Deforestation

The most serious problem facing the world’s forests is deforestation. According to the FAO, forests shrank more than 36 million hectares (89 million acres) between 2000 and 2005. This amounts to a net annual loss of 0.2%. This estimate of forest loss does not take into account remaining forests that have been thinned or degraded by overharvesting, declining biological diversity, and reduced soil fertility.

Causes of deforestation include fires caused by drought, land-clearing practices, expansion of agriculture, construction of roads in forests, tree harvests, and insects and diseases. When forests are converted to other land uses, they no longer make valuable contributions to the environment or to the people who depend on them. Forest destruction, particularly in the tropics, threatens indigenous people whose cultural and physical survival depends on the forests.

Deforestation results in decreased soil fertility through rapid leaching of the essential mineral nutrients found in most forest soils. Uncontrolled soil erosion, particularly on steep deforested slopes, affects the production of hydroelectric power as silt builds up behind dams. Increased sedimentation of waterways caused by soil erosion harms downstream fisheries. In drier areas, deforestation contributes to the formation of deserts. When a forest is removed, the total amount of surface water that flows into rivers and streams actually increases. Because the forest no longer regulates this water flow, the affected region experiences alternating periods of flood and drought.

Deforestation contributes to the extinction of many species. Many tropical species, in particular, have limited ranges within a forest, so they are especially vulnerable to habitat modification and destruction. Migratory species, including birds and butterflies, suffer from deforestation.

Deforestation contributes to regional climate changes. Trees release substantial amounts of moisture into the air; about 97% of the water that roots absorb from the soil is evaporated directly into the atmosphere. This moisture falls back to Earth in the hydrologic cycle (see Chapter 5). When a large portion of forest is removed, rainfall may decline and droughts may become more common in that region. Studies suggest that the local climate has become warmer and drier in parts of Brazil where huge tracts of the rain forest were burned. As the forest continues to shrink, it is possible that the changing regional climate will no longer be able to support the forest. Thus, large parts of what had once been tropical rain forest could become savanna. Deforestation also contributes to an increase in global temperature, ocean acidification, and other global changes by releasing carbon originally stored in the trees into the atmosphere as carbon dioxide. Carbon dioxide enables the air to retain heat. The carbon in forests is released immediately if the trees are burned or more slowly when unburned parts decay. If trees are harvested and logs are removed, roughly one-half of the forest carbon remains as dead materials (branches, twigs, roots, and leaves) that decompose, releasing carbon dioxide. When an old-growth forest is harvested, researchers estimate that it takes about 200 years for the replacement forest to accumulate the amount of carbon that was stored in the original forest.

Why Are Tropical Rain Forests Disappearing?

Several studies show a strong correlation between population growth and deforestation. More people need more food, so the forests are cleared for agricultural expansion. Tropical deforestation is a problem that cannot be attributed simply to population pressures, however. The main causes of deforestation vary from place to place, and a variety of economic, social and government factors interact to cause deforestation.

Government policies sometimes provide incentives that favor the removal of forests. The Brazilian government opened the Amazonian frontier, beginning in the late 1950s, by constructing the Belem- Brasilia Highway, which cut through the Amazon Basin. Such roads open the forest for settlement.

Sometimes economic conditions encourage deforestation. For example, when world demand is high for cattle, farmers and ranchers are more likely to remove forest to expand grazing land. According to the National Institute for Space Research, deforestation in the Amazon Basin increased dramatically from August 2007 to August 2008, largely due to rising demand (and rising prices) for cattle. Keeping in mind that tropical deforestation is complex, three agents—subsistence agriculture, commercial logging, and cattle ranching—are considered the most immediate causes of deforestation. Other reasons for the destruction of tropical rain forests include mining, particularly when ore smelters burn charcoal produced from rainforest trees, and dam construction for hydroelectric power, which inundates large areas of forest.

Subsistence Agriculture.

Subsistence agriculture, in which a family produces just enough food to feed itself, accounts for more than half of tropical deforestation. In many developing countries where tropical rain forests occur, the majority of people do not own the land they live and work on. In Brazil, 5% of the farmers own 70% of the land. Most subsistence farmers have no place to go except into the forest, which they clear to grow food. Subsistence farmers often follow loggers’ access roads until they find a suitable spot. They first cut down the trees and allow them to dry, then they burn the area and plant crops immediately after burning; this is known as slash-and-burn agriculture.

The yield from the first crop is often quite high because the nutrients that were in the burned trees are now available in the soil. However, soil productivity declines at a rapid rate, and subsequent crops are poor. In a short time, the people farming the land must move to a new part of the forest and repeat the process. Cattle ranchers often claim the abandoned land for grazing, because land not rich enough to support crops can still support livestock.

Slash-and-burn agriculture done on a small scale, with plenty of forest to shift around in so that periods of 20 to 100 years occur between cycles, is sustainable. The forest regrows rapidly after a few years of farming. But when millions of people try to obtain a living in this way, the land is not allowed to lie uncultivated long enough to recover. Globally, at least 200 million subsistence farmers obtain a living from slash- and-burn agriculture, and the number is growing. Moreover, only half as much forest is available today as was available 50 years ago.

Commercial Logging. Vast tracts of tropical rain forests, particularly in Southeast Asia, are harvested for export abroad. Most tropical countries allow commercial logging to proceed at a much faster rate than is sustainable. Unmanaged tropical deforestation, rather than contributing to economic development, depletes a valuable natural resource faster than it can regenerate for sustainable use.

