Forest

A forest is an area with a high density of trees. There are many definitions of a forest, based on various criterion. These plant communities cover in the region of 9.4% of the Earth's surface (or 30% of total land area) and function as habitats for organisms, hydrologic flow modulators, and soil conservers, constituting one of the most important aspects of the Earth's biosphere. Historically, "forest" meant an uncultivated area legally set aside for hunting by feudal nobility, and these hunting forests were not essentially forested much if at all (see Royal Forest). However, as hunting forests did often include substantial areas of woodland, the word forest ultimately came to mean wooded land more generally.

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 atmosphere has been impaired by natural processes or by human behavior. As a general rule, forests conquered by angiosperms (broadleaf forests) are more species-rich than those conquered 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 constituent of a forest contains lignin, which is relatively slow to crumble compared with other organic materials such as cellulose or carbohydrate.