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

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.

Logging on shallow soils, on south-facing slopes that bake under the summer sun and on north-facing slopes that don’t warm up enough, has also turned forests in California, Oregon and Washington into scrubby brush fields and tough grasslands. So has clear-cut logging that has removed seed sources and exposed large areas to the elements.

Even selective logging of just the biggest trees can result in an unintended clear-cut. This often happens in the tropics, where removing the larger trees exposes smaller trees they had shaded to fatal overheating.

In the British Isles, the cutting of trees in dry, windy areas with infertile soils has resulted in open heaths covered with pretty but unproductive heather.

In sandy areas where each tree is an oasis anchoring the soil against the clawing wind, removing the tree cover can cause sand to drift ominously. This has happened north of Coos Bay, Ore., on the south shore of Lake Michigan, on Cape Cod and in many other places.

Fires on Mt. Shasta

The hotter, drier, windier conditions that follow deforestation increase the hazard of fire and convert minor fires into major conflagrations. In California, several hundred thousand acres of timberland on the slopes of Mt. Shasta have degenerated into brush fields because of repeated fires since turn-of-the-century logging.

“The timber would have come back after the first fire, but subsequent fires removed the seed source and the seedlings,” explains Richard Harrell, a Forest Service fire-management specialist.

Other forested areas that were repeatedly burned have gone back to forest but grow only stunted trees and commercially inferior species. In Michigan, white and Norway pines, hemlock and valuable hardwoods were almost entirely eliminated after logging and repeated burning, giving the land over to scrubby stands of jack pine, aspen and other less desirable species.

Removing trees in hot, arid regions permits the soil to wash or blow away. If hooves compact the earth, the chances of seeds germinating are remote. Little wonder, then, that as forests shrink, the world’s deserts are spreading, swallowing up arable land, displacing villages, ruining lives.

Deforestation is expanding deserts not only in continental interiors but also along seacoasts where forests used to comb moisture from clouds and fog rolling in from the ocean. In Mexico, the cutting of forests along the Gulf of Mexico north of Veracruz to expand corn cultivation destroyed one of the corn farmers’ most important sources of water, increasing aridity and inadvertently ruining more than 1,000 square miles of land. Similar disasters have taken place in coastal Peru and Chile.

Wet Regions Get Wetter

While making already-dry regions drier, deforestation often makes wet regions even wetter. For trees serve as both umbrellas and water pumps. Their canopies intercept rainfall and evaporate much of it from leaves and twigs before it reaches the ground. Meanwhile, their roots take up moisture from the soil and pump it up to the leaves, which transpire it into the atmosphere.

When trees that used to shed excess moisture are removed, already wet soils tend to become waterlogged. In Britain, removing trees in areas of high rainfall and poorly drained soils has led to waterlogged moors and peat bogs. In the Upper Peninsula of Michigan, cutting trees in black spruce bogs has raised the water table, preventing regeneration.

Waterlogging is less of a problem worldwide than the erosive runoff of water. A certain amount of soil erosion and sedimentation is both natural and desirable, of course. Rivers need a little silt to stabilize their bed and banks and to replenish sandy beaches at their mouth. River valleys become more fertile when sediment is deposited gently over the millennia.

As long as no more soil is washed away than is formed, erosion is generally not a problem. But removing the vegetative cover greatly increases the erosion rate, replacing the beneficial process of geologic erosion with the destructive process called accelerated erosion.

Deforestation accelerates erosion because tree canopies and litter and humus on the forest floor are no longer available to break the impact of downpours. Raindrops that strike the bare soil dislodge fine-textured particles that clog pores in the soil, preventing water from being absorbed.

Danger From Steep Slopes

Water that runs off quickly on steep slopes causes the most destruction. Doubling the velocity of water increases its cutting power fourfold, its carrying capacity 32-fold, and the size of debris it can carry 64-fold. This helps explain how huge boulders can be carried down small streams once they turn into torrents.

A single rainstorm can carry away a layer of fertile topsoil that took centuries to build. Where runoff cuts channels in the earth, gullies are formed, growing deeper, wider and longer with each heavy rain.

In the Southeast, a region of hills, erodible soils and heavy rainfall, replacing the thick natural forest cover with cotton and tobacco fields cut gullies so deep and steep that when the Civilian Conservation Corps reforested in the 1930s, workers with seedlings had to be lowered into the gullies by rope.

For the world’s deepest gullies, however, one must turn to China. The comparatively small percentage of level land in China explains the early extension of cultivation into mountainous areas. When the Chinese cleared the higher plateau lands through which the Yellow River flows, they exposed the fertile, fine-grained soil laid down by wind-blown dust. Predictably, summer rainstorms washed away the soil, in the process carving a lifeless maze of narrow, steep-sided gullies up to 650 feet deep.

The Yellow River has the dubious distinction of being the muddiest river in the world. The huge load of sediment from deforested highlands that it has carried for several thousand years has elevated the river bed in lower reaches. This aggradation, now averaging three to four inches a year, has forced Chinese peasants farming the fertile lowland plains to build levees to confine the river within its banks. Using their bare hands and baskets, the farmers have raised the levees a little higher each year, so that today the river runs 10 to 30 feet above the surrounding countryside.

