Rainforest

Rainforests are temperate or tropical forests, usually occurring as old-growth ecosystems. The world sustains many types of rainforests, which differ geographically in terms of their species composition and the environmental conditions in which they occur. However, the various rainforests have broad ecological similarities. A such, temperate and tropical rainforests are considered to represent biomes, or widespread kinds of natural ecosystems having broad similarities of structure and function.

Rainforests require a humid climate, with more than about 80-100 in/yr (200-250 cm/yr) of precipitation distributed rather equally across the seasons, so there is no pronounced dry period. This sort of precipitation regime does not allow any but the rarest occurrences of wildfire. Other catastrophic events of stand-level tree mortality are also rare in rainforests. As a result, this ecosystem usually develops into old-growth forest containing some extremely old and large trees. However, the population structure of trees in old-growth rainforest is unevenly aged because of the micro-successional dynamics associated with the deaths of individual large trees, which result in canopy gaps below which there are relatively young trees. Old-growth rainforests also have a complex physical structure, with multiple layers within the canopy, and with large, standing dead trees and decomposing logs lying on the forest floor. Although old-growth rainforests support a very large biomass, trees within the ecosystem are dying and decaying about as quickly as new productivity is occurring. Consequently, the net ecosystem productivity of these old-growth forests is very small or zero. Temperate rainforests are dominated by a few species of coniferous trees, while tropical rainforests are characterized by a much greater diversity of tree species, along with an enormous richness of species of other plants, animals, and microorganisms.