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The forest floor, also called detritus, duff and the O horizon, is one of the most distinctive features of a forest ecosystem. It mainly consists of shed vegetative parts, such as leaves, branches, bark, and stems, existing in various stages of decomposition above the soil surface. Although principally composed of non-living organic material, the forest floor also teems with a wide variety of fauna and flora. It is one of the richest components of the ecosystem from the standpoint of biodiversity because of the large number of decomposers and predators present, mostly belonging to invertebrates, fungi, algae, bacteria, and archaea. The major compartments for the storage of organic matter and nutrients within systems are the living vegetation, forest floor, and soil. The forest floor serves as a bridge between the above ground living vegetation and the soil, and it is a crucial component in nutrient transfer through the biogeochemical cycle. Much of the energy and carbon fixed by forests is periodically added to the forest floor through litterfall, and a substantial portion of the nutrient requirements of forest ecosystems is supplied by decomposition of organic matter in the forest floor and soil surface. The sustained productivity of forests is closely linked with the decomposition of shed plant parts, particularly the nutrient-rich foliage. The forest floor is also an important fuel source in forest fires.
The amount of material in the forest floor depends on the balance between inputs from litter production and outputs from decomposition, and amounts also reflect the site's disturbance history. Both litter production and decomposition are functions of the site (e.g., wet versus dry; cold versus warm; nutrient rich versus nutrient poor) and the vegetation that occupies the site (e.g., conifer versus broadleaf). A site's forest floor is determined by its areal weight, depth, and nutrient content. Typically, forest floors are heaviest and deepest in boreal forests and mountain forests where decomposition rates are slow. In contrast, the lightest and thinnest forest floors usually occur in tropical rain forests where decomposition rates are rapid, except on white sands where nutrients could not be supplied from mineral weathering.