Still cradles of life, forests also perform all kinds of practical services that benefit modern humans. They produce oxygen we breathe and suck up air pollution. In the United States, 80 percent of fresh water originates in forested areas. Forests purify water and refill underground aquifers; in addition, they absorb rain, and slow down floods and water runoff.

Forests and woodlands over the world have changed over the millennia due to changes in climate and geology. In the modern world, forests are classified into various groups, including temperate-zone and tropical forests. Not all rain forests are in the tropics -- some are in cooler climates. And there are other kinds, such as riparian forests, that separate interior areas from coastlines.

Each part of the forest supports life. The soil is full of uncounted numbers of microbes, insects, and fungi, essential to recycling organic matter, and thus to the survival of all life on earth. Larger animals live on the forest floor, and the shrub and tree canopy layers are vital to birds. There are about 1.5 million known species in the world, and the true number of species may be ten times more than that. Many of these spend their lives growing, burrowing, wriggling, or plodding along in forests -- or flying through trees. The extent of forested lands has made it possible for birds and animals to range freely in search of food and appropriate climate; the resulting horizontal and vertical complexity of the forest and its density of life creates biodiversity.

Tropical forests generate the richest biodiversity, as the energy generated by the equatorial sun encourages life to proliferate amid abundant nutrients. Unfortunately, these forests are quite fragile, and over the past half-century have succumbed in large numbers to human clearing and logging. Global forests themselves, as well as their diverse reserves of plants and animals, are threatened as never before. It has been estimated that by the late 1980s three quarters of old-growth forests on the planet had been destroyed, including about half of tropical and temperate rain forests; and human population expansion continues to lead to the clearing of new lands.

According to the U.S. State Department, "one of every six known bird species, one of every 11 mammals, and one of every 15 reptiles" makes the Amazon rainforest its home. Unfortunately, as David B. Sandalow, assistant secretary of state for oceans and international environmental and scientific affairs, recently noted: "Tropical forests are disappearing at an alarming rate. Saws and bulldozers are leveling roughly 200 hectares per minute. A soccer field is close to two hectares, so we are losing about 120 soccer fields of tropical forest per minute, more than 7,000 soccer fields per hour, more than 170,000 soccer fields per day."

Around the globe, forests that are not totally destroyed are being fragmented by roads and human development, a change that threatens the health and survival of the indigenous plants and animals. Biologists believe that destroying 90 percent of a wooded habitat reduces local species by about half. Harvard University biologist Edward O. Wilson has noted that "the poorest people with the fastest-growing populations live next to the richest deposits of biological diversity" and that a single "farmer clearing rain forest to feed his family…will cut more kinds of trees than are native to all of Europe."

As the configuration of forests changes, other factors come into play as well. For instance, some forest dwellers require "edge" habitat, areas of woods near glades or grassland, to thrive, while others require the forest interior. Highly fragmented woodland areas diminish the proportion of interior to edge habitat and alter the balance of species. Greater exposure of once-sheltered trees to "edges" may cause them to dry out and become prey to invaders. Global climate change is liable to pose new challenges to the survival of forest ecosystems; habitat fragmentation could hinder the ability of species to adapt.

Once a forest habitat has been weakened, it is more susceptible to disease and to alien species invasion. For instance, following World War II, most songbirds on the Pacific island of Guam were wiped out by a single species of snake, not native to the island. In the United States, some native birds have declined due to the thinning out of forests, which has made it easier for cowbirds to penetrate into woods and parasitize their nests. Many factors affect species decline. When a local population, or a subspecies, becomes diminished, its members breed with one another, amplifying genetic weaknesses and quite possibly precipitating a final decline to extinction.

Effective forest conservation requires a commitment to look beyond the short term and to retain large forest areas for the bounty they can provide future generations. The greater the biodiversity of an environment, the greater its ability to withstand environmental stress and produce new and useful forms of life. Properly managed, forest animals and plants may produce more valuable medicine, food, and construction material over the years and decades than can be procured by clear-cutting forests and destroying them in their current form. One frequently cited example of the rewards of maintaining biodiversity for future generations is the recent discovery of the Madagascar rosy periwinkle, which was found to produce chemicals that can cure Hodgkin's disease and childhood leukemia. About 3 percent of the world's flowering plants, so far, have been examined for anticancer chemicals similar to those of the periwinkle. In the United States, 25 percent of pharmaceutical prescriptions are derived from plant extracts and another 13 percent are from microorganisms. The venerable neem tree of South Asia has been thought for centuries to have all kinds of health-giving effects; yet, scientists are just beginning to study it systematically. Little-known plants and animals can be potent sources of pharmaceuticals because they have evolved a range of chemical strategies over the millennia to defend themselves from predators, survive, and thrive.

