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Global warming could cut the rate at which trees in tropical rainforests grow by as much as half, according to more than two decades' worth of data from forests in Panama and Malaysia. The effect — so far largely overlooked by climate modellers — could severely erode or even remove the ability of tropical rainforests to remove carbon dioxide from the air as they grow.

The study shows that rising average temperatures have reduced growth rates by up to 50% in the two rainforests, which have both experienced climate warming above the world average over the past few decades. The trend is shown by data stretching back to 1981 collected from hundreds of thousands of individual trees.

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, says Ken Feeley of Harvard University's Arnold Arboretum in Boston, who presented the research at the annual meeting of the Ecological Society of America in San Jose, California.

Losing their balance

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.

That optimism will have to be reassessed, though, if photosynthesis becomes less productive in the tropics. The trends measured by Feeley suggest that entire tropical regions might become net emitters of carbon dioxide, rather than storage vessels for it. "The Amazon basin as a whole could become a carbon source," Feeley says.

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 1ºC over the past 30 years, and both showed dramatic decreases in rates of tree growth. At Pasoh, as many as 95% of tree species were affected, Feeley and his colleagues report. The research has also been published in the journal Ecology Letters¹.

Sinking feeling

Feeley suspects that the effect occurs because plant photosynthesis is impaired if the temperature rises above a certain threshold. The effect, he adds, has not been included in models of the global carbon cycle, meaning that predictions of the future performance of tropical forests as carbon stores may be unduly optimistic.

That said, he stresses that the effect is far from proven, and could be due to other factors. "Under increasing carbon dioxide alone, we know the growth rate will increase," he says. "But there are lots of factors — it's naïve to think of any one in isolation." The study acknowledges that increased cloudiness — or even a growing role for lianas — may account for some of the results.

Yet ultimately, those changes are also related to climate change, which can be expected to have effects all over the tropics. "If we're correct and the temperature is driving these changes, this is something we're going to see in a lot more places," Feeley predicts. "It has very important implications — we may need to look elsewhere for our excess carbon sink."

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 2ºC over the coming century, most rainforests could feel the effect before too long.

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• References

  1. Feeley, K. J. et al. Ecol. Lett. 10, 461-469 (2007).