By Jan TenBruggencate
Advertiser Columnist

Almost everyone agrees that healthy forests are a good thing, but a lot of folks are wrong about some of the ways in which forests work as watersheds, according to a University of Hawai'i professor who studies these things.

Thomas Giambelluca, a geography professor specializing in climate issues, said there are persistent myths about the benefits of forests.

One is that forests attract rainfall.

"There are some instances where forest cover can interact with the weather, but probably mainly in cases like the entire Amazon Basin, and probably not in small areas like Hawai'i," he said.

There also are strange things about forests that you might not expect. For instance, they might be able to change the elevation of clouds on a mountainside.

State forester Michael Buck said that forested areas tend to be cooler than bare soil, and clouds or fog form more readily in cooler temperatures. As a result, if a particular mountainside is covered with forest, clouds can form at lower elevations than if the landscape is bare.

And since trees can strip the moisture out of fog in a process called fog drip, the forest can increase the amount of moisture that makes it to the ground.

But Giambelluca said there's something else at work, too. This involves the myth that a forested area keeps more water in the ground than an unforested area.

"Forests evaporate more water than anything else. You end up with less water," Giambelluca said.

It might feel moister and cooler under the trees, but deep in the root zone, those trees are pulling water out of the soil, he said.

Despite that, forests are generally an overall plus to the watershed for other reasons.

Rainwater can find its way deep into the soil more easily in a forest, since it can travel downward along the roots. The roots and leaf debris filter the water, reducing the amount of sediment that gets into streams. And the forest soil holds the water like a sponge, releasing it slowly.

The slow release of water from forested areas is one reason streams can continue to flow when it's not raining.

The Nature Conservancy of Hawai'i, with the help of the state Department of Land and Natural Resources, the U.S. Fish and Wildlife Service and Malama Hawai'i, has produced a beautiful booklet on the Hawaiian forest and why it is important.

"Last Stand: The Vanishing Hawaiian Forest" is filled with information and stunning photography. It's available free by calling the conservancy at 537-4508.

Jan TenBruggencate is The Advertiser's Kaua'i bureau chief and its science and environment writer. Reach him at (808) 245-3074 or jant@honoluluadvertiser.com.