A forest is a terrestrial ecosystem dominated by trees and the interactions among other vegetation, animals, microorganisms, soil, and the physical environment. Forests range from humid tropical communities to cold northern woodlands, and include both naturally regenerating and planted stands. They support a substantial share of terrestrial biodiversity, influence carbon and water cycles, and supply materials, food, and livelihoods. The Food and Agriculture Organization of the United Nations (FAO) estimated global forest area at 4.14 billion hectares, approximately 32 percent of land area, in its 2025 assessment. (fao.org)
Definitions and measurement
Definitions of forest vary according to ecological, statistical, and administrative purposes. FAO’s international reporting definition uses three thresholds: an area exceeding 0.5 hectares, trees taller than 5 metres, and canopy cover greater than 10 percent, or trees capable of reaching those height and cover thresholds at the site. Land predominantly used for agriculture or urban purposes is excluded, even when it contains considerable tree cover. Consequently, orchards and oil-palm plantations are not classified as forests under this definition. (forest.eea.europa.eu)
Forest land need not carry a mature tree canopy continuously. Young stands and temporarily unstocked areas expected to regenerate can remain forest in land-use accounting. This distinction separates forest area, a land-use measure, from tree cover, a measure of vegetation visible at a particular time. It also explains why a harvested site is not necessarily classified as deforested. (forest.eea.europa.eu)
Distribution and major types
Forest distribution depends on climate, soil, topography, and human activity. Temperature and the amount and seasonality of rainfall help determine tree growth, leaf characteristics, and community composition. Broad classifications distinguish tropical, subtropical, temperate, and boreal climatic domains, with considerable variation within each. (fao.org)
Tropical forests occur mainly at low latitudes. Tropical rainforests experience abundant moisture and generally retain evergreen vegetation; seasonal and dry tropical forests experience longer dry periods and may contain many deciduous trees. Tropical forests have exceptionally high species diversity, although richness varies among locations and forest types. (fao.org)
Temperate forests occur in middle-latitude regions of both hemispheres. They include deciduous broadleaf, evergreen coniferous, mixed, and temperate rainforest formations. Their composition reflects winter temperatures, growing-season length, moisture availability, and regional environmental conditions. (fao.org)
Boreal forests, also called taiga, extend across northern North America and Eurasia. Long, cold winters and short growing seasons favor extensive conifer forests, including spruce, fir, pine, and larch. Bogs and peatlands occur within many boreal landscapes. Toward the northern treeline, trees become shorter and more sparsely distributed. (fao.org)
Structure and ecological processes
Forest vegetation is frequently arranged in vertical layers: the canopy of tree crowns, an understory of smaller trees and shrubs, and vegetation near the forest floor. These layers provide different habitats for organisms. Forest biodiversity also includes life in soils and dead organic material, rather than simply the diversity of living trees. (fao.org)
Through photosynthesis, plants incorporate carbon into organic matter. Growth builds biomass, while litterfall, root turnover, and decomposition transfer and recycle materials. Soil organisms participate in nutrient transformations, and roots obtain water and mineral nutrients from the soil. Nutrients can leave the ecosystem through leaching, erosion, harvesting, and fire. These processes connect forest growth with the carbon cycle and the continuing fertility of forest soils. (fs.usda.gov)
Forests are dynamic rather than fixed communities. Ecological succession follows disturbances that alter vegetation and growing conditions. Surviving organisms and retained woody debris influence subsequent development. Early-successional communities can support diverse plants and complex food webs before a dominant tree canopy develops; they are not merely ecologically empty intervals between mature stands. (research.fs.usda.gov)
Environmental and human importance
Forests provide ecosystem services through their effects on carbon storage, water movement, soil protection, and biological diversity. Carbon is held in vegetation and other ecosystem pools. Whether a particular forest gains or loses carbon over a period depends on the balance among growth, decomposition, disturbance, and removal of biomass. (fao.org)
Forests influence the water cycle through interception, evaporation, and transpiration. Roots and porous, organic-rich soils encourage infiltration, while vegetation and litter reduce the impact of raindrops and limit erosion. These effects can improve water quality and influence streamflow. Nevertheless, trees also consume water, and forest cover does not prevent every flood: outcomes depend on climate, landscape scale, soil conditions, and storm intensity. (fao.org)
Human uses include timber, fuelwood, food, fodder, and other biological products. Forest resources support household livelihoods and commercial production, while forests also provide recreation. Communities living within forests often depend directly on their biological resources for shelter, subsistence, and income. (fao.org)
Forest change and management
Deforestation is the conversion of forest to another land use. It differs from forest degradation, which concerns deterioration in forest condition without necessarily eliminating forest land. Agricultural expansion is a major driver of forest conversion, while disturbances and management activities can change forest structure and ecological functioning. (fao.org)
FAO estimated annual deforestation at 10.9 million hectares during 2015–2025. Net forest loss was lower—4.12 million hectares annually—because forest expansion offset part of the loss. These measures are not interchangeable: net area change does not describe all changes in forest composition or condition. (fao.org)
Forestry encompasses management for production, protection, and other objectives. Conservation includes protected areas and management outside them, while ecological restoration seeks to recover degraded forest ecosystems. Regeneration outcomes depend on previous land use, site degradation, and the availability of species able to establish. Increasing tree cover alone therefore does not fully describe ecological recovery. (fao.org)