River Changes Course

Even then, heavy rainfall has overwhelmed the river’s capacity, causing an average of one flood every other year between 206 BC and AD 1949. The river has changed course eight times since AD 11.

Since the early 1950s, tree-cutting along the Yellow River’s middle and upper reaches has continued, and the siltation rate has increased more than 20%. What’s more, Chinese scientists warn that the Yangtze, which flooded in 1980, 1981 and 1982, is in danger of turning into a second Yellow River because of deforestation and attempts to terrace hillsides for farming in the southwestern highlands.

China is also having problems with reservoirs. It has had to abandon many small reservoirs after just two or three years because of siltation from deforested watersheds. Siltation has similarly reduced the water-holding and power-generating capacity of both dams and reservoirs in many Third World countries, including the Philippines, Thailand, Kenya, Tanzania, Colombia and Costa Rica.

Even the United States is not exempt. Deforestation, mining and other misuse of hilly watersheds in the Tennessee Valley have silted up several Tennessee Valley Authority reservoirs. One of these, on the Ocoee River in Southeastern Tennessee, has lost 90% of its storage capacity. Accumulated silt blocks the intake of the tunnel that carries water from the reservoir to electrical generators. This necessitates periodic sluicing of the silt--but that only transfers the problem downriver to another reservoir where the silt settles out to form mud flats.

Silt Extends Coastlines

Silt from deforested highlands also has a long history of clogging harbors and pushing coastlines out to sea. The northern Adriatic coast of Italy has been silting up and extending seaward for at least 2,000 years. Ravenna, once the chief Roman port on the Adriatic coast, lost its access to the sea long ago and is now six miles inland. Adria, built on an island near the mouth of the Po River, today is 12 miles inland, its streets 15 feet above the foundations of houses that the Etruscans built 2,500 years ago.

“The silt loads of the rivers began to accelerate during the Renaissance period, when the area had its most prosperous and glorious period in history,” Vernon Gill Carter and Tom Dale report in “Topsoil and Civilization.” “In other words, this region followed the familiar pattern. Its high point in civilization was achieved mainly by intensive use of the land and this, in turn, brought on the serious erosion that eventually resulted in decline.”

Today, the same process is at work in many Third World countries. Silt from the Himalayan foothills has created navigational hazards at the port of Calcutta on the Hooghly River. Many ships can no longer reach its docks, and each year Calcutta loses more shipping to other Indian ports.

Silt has created entire new islands in the nearby Bay of Bengal. As soon as the islands are formed, landless peasants settle them, growing rice and grazing cattle. Several thousand of these people were drowned in May, 1985, when a cyclone swept over silt-formed islands off Dhaka, Bangladesh.

Malaria-Carrying Mosquitoes

Silt from deforested uplands deposited in low-lying coastal plains often creates marshes that can become breeding grounds for malaria-carrying mosquitoes. And this, too, has been going on for thousands of years.

Malaria became widespread in Greece around 400 BC and in Italy about 200 BC. The most notorious breeding ground was the Pontine Marshes at the mouth of the Tiber River near Rome. The Romans periodically drained these swamps, but the basic cause remained unsolved, and so did the prevalence of the disease.

Deforestation is also harmful to agriculture. For one thing, it alters the local climate, increasing temperature extremes and windiness and reducing rainfall and soil moisture. These effects were studied some years ago by the Forest Service in the Tennessee Copper Basin, most of which was denuded by smelter fumes and firewood harvesting by the turn of the 20th Century.

The Forest Service found that during the summer, soil temperatures in the open reached 127 degrees, compared with an 82-degree maximum in the nearby woods. Wind velocity was 13 times greater in the open. In the winter, soils in the open froze earlier and more deeply. And year-round, more than three times as much moisture evaporated from open spots as from the forest.

Fewer Rain Clouds Form

Despite greater evaporation from open spots, deforestation reduces rainfall. The enormous quantity of water that trees pump up from underground and transpire into the atmosphere rises as cool, moist air that often condenses into rain. In contrast, warm, dry air rising from deforested areas has more capacity to hold moisture, so it forms rain clouds less often.

With less rain falling on them, some heavily logged tropical rain forests are drying out enough during recurrent drought years to fall victim to a rare phenomenon: fire. Four years ago, a massive fire in the Ivory Coast in Africa destroyed 1,700 square miles of rain forest.

Deforestation can alter regional as well as local climate. The removal of rain forests on the Atlantic Coast of Africa is suspected of reducing the moisture that moves inland to help generate rainfall in the drought-prone Sahel Zone from Senegal to the Sudan. Deforestation in the Amazon Basin may be cutting down rainfall in Venezuela and Colombia to the north and in south-central Brazil to the south.

