Tropic trees tell an age-old story

Image credit: Zach Carter
Date:16 October 2006 Tags:, ,

You can tell how old a tree is simply by counting its rings, which reflect the changing seasons. However, things are less straightforward when it comes to tropical trees, since there are no summers and winters to mark the passage of time. Now scientists have worked out a simple way to tell the age of trees from the tropics, according to Nature.

Currently, factors such as the width of tree rings in temperate zones are used to provide a detailed record of climate conditions for the past 1 000 years or so. But only a few types of trees in the tropics have visible rings: the variances between wet and dry seasons are too slight to leave a distinguishable mark.

Until now, scientists have depended on measurements of oxygen and carbon isotypes to determine their age, but Pascale Poussart, a geochemist at Princeton University in New Jersey, and her colleagues, have shown that an apparently ringless Miliusa velutina tree from Thailand does have rings… they’re just invisible.

Using X-ray beams focused on wood samples, the researchers looked at calcium, a mineral that trees take up during their main growing season. They revealed annual peaks dating back to 1909.

Although the results were in line with carbon isotope measurements, the calcium method is much quicker. “It took us just one afternoon in the synchrotron to produce the record,” says Poussart. “The isotope record cost four months of lab time.”

The team doesn’t yet know whether seasonal calcium cycles are common, or if the feature is specific to just a few types of trees. And it is unclear whether periods of drought, or the modest and patchy dry seasons that feature in some areas of the tropics, will make the signal undecipherable.

In search of answers, Poussart will be working with scientists at the Smithsonian Tropical Research Institute’s Center for Tropical Forest Science, who run long-term forest monitoring programmes in Africa, Asia and Latin America.

At the moment, much about tropical systems is a mystery, so anything that helps measure time will make an important contribution to the understanding of tropical forest dynamics.

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