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Damage Can Be Irreversible : Drought, Floods, Erosion Add to Impact of Tree Loss

THE VANISHING FORESTS: Third in a Series. Next: Reforesting--the benefits and problems.

June 19, 1987|A. KENT MacDOUGALL | Times Staff Writer

MAUNA KEA, Hawaii — When David Douglas, the Scottish botanist after whom the Douglas fir tree is named, hiked up this 13,784-foot volcanic mountain in 1834, he noted that the "highly picturesque and sublime" scene included a forest of mamane trees stretching up to 9,300 feet.

Today, there is no mamane forest at 9,300 feet, or even at 6,300 feet. Here and there, a lone tree stands out. But mostly this is a stark wasteland. The forest is gone, and much of the rich volcanic soil with it. Nothing has taken its place.

Mauna Kea's baldness is one of the temperate world's legacies here in the tropics. For it was English and Yankee ship captains who, eager to provide fresh meat for seafarers, loosed upon this island the cattle, sheep and goats largely responsible for eating the forest to death.

The subsequent slowness of the mamane forest to regenerate, even after removal of nearly all the herbivores, points up the fragility of island ecosystems. "On a small island it doesn't take much to destroy an entire ecological zone, and with it all the native plants and animals that have evolved over millions of years," says James Juvik, professor of geography at the University of Hawaii at Hilo.

Far from providing a model for tropical Third World countries of how economic development can go hand in hand with environmental protection, the no-longer paradisiacal Hawaii exemplifies the adverse impact of deforestation on soil, water, plants and wildlife. For one thing, a third of the islands' native bird species and subspecies have become extinct in the two centuries since the Europeans arrived. And most of those remaining, including the palila, a small bird that eats mamane seeds and is found only in Hawaii, are considered officially endangered.

Island ecosystems pose special problems, of course. Their plant and animal populations are smaller and more vulnerable to disruption. Having evolved without natural predators, many species are easy marks for predators that are introduced. Hawaii's native trees, for instance, evolved without thorns, poisonous sap, bitter-tasting bark or other protective defenses against goat and sheep attacks.

With plenty to eat and nothing to fear except occasional hunters, the few cattle, sheep and goats set free by Capt. James Cook and his successors soon turned into huge herds that roamed freely on this, the largest island in the Hawaiian chain, as on others. While the cattle trampled undergrowth and cut shallow tree roots with their sharp hooves, the sheep and especially the goats gobbled leaves and twigs and chewed sprouts and seedlings down to the ground, preventing regeneration. As the forest thinned, the land dried out, deteriorating in many areas to scrubby brushland.

As the creation of unproductive brushland indicates, deforestation usually entails more than just losing trees. Other environmental damage that in turn causes economic and social distress includes accelerated soil erosion, flooding and siltation of waterways.

Soil Loses Porosity

Consider the impact of deforestation on water flow. Forest soils rich in decomposed organic matter absorb and store more water than cultivated fields, grass-covered pasture and, especially, bare mineral soil, which tends to become hard and impermeable when exposed in the open. Tree roots increase the porosity of the soil by pushing into and loosening up new areas. When the roots die they add organic matter to the soil and leave channels through which water can percolate downward to recharge underground aquifers and to emerge downhill as springs.

Spongy forest soils help even out the flow of water, retarding runoff from heavy spring rains and melting snow and increasing the seasonally low flow in the summer. When a forest is cleared, streams often become roaring, destructive torrents in the rainy season and parched channels in dry periods.

In the Mediterranean climate, soil moisture is barely sufficient to sustain forests through the long, dry summer. When the forests are cut and subjected to the added stress of grazing and burning, they have a habit of never coming back. That happened in Ancient Greece, where eroded land too stony, hilly and dry to go back to forest was given over to olives and grapes. The oil and wine was then traded for timber and grain from the still-wooded Black Sea Coast.

California Forests

Low soil moisture, which experts say is a more critical determinant of drought than low rainfall, has also prevented the regeneration of forests in California. The Plumas National Forest north of Lake Tahoe, for one, has 56,000 fewer acres in timber than 40 years ago because of the failure of cut-over, dried-out forests to regenerate.

"California has places at high elevations where we harvested timber, but we're not getting it back, because of short growing seasons, drought, high soil temperatures and the harsh climate," concedes Zane G. Smith Jr., Pacific Southwest regional forester for the U.S. Forest Service.

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