Forests are stores of food. About a dozen fruits - apples, peaches, strawberries, bananas, etc. - dominate world consumption. There are probably about 3,000 more kinds of fruits in the tropics, of which 200 are widely eaten. Tens of thousands of other grains, vegetables, and forms of plant food are out there waiting to cure starvation and create greater variety on the dinner table, if they are allowed to survive. The winged bean of New Guinea, for instance, is full of protein, is entirely edible, and can be fried, roasted, ground into flour, or served as a hot beverage. And it grows to a length of 4 meters in a few weeks. The Amazonian babassu palm, still found in a natural state, offers the world's highest yield of vegetable oil from its fruit. It can also feed livestock, produce thatching materials, and be burned for charcoal. Iguana meat is prized by many in the Southern Hemisphere. Scientists estimate forest-ranched iguanas can yield ten times the amount of meat as cattle on the same acreage of cleared land. Other less well-known, yet tasty, animals could produce much food without destroying their forest cover.

Much loss of forest cover, including loss of ancient forests, is due to harvesting for paper, at the rate of hundreds of millions of metric tons per year. There are less environmentally destructive ways to produce paper, including recycling, or the use of crops such as kenaf. Once an acre of forest is cut down and the trees sold for timber, its value of the land is often diminished. Some studies have shown that harvesting fruits, chocolate, substances such as latex, and vegetables can create more sustainable yield for a tropical farmer than a one-time timber harvest.

One approach currently being used around the world is to identify biodiversity "hot spots" and to concentrate on saving those areas first. In a tropical forest, areas with the most diverse species of trees also tend to harbor the most diverse groups of shrubs, plants, birds, insects, amphibians, fish, and other creatures. Hotspots are not all in forests: they can vary in terms of geography and habitat and in the kinds of organisms they shelter. However, they are useful in delineating and protecting biodiversity. In 1997, one conservation group estimated that the 17 most vibrant hot spots in the world occupied 1.3 percent of the planet's land, yet protected 25 percent of terrestrial vertebrate species and 40 percent of plants. Clearly, well-maintained hot spots can begin to salvage the biological richness and potential of many nations, while mitigating the destruction of remaining wild areas. The Internet may become a tool in the service of forest conservation, posting and correlating the best data on deforestation and making it available worldwide. Global Forest Watch, an arm of the World Resources Institute, is attempting to create an international data and mapping network to track the pace of the destruction. Similar efforts may be made by other groups.

The world is beginning to wake up to the need to save forests. Governments are increasingly committed to forest conservation. In 1992 at the Rio Earth Summit, nations adopted the Forest Principles, the first-ever global consensus on the importance of forest conservation. In 1995, the United Nations established the Intergovernmental Panel on Forests. David Sandalow adds, "Illegal logging may be the single greatest threat to tropical forests," and commends some nations for having "made major commitments to address the problems."

The United States is implementing its own Tropical Forest Conservation Act. In March, 2000, the United States committed itself to a debt-for-nature swap with Bangladesh, to help conserve Bangladeshi tropical forests. Shortly thereafter, the U.S. government added 262,400 hectares of giant sequoia trees in California to its extensive network of national parks and nature reserves. Corporations are beginning to see the value of sustainable use of forests. Home Depot, a major U.S. retailer, has announced it will stop selling wood products from environmentally sensitive areas. The American Forest and Paper Association has committed itself to a focus on sustainable forest management.

In 1997, the government of Bolivia, the Nature Conservancy, a U.S.-based not-for-profit organization, and American Electric Power provided funds and worked together to expand and protect an ecologically rich national park. The same year, a public-private partnership worked together to protect 4 million acres of rainforest in Suriname. In general, such partnerships involving many stakeholders, targeted to local conditions, seem to work the best. Agronomists are striving to continue to improve agricultural productivity, in order to make it less necessary to cut down forests for cropland. Around the world, 500 million people are thought to depend on forests for their livelihood - an incentive to preserve the health of forests and to protect them as a sustainable resource for future generations.