Forests and woodlands are being degraded or destroyed and natural ecosystems reduced or fragmented, further threatening biodiversity. Many factors contribute to deforestation: timber production and clearance for agriculture being the principal; also cutting for firewood and charcoal, fires, droughts, strip mining, pollution, hydroelectric power development, mining, industrial infrastructure and railways, urban development, cyclones and war. The quality of the remaining forest is threatened by a range of pressures including acidification, climate change, water abstraction, fire, population pressures, diseases and invasive species.
Forests take in carbon dioxide gas, provide oxygen and clean the air. Their water-holding capacity maintains soil and water levels, preventing disasters such as landslides, floods and droughts. Tropical forests, the most important surviving forest lands, contain about two thirds of all plant and animal species. Tropical plants are the basis for many useful drugs but vast numbers have not been tested for their medical properties. Tropical forests are also stores of genetic material to feed back into cultivated plants susceptible to disease and pests. At the present rate of deforestation, an estimated 15% of all forest species could disappear within the next two decades.
Deforestation accelerates erosion, changes local hydrological cycles and precipitation patterns, and decreases the land's ability to retain water during rainy periods. Resulting flash floods destroy irrigation systems and plug rivers and resources with silt. And when silted coastlines decimate fisheries, fishermen turn to agriculture; they join land-starved farmers in cutting down more forests, completing a vicious circle.
In many countries, and especially in the developing countries of the southern hemisphere where most uncleared forest land remains, systematic burning, grazing and cutting of forest land is still carried out in order to provide new land for crop agriculture or livestock. It is often done without factors such as climate and topography having been sufficiently studied and on lands where slope, nature of the soil or other physiographic characteristics clearly indicate that the land involved is suitable only for forest. Although this practice may lead to a temporary increase in productivity, in the long run there is usually a decrease in productivity per unit of surface and that erosion and irreversible soil deterioration often accompany this process.
The underlying driving forces behind deforestation are poverty, population and economic growth, urbanization and expansion of agriculture lands. Clearance for agriculture is the largest cause of tropical deforestation; logging, however, is responsible for an estimated one-third of the total, the proportion rising to about one-half in Asia, and possibly higher still in parts of South America.
Of the 29 million square kilometres of closed forests, 32% are boreal (subarctic), 26% temperate (in both hemispheres) and 42% tropical. Three-quarters of the open forests and shrubland are in the tropics.
Since pre-agricultural times, the world has lost 20% of its forest resources, with a reduction from 12 billion to 10 billion acres. In the past, most of forest losses were in the temperate forests of Europe, Asia and North America. In recent years, it is the tropical forests of Latin America, Asia and Africa that have been disappearing most rapidly.
Globally, between 1970 and 2002 forest cover dwindled by 12 per cent. Between 1990 and 1995, 65 million hectares of forests were lost, a total loss of 65 million hectares in developing countries being partially offset by an increase of nearly 9 million hectares in the developed world.
Deforestation continues at high rates in developing countries, mainly driven by the demand for wood products and the need for land for agriculture and other purposes. Despite increased public awareness and a large number of initiatives, deforestation is still continuing in most of Africa, Latin America, and Asia and the Pacific. In the early 1990s, the rate of deforestation in primary forests in South America was 2.2 million hectares per year. This is an increase of approximately 80 percent since the early 1970s. During 1980-90 alone, the Latin American region lost 62 million hectares (6.0 per cent) of its natural forest, the largest loss in the world during those years, with a further 5.8 million hectares a year lost during 1990-95.
The USA cleared most of its forests in the 19th century and is still felling trees. Roughly 16 per cent of the world's forests – some 565 million hectares – are in the Asia/Pacific region. These forests harbour some of the world's rarest and unique animals and plants: the tiger, giant panda, Asian elephant, orangutan, rhino, Rafflesia (the world's biggest flower) and many variety of orchids. In the early 1990s, the rate of deforestation in primary forests in the Asia Pacific region was 1.8 million hectares per year. This is an increase of approximately 80 percent since the early 1970s. About 838,000 ha of the region's forests are lost annually to deforestation and degradation. The Himalayan watershed covering Northern India, Nepal, and Bangladesh had lost 40% of its forest by 1980, where the lowland terai forests are rapidly being logged and cleared, often illegally. Forest cover shrank at an average rate of 0.6 per cent a year throughout the 1980s in India and the rate was increasing. With barely a quarter of the land mass recorded as forest, the country fell well short of the government's aim to maintain a third of its area as forest. In the Himalayan-Hindukush-Karakorum mountain region from Afghanistan to Bangladesh, forests logged and degraded by excessive grazing, fodder and fuelwood collection, and pressure from the influx of refugees and political disturbance. China's forests have also been badly depleted and losses continue, especially within Tibet.
In the Philippines, rainforests and mangroves are still being illegally logged despite already being reduced to fragments. Forests in Indonesia, are rapidly being logged and cleared often to establish exotic plantations. In addition, the country's forests have been burnt, much of this occurring in Kalimantan with particularly bad results in the late 1990s. In Thailand, forests have already been fragmented, and a Thai Government logging ban has increased pressure on the forests of neighbouring countries such as Cambodia, Laos, Myanmar and Vietnam. In the South Pacific, the forests of Papua New Guinea, the Solomon Islands and Vanuatu are being logged, much of which illegally.
Where it has not yet been ruined by modern man – forests devastated to set up pastures, industrial coffee plantations or vast plantations of other luxury crops for the industrialized world – the exuberant tropical vegetation is rich in plants, grasses and trees which are or could be used for food.