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.