The sheer diversity of tropical forests -- where 130 acres can contain as many as 1,100 tree species and 366,000 individual trees -- has long clouded the basic ecological question of whether tropical trees of the same species are "aggregated" or dispersed randomly across the landscape. A census of six large plots of 25 to 52 hectares (60 to 130 acres) in five South American and Asian countries is described in this week's Science and shows most tropical trees are aggregated, or clumped. The size of the data set should help end decades of debate on this subject, according to Patrick Baker, co-author and doctoral student at the University of Washington's College of Forest Resources.
The findings have implications for environmental decision-makers interested in such things as designing nature reserves, selecting native trees to reforest degraded areas and determining biologically sustainable harvest rates for timber, Baker says, "provided researchers can next determine if there are key mechanisms that drive these patterns."
The notion that species were widely dispersed dominated theoretical tropical ecology from 1853 until 1979 when Stephen Hubbell, another co-author on this week's Science paper, published results contrary to popular wisdom based on work at a 13-hectare plot in Costa Rica. Hubbell's results have been the subject of debate since then.
Much larger plots from Panama, Malaysia, Thailand, India and Sri Lanka have now been surveyed in work funded mainly by the National Science Foundation and the MacArthur Foundation, and coordinated through the Center for Tropical Forest Science, a part of the Smithsonian Tropical Research Institute.
Nearly every species was aggregated when scientists considered trees of 1 cm or more in diameter, according to calculations by the paper's lead author Richard Condit of the Center for Tropical Forest Science. Rare species were substantially more aggregated than common species at all but one site. Clumping was discerned even for species so rare that researchers could find only 10 individuals across 50-hectare plots.
The mechanisms responsible for like trees to be in the same vicinity could include the ways seeds are dispersed or factors in the habitat that favor certain species.
Baker worked at the Huai Khae Khaeng plot in Thailand, the site with the most tightly aggregated species the scientists found: 51 of 59 individuals of Lagerstroemia sp. (Lythraceae) were within 20 meters of each other. Baker said the mechanism for clumping for that species appeared to be the strong sucker growth. He recalled standing at a dead tree and being able to discern its root pattern 10 meters in various directions because of the lines of two- to three-meter-tall juveniles radiating away from the stump.
Other co-authors on the paper include researchers from Harvard University, University of Georgia, the Field Museum of Natural History, Thailand, Sri Lanka, Japan, Singapore, Malaysia and India.
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