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Spring 2000

Research Magazine > ARCHIVE > Summer 99 > Article

Fragmented Forests
by Paul Karr

For up to a month at a time, he will collect and extract thousands of seeds from pods in the hot tropical sun. Each tree must be carefully mapped, each seed packaged and sent to a laboratory in Athens. Then it may take up to a year to fully analyze the samples genetically.

But when he's done, botany professor Jim Hamrick can tell you the parents of every seed taken from a small patch of Costa Rican forest and, just maybe, what that means for the future of that forest.

Hamrick has spent hundreds of hours studying small, cut-over patches of trees in Costa Rica to determine how logging and land-clearing are affecting the gene pool of the trees left standing.

And the news isn't always good.

Hamrick's work involves a combination of old-fashioned field collection and a careful lab examination of tiny bits of genetic information. He's studying gene flow - the process by which variation is introduced into seeds' genes - as well as genetic drift and inbreeding, which reduces that variation. Just as with humans and animals, this can mean the difference between healthy and unhealthy individuals or populations.

"Logging and other changes in forest structure can have direct and negative effects on the pollinators" like birds and bats, he said. "And this, over time, can change the genetic composition of the population."

In other words, logging a forest gives pollinators fewer choices of places to spread pollen. That increases inbreeding and weakens the next generation of trees - though the extent, he has found, is quite unpredictable.

In his Athens lab, Hamrick, with a technician and a team of graduate students, uses a variety of genetic markers to determine the parentage of the seeds and seedlings. Former graduate student Preston Aldrich developed a technique using microsatellites - tiny segments of DNA also known as "short sequence repeats" - to determine the pedigree of Symphonia globulifera seedlings growing in moist forests of the foothills of southwestern Costa Rica.

Aldrich's work indicated more than 50 percent of the forest seedlings originated from just two trees growing in a nearby pasture. Despite their healthy appearance, he found these seedlings actually represented a very limited gene pool - one that could eventually give rise to inbreeding weaknesses such as disease susceptibility in those trees.

More recently, Hamrick has moved his work to another area of Costa Rica: a drier northern region called Guanacaste, where his focus species is the guanacaste tree (Enterolobium cyclocarpum). The tree's pods, shaped like ears - the common name is ear-tree - make especially good study subjects because of the way the tree is pollinated: A dense ball of pollen ensures that every seed within a pod has the same father.

Hamrick will sample these stands, which are pollinated by hawk-moths and bees, for another five years to get a better grasp of their long-term genetics.

Early results seem to indicate that genetic drift also is occurring in northern Costa Rica, though for somewhat different reasons and to a lesser degree than it is in the southern foothills.

"This work points out that you need to understand each individual species in order to make predictions," Hamrick said. "It's difficult to make a blanket statement. You need to understand each tree's biology and natural history to understand how [disturbances like logging] affect it."

Another by-product of these studies has been a better understanding of the role of lone guanacaste trees standing in pastures or cleared areas. Hamrick's work indicates that such trees play a significant role in the breeding of nearby forests. Without them, levels of genetic variation would be reduced - perhaps to critical levels - and trees in the thinned forest might become even more inbred than they already are.

The lesson? Even a single tree in a field can be vital - and if that tree falls, the forest will hear it.

E-mail hamrick@dogwood.botany.uga.edu
for more information.


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Guanacaste tree