Scientists have long known the watershed values of a diverse native Hawaiian forest, and new research suggests that very complex tropical forests may also be exceptional at storing carbon.

There is a fundamental difference between the structure of most native forests in the Islands and that of many stands of foreign trees.

Walk through a stand of Australian-native eucalyptus, the strawberry guava from Brazil or Central American koa haole, and you find that there's not much else growing there. During heavy rains, such forests can allow much of the water to run off quickly.

A native upland forest is quite different. Under the canopy of giant koa and 'ohi'a, there is an understory of middle-height trees and tall tree ferns, and below that a low growth of shrubs, mosses, smaller ferns and other species. Such a forest tends to trap much of the moisture from heavy rains, soaking it up like a sponge, maintaining high humidity near the ground and keeping the soil moist.

"Prolonged downpours challenge a simple forest, while in a diverse forest the understory community is really, really rich and soaks it up," said Sam Gon III, director of science at The Nature Conservancy of Hawai'i. The dense foliage also blocks the sun and wind, limiting evaporation, he said.

In new research conducted mainly in South American tropical forests, researchers are finding that diverse tropical forests often store a great deal more carbon than many simpler forests. Their work included studies of diversity loss associated with logging.

A Columbia University scientist said it's not inevitably the case that a complex forest stores more carbon than a simple one, but "species diversity provides for a more predictable carbon trap."

Also, "some simpler species combinations may store more carbon, but it is very likely that these species would not be best for other critical functions, such as providing water quality, flood control, fire control, fruit production, etc.," he said.

The scientist and his co-researchers, in a paper published last week by the journal Science at the Science Express Web site, said the work they're doing shows that tropical forest biodiversity plays an important role in global climate, since carbon trapped in the forest is not available to the atmosphere as the greenhouse gas carbon dioxide.

In some cases, they found, a complex forest can trap six times more carbon than one deprived of many of its native species.

If you have a question or concern about the Hawaiian environment, drop a note to Jan TenBruggencate at P.O. Box 524, Lihu'e, HI 96766 or jant@honoluluadvertiser.com. Or call him at (808) 245-3074.