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Seeing Forest Biodiversity for the Trees

6 March 2008 (All day)
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(inset) Christian Ziegler; (forest) Marcos Guerra/STRI

Gauging growth.
A researcher measures a tree in a Panamanian forest (top).

Climate change may not be as serious a threat to tropical forest biodiversity as previously thought, according to a new study. The question of whether rising carbon dioxide levels act as a forest fertilizer remains unresolved, however.

Global climate change is likely to be bad news for some humans, but the effect on tropical forests is less clear. Previous work has suggested that higher carbon dioxide levels and warmer temperatures benefit fast-growing trees. As a result, these species might eventually crowd out slower growing tree species, leading to a loss in overall forest diversity. Alternatively, scientists have wondered whether climate change represents an overall boon for tropical forests by helping all species grow faster: the so-called "fertilization effect."

In an attempt to clarify the issue with more comprehensive data, an international collaboration of more than 30 forest scientists collected more than 20 years' worth of tree measurements in 12 tropical forests ranging from Panama to India and Malaysia. The researchers took more than 2 million measurements of the width of the trees. This led to a much larger data set--by a factor of 10 or more--than previous efforts had compiled. The results, published this week in PLoS Biology, show that both fast- and slow- growing trees increased roughly equally in biomass during the years of the study.

The results counter the theory that fast-growing species crowd out slower growing trees in response to climate change, says co-author Helene Muller-Landau of the Smithsonian Tropical Research Institute in Panama. But the study is less clear on the larger question of whether climate change is making tropical rain forests grow faster. Although overall forest biomass increased by roughly half a metric ton in aboveground biomass per hectare, it may have simply been a recovery from previous stressors such as logging or El Niño events. "It could have nothing to do with carbon dioxide in the atmosphere," says Muller-Landau.

Ecologist Scott Saleska of the University of Arizona, Tucson, applauds the scientists involved for tackling the problem "at the largest scale that has been measured." To help answer the question of fertilization, Saleska plans a study in the Amazon this summer that will use laser-imaging equipment on planes to calculate tree heights to get much more data on tree growth. That would offer a much larger total acreage for study and would include the Amazon, which Muller-Landau and her colleagues did not survey.

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