“Brazil’s efforts to resettle the excess population from its northeast and south and to expand beef production by converting the Amazon rain forest to grassland may indirectly threaten food production in the country’s agricultural heartland,” Lester R. Brown, president of Worldwatch Institute, notes in “State of the World 1985.”

The same kind of climatic deterioration that Brown sees reinforcing environmental deterioration in Brazil and Africa helped dry out the entire Mediterranean area long ago. Or so many experts believe.

‘Land of Milk and Honey’

“When Israel was the land of milk and honey, it was moister than today, and probably cooler,” says Eric Bourdo, retired dean of the School of Forestry at Michigan Technological University.

The desiccation process triggered by deforestation can be seen even in remote Soviet Central Asia. There, deforested mountain slopes heat up in summer, melting nearby glaciers. One such glacier, the Zeravashan, has retreated 50 miles in the last century.

“There’s no need explaining where this process can lead,” a Tadzhik agricultural official told Pravda. “At present, it’s fairly slow, but it’s time we thought about stopping it. Otherwise, Central Asia may eventually lose its rivers.”

The Soviets are worried about their lakes as well as their rivers. Again, their own timber harvesting practices are partly to blame. Soviet loggers commonly float individual logs down rivers to sawmills, rather than lashing them together in rafts. And they often fail to strip the bark off first. Many individual logs sink and many lose their bark. The bark rots on the bottom, poisoning the water, reducing its oxygen content and smothering fish-spawning beds. Sunken logs in rivers flowing into Lake Baykal have contributed to a sharp decline in the catch of the lake’s most important commercial fish, the omul.

The United States has its share of logging-caused fisheries problems, too. Salmon spawning grounds on the South Fork of the Salmon River in Idaho were nearly wiped out in 1965 when heavy rains crumbled hillside logging roads into the river. Now at minimum survival levels, the salmon population will take a hundred years to recover, according to the Wilderness Society.

Logs Prevent Fish Migration

Silt has clogged spawning grounds in other Western rivers as well. Logs felled into streams have blocked upstream salmon migration. And trees removed from the banks of some streams have increased solar radiation and stream temperatures enough to disrupt spawning and kill fish eggs.

In many tropical regions, deforestation not only has despoiled fish habitats but deprived fish of food. Many species feed primarily on fruits and seeds that fall from trees into streams and rivers. When tropical forests are cleared, tree food declines, and fish populations with it.

Deforestation also damages offshore fisheries. Around the Caribbean, deforestation washes silt into clear coastal waters, reducing the waters’ transparency and transmission of sunlight, and killing off grasses and coral reefs on which much marine life depends. Along the coast of Chile, shellfish beds have been smothered by sludge deposited following deforestation.

Deforestation’s effects on wildlife are mixed. Deer and bear thrive on the new growth that follows clear-cutting of forests. Large clearings also benefit some small mammals, quail and other game birds, as well as birds of prey.

Birds Go the Way of Forests

But most animals lose out. Elk go hungry when tree canopies no longer reduce snow accumulation over winter forage. Martens and fishers have declined along with the tall dense forests in the Pacific Northwest on which they depend. So have spotted owls. And in the South, ivory-billed woodpeckers have gone the way of mature hardwoods.

Several species of U.S. songbirds have declined in numbers because of the devastation of the forests in Central and South America to which they migrate for the winter.

And here in Hawaii, which has more endangered species than any other state, the palila and 28 other native birds on the federal list of endangered or threatened species barely hang on in what’s left of the native forest. With many native plants extinct and others endangered, and non-native plants firmly in their niches, restoring the original biotic community is considered impossible.

Even the oysters that gave Pearl Harbor its name are gone, long ago smothered by silt from the harbor’s deforested, eroded watershed. In both big and little ways, Hawaii is still paying for the deforestation that began when the Polynesians cleared lowland forests to plant crops and accelerated after the Europeans arrived in 1778.

In land usage, as in life, there is no free lunch. RAVAGES OF DEFORESTATION

1. Deforestation entails more than just the loss of trees. The effects extend to soil, water, plant and animal life. the most visible immediate scars are on the land: a sea of stumps left after logging, access roads built for equipment, and tracks left by heavy machinery. But in fragile ecosystems, these are only the begining.

2. Rain falling on denuded hillsides washes past the spindly remains of inferior trees and creates channels. Spongy forest soils help even out the flow of water; when a forest is cleared, streams often become roaring, destructive torrents in the rainy season and parched channels in dry periods. With the runoff flows precious topsoil.

3. Eventually, the treeless slopes can no longer absorb the rush of water, and the hillside becomes undermined. Downstream, soil eroded from the hills and river banks settles out to form mud flats that somother fish spawning beds. The silt also builds up behind dams, blocking intake tunnels and reducing power-generation capacity, and clogs harbors.

4. Over the years, the gullies widen and deepen. New trees, most of them spindly and of commercially undesirable species, dot the landscape. The microclimate also has been altered: Temperatures and wind increase, and rainfall is reduced. The area has become desert-like, with barren soil that supports mostly drought-tolerant bushes and coarse grasses.