Over the past 20 years, the delicate balance of tree species growing in remote regions of the Amazon has been altered significantly. Scientists
believe it is a direct result of increases in carbon dioxide emissions caused by humans. The discovery comes from a study of 18 undisturbed plots in
the heart of the Brazilian Amazon that researchers have been monitoring since the 1980s.
Plants need carbon dioxide in the way that animals need oxygen - but the 30 per cent extra carbon dioxide in the past 200 years has begun to
accelerate growth and change the composition of the world's biggest rainforest, says a study published yesterday in the magazine 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 seemingly pristine forests can change so quickly and dramatically."
Dr Laurance and his team noticed that the growth of large trees in the Amazonian rainforests have accelerated over the past two decades while the
growth of smaller ones has slowed.
The levels of carbon dioxide (CO2) have risen because of emissions from cars and industry and rapid forest burning, particularly in the tropics. Much
of the increase in CO2, which plants use from the air for photosynthesis, has occurred since 1960.
The scientists suspect the rising CO2 levels are fertilising the rainforests and increasing competition for light, water and nutrients in the soil. So
the big fast-growing trees have an advantage and are outpacing the smaller ones.
The odd change in growth patterns could also be a signal for an overall change in rainforest ecology, scientists say.
Researchers have studied nearly 14,000 trees in the central Amazon, scattered across a 31 square kilometre landscape. During the 20-year 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.
Alexandre Oliveira of the University of Sao Paulo, Brazil, said: "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."
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
"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 wind storms. The next step is to see whether such changes were mirrored in other tropical forests
around the world.
- New Scientist
- Sydney Morning Herald
- ABC News