During photosynthesis, plants absorb carbon dioxide and convert it to carbon (stored as plant tissue) and oxygen. Forests therefore have an important ecological function in fixing and storing carbon from the atmosphere. Increasing concentrations of carbon dioxide in the atmosphere appears to be one of the factors leading to observed changes in the global climate, so that there is growing interest in the role of forests as a possible factor in mitigating climate change.
"Each year, as forests grow and increase their biomass, they absorb carbon from the atmosphere and store it in plant tissue. This process is known as carbon sequestration. Despite constant exchanges of carbon between forest biomass, soils, and the atmosphere, a large amount is always present in leaves and woody tissue, roots, and soil nutrients. This quantity of carbon is known as the carbon store. Carbon sequestration and storage slow the rate at which carbon dioxide accumulates in the atmosphere and mitigate global warming. Forests sequester and store more carbon than any other terrestrial ecosystem, and constitute an important natural defense against climate change."
Tropical forests have a special role in the conservation of biodiversity. They are the home to 70 percent of the world's plants and animals -- more than 13 million distinct species (Anon., 1996). The tropical forests contain 70 per cent of the world's vascular plants, 30 per cent of all bird species, and 90 per cent of invertebrates. Many of the mammals are among the most famous icons of natural history -- the great cats, the primates, and the ungulates of the East African woodlands. In tree species alone, tropical rain forests are extremely diverse, often having more than 200 species per hectare. Boreal forests, on the other hand, are biologically much simpler, with as few as one species per hectare for fire-regenerated stands like lodgepole pine in North America.
Forests influence the local and probably global climates. They moderate the diurnal range of air temperatures and maintain atmospheric humidity levels. Forests absorb atmospheric carbon and replenish the oxygen in the air we breathe. The conservation of forest resources in the watersheds that supply water for irrigation, sanitation, and human consumption is an important component of water supply strategies. When tropical watersheds have balanced land use, their forests absorb excessive rainfall that is gradually released later. Forests regulate stream flows by intercepting rainfall, absorbing the water into the underlying soil, and gradually releasing it into the streams and rivers of its watershed. This minimizes both downstream flooding and drought conditions. Tree cover conserves moisture in the soil by providing shade that reduces the evaporative loss from radiant energy exchange with the atmosphere. Tree roots enhance soil porosity, reduce compaction, and facilitate infiltration. Trees act as windbreaks, reducing the force of desiccating, eroding winds at ground level.