbon dioxide build-up.
Feeley and his colleagues analysed data on climate and tree growth for 50-hectare plots in each of the two rainforests, at Barro Colorado Island in Panama, and Pasoh in Malaysia. Both have witnessed temperature rises of more than one degree Centigrade over the past 30 years, and both showed dramatic decreases in rates of tree growth.
At Pasoh, as many as 95 percent of tree species were affected.
Feeley suspects that the effect occurs because plant photosynthesis is impaired if the temperature rises above a certain threshold.
"If we're correct and the temperature is driving these changes, this is something we're going to see in a lot more places. It has very important implications - we may need to look elsewhere for our excess carbon sink," Feeley predicts.
So far, the Amazon rainforest - the world's biggest - has not suffered significant climate warming. But with even the most optimistic predictions of climate analysts asserting temperatures are to rise by two degree Centigrade over the coming century, most rainforests could feel the effect before too long.