In the Jungle, the Clumpy Jungle

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Menno Schilthuizen
2000-05-26 17:00
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Nineteenth century naturalist and adventurer Alfred Russel Wallace noted how hard it is to find two trees of the same species in a tropical rainforest. Ever since, conventional wisdom among botanists has held that tropical trees are widely and randomly scattered. Now, an exhaustive tree census has revealed that there is indeed a pattern in the tropical chaos.

The census takers, in what is by far the largest effort of its kind, identified and plotted the locations of trees in six tropical rainforests. Botanist Richard Condit of the Smithsonian Tropical Research Institution in Balboa, Panama, and colleagues systematically worked through six plots of 25 or 50 hectares each--for a grand total of about 715 soccer fields worth of terrain--in rainforests of Asia and Central America, keying out every tree with a trunk thicker than 1 centimeter. That's 1.22 million trees.

One plot alone, in the Lambir forest of Borneo, occupied 50 to 60 surveyors for more than 2 years. Identifying the smallest saplings was "a bit tricky," says co-author Hua Seng Lee of the Sarawak Forest Department in Kuching, Malaysia, but in the end his team managed to identify 366,121 trees, from 1174 species. Colleagues surveyed plots in Thailand, Peninsular Malaysia, Sri Lanka, India, and Panama.

To the researchers' surprise, the distribution maps they eventually drew up did not show the expected random scatter. Instead, practically every species was distributed in clumps, they report in the 26 May issue of Science. Certain kinds of trees grew only in gullies, while some preferred only ridge-tops. Other species cluster for unclear reasons: one Panaman species grew in groups of up to 100 meters in diameter, possibly circled around long-gone grandmother trees. Whatever the causes, the vast scale of the new study has revealed distribution patterns that had remained hidden from earlier, smaller studies.

Other tropical rainforest ecologists are happy to have this long-awaited bird's-eye view of the jungle. "This finding may lay to rest a long-standing debate in tropical forest ecology," says ecologist Steve Turton of James Cook University in Cairns, Australia. He adds that similar projects are in the making in Australia and Africa, which eventually should complete the picture.

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