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by Larry

March, 2004

Are Rainforests Losing Their Capacity to Absorb Excess Carbon Dioxide?

Tropical rainforests far from human development and habitation, long thought capable of using and holding extra atmospheric carbon dioxide, believed to be a major culprit in greenhouse effect causes of global warming, are being themselves affected by increased levels of carbon dioxide in ways that may limit their long-term ability to be net removers of atmospheric CO2. rainforest canopy

Up till now it was thought that the world's remaining rainforests would offset the increase in greenhouse gases in the atmosphere, since their trees could use surplus CO2, and the generally slower growing hardwood trees in the globe's pristine, usually equatorial, forests would on average not be dying, decaying, and releasing their own carbon dioxide back into the planet's exterior layer of gas for scores or even hundreds of years.

But recent researches from remote Panamanian rainforests show that over the past twenty years or so the fastest growing tropical forest trees are being stimulated by the excess atmospheric carbon dioxide to even faster growth, 50% faster in some cases, while the hardwoods, though having increased growth too, are responding with much slower increases than the softwoods.

The researchers studied 69 different remote plots in Panama over twenty years, intending that these be control areas, for comparison with regions close to human activity, assuming initially that the latter would show significant changes relative to the expected stable conditions of the controls. Instead, they found that fast growing trees in their "controls" were dramatically spreading their forest canopy coverage at the expense of the slow growers. The fast growers were becoming abundant, while the number of slow growers was declining.

If these findings are borne out in other tropical rainforests and persist, as the researchers conjecture, there are negative implications. The fastest growing trees seem likely now to win in their ongoing competition for sunlight and space with the previously dominant slow growers. But these rapid growers also die much sooner than the slow growers, releasing carbon dioxide back into the atmosphere more quickly. Unfortunately, the slow growing trees have the most foliage and are more efficient than the fast growers at removing CO2 from the atmosphere. As a consequence of both these factors, the capacity of rainforests to be net absorbers of excess carbon dioxide apparently has now been greatly reduced.

Meanwhile the biodiversity dependent on the formerly vast stands of tropical hardwoods is now in jeopardy not merely from active destruction of these wild areas to make room for human communities and agriculture but also from a major change in the composition of the remaining forests themselves.

It seems clear that even the tropical regions least directly affected by people are now subject to the worldwide increase in CO2 levels and may be undergoing irreversible alterations as a result.


Nature Science Update


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