AS MORE and more of the tropical rainforest 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 rainforest 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 plant’s status each month for a year.
The results were dramatic. The seeds in the forest fragment were 3-7 times less likely to germinate than those in the continuous forest. “In terms of population growth, 7% could be a really big difference,” Bruna says. “Just think of a bank account earning 7% less interest 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 rainforest seed would land on the bottom of the rainforest 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 dries them out.
Another obstacle comes from trees at the edge of the forest, who are stressed and tend to lose more leaves. The excess leaves cover the seeds causing them to mold or to receive no sunlight at all.
“If the effects were felt in plants that were common, this might suggest rarer plants are at an even greater risk,” Bruna says.
Bruna’s research is part of the Biological Dynamics of Forest Fragments Project, a partnership between Brazil’s National Institute for Research in the Amazon (INPA) and the US-based Smithsonian Institution.
This unique project began 20 years ago when ranchers began to cut down the rainforest around Manuas, Brazil. Seeing a scientific opportunity, INPA scientists asked the ranchers to leave isolated strips of forest around their land. Studies both before and after isolation have resulted in a wealth of information about the effects of forest fragmentation on plants, birds, insects, and animals.
The results of Bruna’s research show that continuous forests are just as important for plant reproduction as they are for animals.
“Fragmentation can influence plant reproduction in unexpected ways. It suggests that even if pollination and seed dispersal—two interactions thought to be very susceptible to the effects of fragmentation because they usually depend on animals—are successful, environmental conditions may cause reproduction to fail,” he explains.
“If this goes on for long enough, plants in patches of rainforest may find their population sizes dwindling away, perhaps ultimately leading to extinction.”