Forests are among the most productive of Earth's natural ecosystems. On average, tropical rain forests have a net primary productivity of about 4.8 lb/ft2/year (2.2 kg/m2/yr), compared with 3.5 lb/ft2/year (1.6 kg/m2/yr) for tropical seasonal forests, 2.9 lb/ft2/year (1.3 kg/m2/yr) for temperate conifer forests, 2.6 lb/ft2/year (1.2 kg/m2/yr) for temperate angiosperm forests, and 1.8 lb/ft2/year (0.8 kg/m2 yr) for conifer subarctic forests.
Although tropical rain forests have relatively high rates of net primary productivity, their net ecosystem productivity is very small or zero. This occurs because these forests are typically in an old-growth condition, so that there are always some individual trees that are dying or recently dead, resulting in a relatively large number of standing dead trees and logs lying on the forest floor. These deadwood components decompose rather quickly under the warm and humid conditions of tropical rain forests, as does the rain of leaf litter to the forest floor. Because the rate of decomposition approximately counterbalances the positive net primary production, the net ecosystem productivity is more-or-less zero in these old-growth forests. Old-growth temperate rain forests are less common than tropical ones, but these also typically have a small or zero net ecosystem productivity.
Mature forests store more carbon (in biomass) than any other kind of ecosystem. This is especially true of old-growth forests, which typically contain large trees and, in temperate regions, a great deal of dead organic matter. Because all of the organic carbon stored in forests was absorbed from the atmosphere as carbon dioxide (CO2), these ecosystems are clearly important in removing this greenhouse gas from the atmosphere. Conversely, the conversion of forests to any other type of ecosystem, such as agricultural or urbanized lands, results in a large difference in the amount of carbon stored on the site. That difference is made up by a large flux of CO2 to the atmosphere. In fact, deforestation has been responsible for about one-half of the CO2 emitted to the atmosphere as a result of human activities since the beginning of the industrial revolution.
Because they sustain a large biomass of foliage, forests evaporate large quantities of water to the atmosphere, in a hydrologic process called evapotranspiration. Averaged over the year, temperate forests typically evapotranspire 10-40% of their input of water by precipitation. However, this process is most vigorous during the growing season, when air temperature is highest and the amount of plant foliage is at a maximum. In fact, in many temperate forests evapotranspiration rates during the summer are larger than precipitation inputs, so that the ground is mined of its water content, and in some cases streams dry up.
Intact forests are important in retaining soil on the land, and they have much smaller rates of erosion than recently harvested forests or deforested landscapes. Soil eroded from disturbed forests is typically deposited into surface waters such as streams and lakes, in a process called sedimentation. The resulting shallower water depths makes flowing waters more prone to spilling over the banks of rivers and streams, causing flooding.
Forests are also important in moderating the peaks of water flow from landscapes, both seasonally and during extreme precipitation events. When this function is degraded by deforestation, the risk of flooding is further increased.