Ecosystem Processes & Environmental Protection


Forests are an integral part of Earth's life-support system. They play an important role in regulating climate and water cycles, as well as in maintaining and conserving soil. Because of the unique nature of rainforests, the environmental effects of clearing them are much more severe compared to temperate forests.

Deforesting hillsides allows heavy rains to wash away the existing soil. A study in Cote d'Ivoire showed that the annual loss of soil on a forested slope was 30 kilograms per hectare (160 pounds per acre); in contrast, a similar deforested slope lost a staggering 183 tons (121 US tons) of soil annually.

Deforestation is also responsible for flooding and droughts in many countries with tropical rainforests. Rainforests, with their thick foliage and complex root systems, regulate water supplies. Typically, in a well-forested watershed, 95% of the annual rainfall is detained in the sponge-like network of roots. Much of this water is released back into the atmosphere by evaporation and transpiration (the process by which water is drawn up from the roots of a plant and evaporated from its leaves), thus reducing the total water run-off. The remainder is released slowly throughout the year, keeping streams and rivers flowing even during the dry seasons. Globally, more than one billion people depend on water from tropical forests for drinking and crop irrigation. Without the regulatory function of the rainforests, heavy rains lead to floods and landslides, whereas rivers dry up if the rains are poor.

Rainforests also affect the local climate. They are fundamental in maintaining rainfall patterns by returning huge amounts of water to the atmosphere. One study found that 10-20% less water evaporates from cleared areas than from forested areas. Cutting down trees therefore reduces atmospheric humidity and also reduces rainfall.
A research team headed by Aneas Salati at the Brazilian Space Research Institute concluded that nearly 50% of the rain falling over the Amazon Basin is returned to the atmosphere from the forest. The recycling of water forming East to West across the Amazon Basin plays an important part in keeping the area wet. The western regions are thousands of kilometers from the Atlantic Ocean, and rely on water "passed on" through the forest-atmosphere system. If large areas of forest in the East are destroyed, this recycling "conveyor" could break down and lead to a gradual drying out and ultimate death of the forests in the far West of the basin.

The climatic effects of tropical rainforests are so powerful that they are felt thousands of miles away. Plants and animals release huge amounts of carbon. When forests are burned for land clearing and conversion, this carbon is released into the air as carbon dioxide (CO2), which is one of the several greenhouse gases that occur naturally in the atmosphere and help to regulate temperatures on the earth's surface. During the last 100 years, the amount of CO2 in the atmosphere has increased steadily. Most of this increase can be accounted for by the burning of fossil fuels (coal, gas, and oil). These fuels are essentially carbon compounds created thousands of years ago from dead plants and animals. Currently, about five billion tons of carbon as CO2 are pumped into the atmosphere from chimneys and exhaust pipes. The contribution made by the burning of forests is much harder to calculate, but may account for a further one billion tons a year.
Large-scale deforestation in the tropics thus threatens to change global climatic systems by altering the mechanisms by which heat is transferred to higher latitudes. No one can predict the outcome for the global climate if this were to occur. If deforestation continues at the present rate, however, the amount of forest burning will increase, adding to the greenhouse effect.

One critical short-term way of decreasing the concentrations of atmospheric CO2 is to plant more trees. As a tree grows, it absorbs CO2 and incorporates the carbon into its cells. When mature, it is in balance with the atmosphere, releasing about the same amount of carbon dioxide into the atmosphere through respiration as it absorbs through photosynthesis.