Changes in tree communities underline humans' pervasive influence. 

Supposedly pristine regions of the Amazon rainforests are being affected by changes in the environment, say researchers in Panama.  The species composition of ecosystems is changing even in remote areas, they report in Nature1.

Human activities such as logging, burning and clearing are known to affect the delicate balance between the many thousands of species in tropical rainforests.  But the latest discovery suggests that these communities may not be safe even when they are far from the nearest humans.

The balance has been altered because the fastest-growing tree species are now growing even faster, says William Laurance of the Smithsonian Tropical Research Institute in Balboa, who led the study.  If the trend continues, more slowly growing trees could find themselves outcompeted, which could be bad news for biodiversity.

The researchers stumbled across the trend when studying the effects of rainforest clearance.  They surveyed tree growth over the past two decades in 69 remote one-hectare plots, intended as pristine 'controls' to highlight changes in other areas, where humans are active.  "But suddenly the controls weren't acting like controls," Laurance says.

All the tree groups that increased in abundance were fast growers that make up the forest canopy, whereas half of those that declined were sub-canopy trees.  "It's clear that this is not random variation," says Laurance.  "Rainforest dynamics are changing."

What is driving this change?  Laurance suspects that carbon dioxide is the culprit.  The gas is a staple requirement for plant growth, and as atmospheric carbon dioxide levels have climbed through the burning of fossil fuels, it seems that fast-growing trees have seized the initiative and begun to dominate the forest.

Carbon dioxide is the most likely cause, agrees Oliver Phillips, a rainforest expert at the University of Leeds, UK.  If the effect were due to rainfall changes, for example, the researchers would have seen a rise or fall in drought-resistant species, he points out.

The trend is worrying news for conservationists, Phillips says.  "It's a concern that even the most intact wilderness in the world is affected by CO2," he says.

Similar changes are probably happening in rainforests across the world, Phillips warns.  "There will almost certainly be biodiversity changes," he says.  He is urging researchers to mount large-scale efforts to deduce the scale of the problem.

Indeed, the problem could reach beyond biodiversity, warns Laurance.  The demise of more slowly growing trees, which tend to produce denser wood and foliage, could eventually lead to a drop in the amount of carbon dioxide that the rainforest removes from the atmosphere.

The Amazon rainforest currently mops up around 0.6 gigatonnes of carbon dioxide per year — 8–10% of that belched out by fossil-fuel burning — says Phillips.  This role as a 'carbon sink' is thought to be an important check on the greenhouse effect of carbon dioxide.

"My gut feeling is that the rainforests will still function as a sink," says Phillips.  "But this may well be under threat across the world."

This prognosis underscores the importance of measures such as the Kyoto Protocol, which seeks to limit carbon dioxide emissions, says Laurance.  He is angry at the United States' failure to sign up to the deal.  "As a US citizen, I am chagrined," he says.  "We have enough information to know that we have to do something."

References Laurance, W.F. et al.. Nature, 428, 171 - 175, (2004).

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