The role of rainforests in the global carbon cycle is complex and little known. Plants and animals contain a great deal of carbon, which they take up as carbon dioxide (CO2) during growth and photosynthesis, and which they release to the atmosphere during respiration and decomposition. Although rainforests form less than half of the total forest on earth, their leaf systems comprise approximately 70% of the world’s total leaf surface area. Rainforests have ten times more leaf area than temperate forests of comparable size and fifty times more than grasslands. It is not surprising, then, that they account for between 30% and 50% of total primary productivity (photosynthesis) in terrestrial systems, although they cover only 6% of the total land area of the earth. This means that they store more carbon (as sugars and starches) per unit area than any other type of ecosystem. Rainforests are thought to contain between 40% and 50% of the carbon in the terrestrial biomass (Phillips, et al., 1998), which has been estimated as more than 17 kilograms of carbon per square meter. The rainforests of Amazonia contain between 14 and 40 kilograms of carbon per square meter. The soils lying under rainforests also contain substantial amounts of carbon (in roots, microorganisms, soil fungi and plants), which amounts to about 27% of global soil carbon (Lodge, et al., 1996).
Not all carbon storage occurs within above-ground plant vegetation. At least 40% (and perhaps as much as two-thirds) of the carbon in tropical forests is found below ground in root systems and soil organic matter. Forests (including temperate forests) have been estimated to contain 330 gigatons (1015 tons) of carbon in the vegetation and 660 gigatons of carbon in soil organic matter (Noble and Dirzo, 1997). Amazonian soils contain from four to nine kilograms of carbon in the upper 50 centimeters of the soil layer, while pasturelands contain only about one kilogram per square meter, in contrast. Thus, tropical forests are critical elements in the carbon cycle of the planet. When forests are cleared and burned, 30 – 60% of the carbon is lost to the atmosphere; unburned vegetation decays and is lost within ten years.
The importance of rainforests in the carbon cycle depends on the extent of the forest, the amount of carbon stored per unit area (as plant body or as organic material in the soil), and the rate at which carbon is “fixed” by the plants during photosynthesis.