Cattle Ranching and Agriculture for Export. Some tropical deforestation is carried out to provide open rangeland for cattle (Figure 18.12). Cattle ranching, which employs relatively few local people (ranching does not require much labor), is particularly important in Latin America. Some of the beef raised on these ranches, often owned by foreign companies, is exported to highly developed countries, although much is consumed locally. After the forests are cleared, cattle graze on the land for perhaps 20 years, after which time the soil fertility is depleted. When this occurs, shrubby plants, known as scrub savanna, may take over the range.

A considerable portion of forestland is cleared for plantation-style agriculture, which produces export crops such as citrus fruits, bananas, and soy. (Soy is exported to use as animal feed.) Plantation-style agriculture, often practiced by foreign corporations, is sustainable on forest soil as long as fertilizers and other treatments are applied.

Why Are Tropical Dry Forests Disappearing? Tropical dry forests are being destroyed at a rapid rate, primarily for fuelwood. About half of the wood consumed worldwide is used as heating and cooking fuel by much of the developing world. The unsustainable use of wood has led to a fuelwood crisis in many developing countries. The 2 billion or so people that the FAO says cannot get enough fuelwood to meet basic needs, such as boiling water and cooking, are at risk of waterborne infectious diseases for a list of some human diseases transmitted by polluted water). Often the wood cut for fuel is converted to charcoal, which is then used to power steel, brick, and cement factories. Charcoal production is extremely wasteful: 4.0 metric tons (4.5 tons) of wood produce enough charcoal to fuel an average-sized iron smelter for only six minutes.

Tropical Rain Forests: Lush Equatorial Forests

Tropical rain forests occur where temperatures are warm throughout the year and precipitation occurs almost daily. The annual precipitation of a tropical rain forest is typically from 200 to 450 cm. Much of this precipitation comes from locally recycled water that enters the atmosphere by transpiration of the forest’s own trees.

Tropical rain forest commonly occurs in areas with ancient, highly weathered, mineral-poor soil. Little organic matter accumulates in such soils because bacteria, fungi, and detritus-feeding ants and termites decompose organic litter quite rapidly. Roots and mycorrhizae quickly absorb nutrient minerals from the decomposing material. Thus, nutrient minerals of tropical rain forests are tied up in the vegetation rather than the soil. Tropical rain forests are found in Central and South America, Africa, and Southeast Asia (see Figure 18.10).

Tropical rain forests are very productive—that is, the plants capture a lot of energy by photosynthesis. Of all the biomes, the tropical rainforest is unexcelled in species richness and variety. No single species dominates this biome. A person can travel hundreds of meters without encountering two individuals of the same tree species. The trees of tropical rain forests are typically evergreen flowering plants (Figure 6.11). Their roots are often shallow and concentrated near the surface in a mat. The root mat catches and absorbs almost all nutrient minerals released from leaves and litter by decay processes. Swollen bases or braces called buttresses hold the trees upright and aid in the extensive distribution of the shallow roots.

A fully developed tropical rain forest has at least three distinct stories, or layers, of vegetation. The topmost story consists of the crowns of occasional, very tall trees, which are exposed to direct sunlight. The middle story reaches a height of 30 to 40 m (100 to 130 ft) and forms a continuous canopy of leaves that lets in little sunlight to support the sparse understory. Only 2% to 3% of the light bathing the forest canopy reaches the forest understory. Smaller plants specialized for life in the shade, as well as the seedlings of taller trees, comprise the understory. The vegetation of tropical rain forests is not dense at ground level except near stream banks or where a fallen tree has opened the canopy.

Tropical rainforest trees support extensive epiphytic communities of plants such as ferns, mosses, orchids, and bromeliads. Epiphytes grow in crotches of branches, on bark, or even on the leaves of their hosts, but they use their host trees primarily for physical support, not for nourishment.

Lianas (woody tropical vines), some as thick as a human thigh, twist up through the branches of the huge rainforest trees. Once in the canopy, lianas grow from the upper branches of one forest tree to another, connecting the tops of the trees and providing a walkway for many of the canopy’s residents. Lianas and herbaceous vines provide nectar and fruit for many tree-dwelling animals.

Not counting bacteria and other soil-dwelling organisms, most tropical rainforest organisms live in the upper canopy. Rainforest animals include the most abundant insects, reptiles, and amphibians on Earth. The birds, often brilliantly colored, are varied, with some specialized to consume fruit (parrots, for example) and others to consume nectar (humming-birds and sunbirds, for example). Most rainforest mammals, such as sloths and monkeys, live only in the trees and rarely climb to the ground, although some large, ground-dwelling mammals, including elephants, are also found in rain forests.

Human population growth and industrial expansion in tropical countries may spell the end of tropical rain forests during the 21st century. Biologists know that many rainforest species will become extinct before they are even identified and scientifically described. (See Chapter 18 for more discussion of the ecological impacts of tropical rainforest destruction.)

Forests, important ecosystems that provide many goods and services to support human society, occupy about one-fourth of Earth’s total land area. Timber harvested from forests is used for fuel, construction materials, and paper products. Forests supply nuts, mushrooms, fruits, and medicines and provide employment for millions of people worldwide. They also offer recreation and spiritual sustenance in an increasingly crowded world.