Pristine Amazon forests have begun to change dramatically because of rising levels of carbon dioxide, according to US scientists.
Plants need carbon dioxide in the way that animals need oxygen — but the 30% extra carbon dioxide in the last 200 years has begun to accelerate growth and change the composition of the world’s biggest rainforest, according to a study published today in Nature. ”The changes in Amazonian forests really jump out at you,” said William Laurance of the Smithsonian Tropical Research Institute in Panama. ”It’s a little scary to realise that seemingly pristine forests can change so quickly and dramatically.”
For two decades, researchers have studied nearly 14 000 trees in the central Amazon, scattered across a 190 square kilometre landscape. During the the study, most species began to grow faster. They also died faster, to be replaced more swiftly by new, young trees. But the composition of the forests, too, was beginning to change.
”There clearly are winners and losers. In general, large, fast growing trees are winning at the expense of smaller trees that live in the forest understorey,” said his colleague Alexandre Oliveira of the University of Sao Paulo, Brazil.
Small trees are highly specialised, often with valuable biological properties. They can flower and reproduce in deep shade. But in the spurts of growth triggered by rising carbon dioxide levels, the bigger, faster growing species tend to take the lion’s share of light, water and soil nutrients.
”Sadly, this could be a signal that the forest’s ecology is changing in fundamental ways,” Dr Laurance said. ”Tropical rainforests are renowned for having lots of highly specialised species. If you change the tree communities then other species — especially the animals that feed on and pollinate the trees — will undoubtedly change as well.”
To separate the effects of global warming from other stresses, the researchers based their studies on plots of forest chosen because they had not been logged, burned, used for hunting or damaged by windstorms. The next step is to see whether such changes were mirrored in other tropical forests around the world.
”If they are, then it is likely that even the world’s remotest forests are now being altered by human activities,” said Thomas Lovejoy, now of the Heinz Centre for Science, Economics and Environment, who launched the research two decades ago. – Guardian Unlimited Â