A forest is an area with a high bulk of trees. There are several definitions of a forest, based on a variety of criteria. These plant communities face large areas of the globe and function as animal habitats, and soil conservers, constituting one of the most important aspects of the Earth's biosphere. While frequently thought of as carbon dioxide sinks, grown-up forests are approximately carbon neutral with only troubled and young forests acting as carbon sinks. However mature forests do play a main role in the global carbon cycle as stable carbon pools, and authorization of forests leads to an increase of impressive carbon dioxide levels.
Forests sometimes have many tree species within a small area or comparatively few species over large areas. Forests are frequently home to many animal and plant species, and biomass per unit area is high compared to other plants communities. Much of this biomass occurs below-ground in the origin systems and as partly decomposed plant accumulation. The woody element of a forest contains lignin, which is comparatively slow to decompose compared with other organic materials such as cellulose or carbohydrate.