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Soil nutrients shape tropical forests, large-scale study indicates
Diana Yates,
Life Sciences Editor
217-333-5802; diya@illinois.edu
Released
1/11/07
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CHAMPAIGN, Ill. —
Tropical forests are among the most diverse plant communities on earth,
and scientists have labored for decades to identify the ecological and
evolutionary processes that created and maintain them. A key question
is whether all tree species are equivalent in their use of resources
– water, light and nutrients – or whether each species has
its own niche.
A large-scale study by researchers at the University of Illinois at Urbana-Champaign
and eight other institutions sheds some light on the issue. It indicates
that nutrients in the soil can strongly influence the distribution
of trees in tropical forests. The finding, published this week in
the Proceedings of the National Academy of Sciences, challenges the
theory that at local scales tree distributions in a forest simply
reflect patterns of seed dispersal, said James W. Dalling, a U. of
I. professor of plant
biology and a principal researcher on the study.
The study evaluated three sites: two lowland forests, in central Panama
and eastern Ecuador, and a mountain forest in southern Colombia. The
researchers plotted every tree and mapped the distribution of soil nutrients
on a total of 100 hectares (247 acres) at the sites. The study included
1,400 tree species and more than 500,000 trees.
The researchers compared distribution maps of 10 essential plant nutrients
in the soils to species maps of all trees more than 1 centimeter in
diameter. Each of the sites was very different, but at each the researchers
found evidence that soil composition significantly influenced where
certain tree species grew: The spatial distributions of 36 to 51 percent
of the tree species showed strong associations with soil nutrient distributions.
Prior to the study, the researchers had expected to see some influence
of soil nutrients on forest composition, but the results were more pronounced
than anticipated.
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“The fact that up to half of the species are showing an association with one or more nutrients is quite remarkable,” Dalling said.
“Differences
in nutrient requirements among trees may help explain how so many
species can coexist.”
Although plants in temperate forests influence the soils around them
(through the uptake of nutrients, decomposition of leaf litter on the
forest floor and through root exudates), in tropical forests local neighborhoods
contain so many species that the ability of individual species to influence
soil properties is likely to be small.
“We interpret these plant-soil associations as directional responses
of plants to variation in soil properties,” the researchers wrote.
The team also found that certain soil nutrients that previously had
not been considered important to plant growth in tropical forests had
measurable effects on species distributions.
At the site in Ecuador, calcium and magnesium had the strongest effects.
In the Panamanian forest, boron and potassium were the most influential
nutrients assayed. And in the Colombian mountain forest, potassium,
phosphorous, iron and nitrogen, in that order, showed the strongest
effects on the distribution of trees.
“There are all kinds of minerals out there that plants seem to
be responding to that we didn’t think were likely to be important,”
Dalling said. Further studies are needed, he said, to evaluate these
influences in more detail.
The other principal investigators on the study are Robert John, a post-doctoral
researcher in the U. of I. department of plant biology; Kyle E. Harms,
Louisiana State University; Joseph B. Yavitt, Cornell University; and
Robert F. Stallard of the U.S. Geological Survey.
Researchers on the study also are affiliated with Smithsonian Tropical
Research Institute, Panama; the University of Georgia; Pontifical Catholic
University of Ecuador; Instituto Alexander von Humboldt, Colombia; and
the Field Museum of Natural History, Chicago.
Editor’s note: To reach James W. Dalling
in Panama, call 011-507-314-9311; e-mail: dallingj@life.uiuc.edu.
