The Dangerous Edge of Forests.

• Author: Hardman, Chris
• Position: Ecology of small tree stands - Brief Article

AS MORE AND MORE of the tropical rain forest is clear cut for ranching or development, isolated forest fragments are left where large tracts of continuous forest once stood. Conservationists warn that forest fragments can't sustain healthy animal populations like continuous forests do. Recent research by University of California at Davis scientist Emilio Bruna shows that plants, too, need continuous forests to survive.

In a rain forest near Manaus, Brazil, Bruna compared the progress of seeds from a common type of Heliconia he planted in a continuous forest to seeds he planted in a forest fragment. He checked the plants' status each month for a year. The results were dramatic. The seeds in the forest fragment were three to seven times less likely to germinate than those in the continuous forest. "In terms of population growth, this could be a really big difference," Bruna says. "Imagine a savings account that earns only 1 percent interest a year compared to one that earns 7 percent a year."

Bruna attributes the difference to edge effects--an unnatural environment created by clear cutting part of a forest. Edges put forest seeds in an environment with higher temperatures, increased humidity, and more sunlight. Typically, a rain-forest seed would land on the bottom of the forest floor, a dark and cool environment protected from the sun. Seeds that land in edges are exposed to intense heat, more sunlight, and increased wind. Some seeds die or simply never germinate--the wind and heat dry them out. Another obstacle comes from trees at the edge of the forest, which are stressed and tend to lose more leaves. The excess leaves cover the seeds, causing...