Forest

A forest is an area with a high density of trees (or, historically, a wooded area set aside for hunting). Actually, many definitions of a forest exist [1]. These plant communities cover large areas of the globe and function as animal habitats, hydrologic flow modulators, and soil conservers, constituting one of the most important aspects of the Earth's biosphere. Although often thought of as as carbon dioxide sinks, mature forests are approximately carbon neutral with only disturbed and young forests acting as carbon sinks [2] [3]. Nonetheless mature forests do play an important role in the global carbon cycle as stable carbon pools, and clearance of forests leads to an increase of atmospheric carbin dioxide levels.

Forests can be found in all regions capable of sustaining tree growth, at altitudes up to the tree-line, except where natural fire frequency is too high, or where the environment has been impaired by natural processes or by human activities. As a general rule, forests dominated by angiosperms (broadleaf forests) are more species-rich than those dominated by gymnosperms (conifer, montane, or needleleaf forests), although exceptions exist (for example, species-poor aspen and birch stands in northern latitudes). Forests sometimes contain many tree species within a small area (as in tropical rain and temperate deciduous forests), or relatively few species over large areas (e.g., taiga and arid montane coniferous forests). Forests are often home to many animal and plant species, and biomass per unit area is high compared to other vegetation communities. Much of this biomass occurs below-ground in the root systems and as partially decomposed plant detritus. The woody component of a forest contains lignin, which is relatively slow to decompose compared with other organic materials such as cellulose or carbohydrate.

Forests are differentiated from woodlands by the extent of canopy coverage: in a forest the branches and foliage of separate trees often meet or interlock, although there can be gaps of varying sizes within an area referred to as forest. A woodland has a more continuously open canopy, with trees spaced further apart, which allows more sunlight to penetrate to the ground between them (see also savanna).

Among the major forested biomes are: