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A forest is an area of land dominated by trees. Hundreds of definitions of forest are used throughout the world, incorporating factors such as tree density, tree height, land use, legal standing, and ecological function. The United Nations' Food and Agriculture Organization (FAO) defines a forest as, "Land spanning more than 0.5 hectares with trees higher than 5 meters and a canopy cover of more than 10 percent, or trees able to reach these thresholds ''in situ''. It does not include land that is predominantly under agricultural or urban use." Using this definition, '' Global Forest Resources Assessment 2020'' (FRA 2020) found that forests covered , or approximately 31 percent of the world's land area in 2020.

Forests are the predominant terrestrial ecosystem of Earth, and are found around the globe. More than half of the world's forests are found in only five countries (Brazil, Canada, China, Russia, and the United States). The largest share of forests (45 percent) are in the tropical latitudes, followed by those in the boreal, temperate, and subtropic domains.

Forests account for 75% of the gross primary production of the Earth's biosphere, and contain 80% of the Earth's plant biomass. Net primary production is estimated at 21.9 gigatonnes of biomass per year for tropical forests, 8.1 for temperate forests, and 2.6 for boreal forests.

Forests at different latitudes and elevations, and with different precipitation and evapotranspiration form distinctly different biomes: boreal forests around the North Pole, tropical moist forests and tropical dry forests around the Equator, and temperate forests at the middle latitudes. Areas at higher elevations tend to support forests similar to those at higher latitudes, and the amount of precipitation also affects forest composition.

Almost half the forest area (49 percent) is relatively intact, while 9 percent is found in fragments with little or no connectivity. Tropical rainforests and boreal coniferous forests are the least fragmented, whereas subtropical dry forests and temperate oceanic forests are among the most fragmented. Roughly 80 percent of the world's forest area is found in patches larger than . The remaining 20 percent is located in more than 34 million patches around the world – the vast majority less than in size.

Human society and forests influence each other in both positive and negative ways. Forests provide ecosystem services to humans and serve as tourist attractions. Forests can also affect people's health. Human activities, including unsustainable use of forest resources, can negatively affect forest ecosystems.

Definition

Although the word ''forest'' is commonly used, there is no universally recognised precise definition, with more than 800 definitions of forest used around the world. Although a forest is usually defined by the presence of trees, under many definitions an area completely lacking trees may still be considered a forest if it grew trees in the past, will grow trees in the future, or was legally designated as a forest regardless of vegetation type.

There are three broad categories of definitions of forest in use: administrative, land use, and land cover. Administrative definitions are based primarily upon the legal designations of land, and commonly bear little relationship to its vegetation: land that is legally designated as a forest is defined as such even if no trees are growing on it. Land-use definitions are based on the primary purpose that the land serves. For example, a forest may be defined as any land that is used primarily for production of timber. Under such a land-use definition, cleared roads or infrastructure within an area used for forestry—or areas that have been cleared by harvesting, disease, or fire—are still considered forests, even if they contain no trees. Land-cover definitions define forests based upon the type and density of vegetation growing on the land. Such definitions typically define a forest as an area growing trees above some threshold. These thresholds are typically the number of trees per area (density), the area of ground under the tree canopy (canopy cover) or the section of land that is occupied by the cross-section of tree trunks (basal area). Under such land-cover definitions, an area of land can only be known as forest if it is growing trees. Areas that fail to meet the land-cover definition may be still included while immature trees are present that are expected to meet the definition at maturity.

Under land-use definitions, there is considerable variation on where the cutoff points are between a forest, woodland, and savanna. Under some definitions, to be considered a forest requires very high levels of tree canopy cover, from 60% to 100%, which excludes woodlands and savannas, which have a lower canopy cover. Other definitions consider savannas to be a type of forest, and include all areas with tree canopies over 10%.

Some areas covered with trees are legally defined as agricultural areas, e.g. Norway spruce plantations, under Austrian forest law, when the trees are being grown as Christmas trees and are below a certain height.

Etymology

The word '' forest'' derives from the Old French ''forest'' (also ''forès''), denoting "forest, vast expanse covered by trees"; ''forest'' was first introduced into English as the word denoting wild land set aside for hunting without necessarily having trees on the land. Possibly a borrowing, probably via Frankish or Old High German, of the Medieval Latin , denoting "open wood", Carolingian scribes first used ''foresta'' in the capitularies of Charlemagne, specifically to denote the royal hunting grounds of the king. The word was not endemic to the Romance languages, e.g., native words for ''forest'' in the Romance languages derived from the Latin ''silva'', which denoted "forest" and "wood(land)" ( cf. the English ''sylva'' and ''sylvan''; the Italian, Spanish, and Portuguese ''selva''; the Romanian ''silvă''; the Old French ''selve''). Cognates of forest in Romance languages—e.g., the Italian ''foresta'', Spanish and Portuguese ''floresta'', etc.—are all ultimately derivations of the French word.

The precise origin of Medieval Latin is obscure. Some authorities claim the word derives from the Late Latin phrase ''forestam silvam'', denoting "the outer wood"; others claim the word is a latinisation of the Frankish *''forhist'', denoting "forest, wooded country", and was assimilated to ''forestam silvam'', pursuant to the common practice of Frankish scribes. The Old High German ''forst'' denoting "forest"; Middle Low German ''vorst'' denoting "forest"; Old English ''fyrhþ'' denoting "forest, woodland, game preserve, hunting ground" (English '' frith''); and Old Norse ''fýri'', denoting "coniferous forest"; all of which derive from the Proto-Germanic *''furhísa-'', *''furhíþija-'', denoting "a fir-wood, ''coniferous'' forest", from the Proto-Indo-European *''perk^wu-'', denoting "a ''coniferous'' or mountain forest, wooded height" all attest to the Frankish *''forhist''.

Uses of ''forest'' in English to denote any uninhabited and unenclosed area are presently considered archaic. The Norman rulers of England introduced the word as a legal term, as seen in Latin texts such as the ''Magna Carta'', to denote uncultivated land that was legally designated for hunting by feudal nobility (see Royal Forest).

These hunting forests did not necessarily contain any trees. Because that often included significant areas of woodland, "forest" eventually came to connote woodland in general, regardless of tree density. By the beginning of the fourteenth century, English texts used the word in all three of its senses: common, legal, and archaic. Other English words used to denote "an area with a high density of trees" are ''firth'', ''frith'', ''holt'', ''weald'', ''wold'', ''wood'', and ''woodland''. Unlike ''forest'', these are all derived from Old English and were not borrowed from another language. Some present classifications reserve ''woodland'' for denoting a locale with more open space between trees, and distinguish kinds of woodlands as ''open forests'' and ''closed forests'', premised on their crown covers. Finally, ''sylva'' (plural ''sylvae'' or, less classically, ''sylvas'') is a peculiar English spelling of the Latin ''silva'', denoting a "woodland", and has precedent in English, including its plural forms. While its use as a synonym of ''forest'', and as a Latinate word denoting a woodland, may be admitted; in a specific technical sense it is restricted to denoting the ''species'' of trees that comprise the woodlands of a region, as in its sense in the subject of silviculture. The resorting to ''sylva'' in English indicates more precisely the denotation that the use of ''forest'' intends.

Evolutionary history

The first known forests on Earth arose in the Late Devonian (approximately 380 million years ago), with the evolution of ''Archaeopteris'', which was a plant that was both tree-like and fern-like, growing to in height. It quickly spread throughout the world, from the equator to subpolar latitudes; and it formed the first forest by being the first species known to cast shade due to its fronds and by forming soil from its roots. ''Archaeopteris'' was deciduous, dropping its fronds onto the forest floor, the shade, soil, and forest duff from the dropped fronds creating the first forest. The shed organic matter altered the freshwater environment, slowing its flow and providing food. This promoted freshwater fish.

Ecology

Forests account for 75% of the gross primary productivity of the Earth's biosphere, and contain 80% of the Earth's plant biomass.

The world's forests contain about 606 gigatonnes of living biomass (above- and below-ground) and 59 gigatonnes of dead wood. The total biomass has decreased slightly since 1990, but biomass per unit area has increased.

Forest ecosystems can be found in all regions capable of sustaining tree growth, at altitudes up to the tree line, except where natural fire frequency or other disturbance is too high, or where the environment has been altered by human activity.

The latitudes 10° north and south of the equator are mostly covered in tropical rainforest, and the latitudes between 53°N and 67°N have boreal forest. 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.

Forests sometimes contain many tree species within a small area (as in tropical rainforests 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.

The biodiversity of forests varies considerably according to factors such as forest type, geography, climate, and soils – in addition to human use. Most forest habitats in temperate regions support relatively few animal and plant species, and species that tend to have large geographical distributions, while the montane forests of Africa, South America, Southeast Asia, and lowland forests of Australia, coastal Brazil, the Caribbean islands, Central America, and insular Southeast Asia have many species with small geographical distributions. Areas with dense human populations and intense agricultural land use, such as Europe, parts of Bangladesh, China, India, and North America, are less intact in terms of their biodiversity. Northern Africa, southern Australia, coastal Brazil, Madagascar, and South Africa are also identified as areas with striking losses in biodiversity intactness.

Components

A forest consists of many components that can be broadly divided into two categories: biotic (living) and abiotic (non-living). The living parts include trees, shrubs, vines, grasses and other herbaceous (non-woody) plants, mosses, algae, fungi, insects, mammals, birds, reptiles, amphibians, and microorganisms living on the plants and animals and in the soil, connected by mycorrhizal networks.

Layers

A forest is made up of many layers. The main layers of all forest types are the forest floor, the understory, and the canopy. The emergent layer, above the canopy, exists in tropical rainforests. Each layer has a different set of plants and animals, depending upon the availability of sunlight, moisture, and food.
* Forest floor contains decomposing leaves, animal droppings, and dead trees. Decay on the forest floor forms new soil and provides nutrients to the plants. The forest floor supports ferns, grasses, mushroom, and tree seedlings.
* Understory is made up of bushes, shrubs, and young trees that are adapted to living in the shade of the canopy.
* Canopy is formed by the mass of intertwined branches, twigs, and leaves of mature trees. The crowns of the dominant trees receive most of the sunlight. This is the most productive part of the trees, where maximum food is produced. The canopy forms a shady, protective "umbrella" over the rest of the forest.
* Emergent layer exists in a tropical rain forest and is composed of a few scattered trees that tower over the canopy.

In botany and countries like Germany and Poland, a different classification of forest vegetation is often used: tree, shrub, herb, and moss layers (see stratification (vegetation)).

Types

Forests are classified differently and to different degrees of specificity. One such classification is in terms of the biomes in which they exist, combined with leaf longevity of the dominant species (whether they are evergreen or deciduous). Another distinction is whether the forests are composed predominantly of broadleaf trees, coniferous (needle-leaved) trees, or mixed.
* Boreal forests occupy the subarctic zone and are generally evergreen and coniferous.
* Temperate zones support both broadleaf deciduous forests (e.g., temperate deciduous forest) and evergreen coniferous forests (e.g., temperate coniferous forests and temperate rainforests). Warm temperate zones support broadleaf evergreen forests, including laurel forests.
* Tropical and subtropical forests include tropical and subtropical moist forests, tropical and subtropical dry forests, and tropical and subtropical coniferous forests.
* Forests are classified according to physiognomy based on their overall physical structure or developmental stage (e.g. old growth vs. second growth).
* Forests can also be classified more specifically based on the climate and the dominant tree species present, resulting in numerous different forest types (e.g., Ponderosa pine/Douglas fir forest).

The number of trees in the world, according to a 2015 estimate, is 3 trillion, of which 1.4 trillion are in the tropics or sub-tropics, 0.6 trillion in the temperate zones, and 0.7 trillion in the coniferous boreal forests. The 2015 estimate is about eight times higher than previous estimates, and is based on tree densities measured on over 400,000 plots. It remains subject to a wide margin of error, not least because the samples are mainly from Europe and North America.

Forests can also be classified according to the amount of human alteration. Old-growth forest contains mainly natural patterns of biodiversity in established seral patterns, and they contain mainly species native to the region and habitat. In contrast, secondary forest is forest regrowing following timber harvest and may contain species originally from other regions or habitats.

Different global forest classification systems have been proposed, but none has gained universal acceptance. UNEP- WCMC's forest category classification system is a simplification of other, more complex systems (e.g. UNESCO's forest and woodland 'subformations'). This system divides the world's forests into 26 major types, which reflect climatic zones as well as the principal types of trees. These 26 major types can be reclassified into 6 broader categories: temperate needleleaf, temperate broadleaf and mixed, tropical moist, tropical dry, sparse trees and parkland, and forest plantations. Each category is described in a separate section below.

Temperate needleleaf

Temperate needleleaf forests mostly occupy the higher latitudes of the Northern Hemisphere, as well as some warm temperate areas, especially on nutrient-poor or otherwise unfavourable soils. These forests are composed entirely, or nearly so, of coniferous species ( Coniferophyta). In the Northern Hemisphere, pines ''Pinus'', spruces ''Picea'', larches ''Larix'', firs ''Abies'', Douglas firs ''Pseudotsuga'', and hemlocks '' Tsuga'' make up the canopy; but other taxa are also important. In the Southern Hemisphere, most coniferous trees (members of Araucariaceae and Podocarpaceae) occur mixed with broadleaf species, and are classed as broadleaf-and-mixed forests.

Temperate broadleaf and mixed

Temperate broadleaf and mixed forests include a substantial component of trees of the Anthophyta group. They are generally characteristic of the warmer temperate latitudes, but extend to cool temperate ones, particularly in the southern hemisphere. They include such forest types as the mixed deciduous forests of the United States and their counterparts in China and Japan; the broadleaf evergreen rainforests of Japan, Chile, and Tasmania; the sclerophyllous forests of Australia, central Chile, the Mediterranean, and California; and the southern beech Nothofagus forests of Chile and New Zealand.

Tropical moist

There are many different types of tropical moist forests, with lowland evergreen broad-leaf tropical rainforests: for example várzea and igapó forests and the terra firme forests of the Amazon Basin; the peat swamp forests; dipterocarp forests of Southeast Asia; and the high forests of the Congo Basin. Seasonal tropical forests, perhaps the best description for the colloquial term " jungle", typically range from the rainforest zone 10 degrees north or south of the equator, to the Tropic of Cancer and Tropic of Capricorn. Forests located on mountains are also included in this category, divided largely into upper and lower montane formations, on the basis of the variation of physiognomy corresponding to changes in altitude.

Tropical dry

Tropical dry forests are characteristic of areas in the tropics affected by seasonal drought. The seasonality of rainfall is usually reflected in the deciduousness of the forest canopy, with most trees being leafless for several months of the year. Under some conditions, such as less fertile soils or less predictable drought regimes, the proportion of evergreen species increases and the forests are characterised as " sclerophyllous". Thorn forest, a dense forest of low stature with a high frequency of thorny or spiny species, is found where drought is prolonged, and especially where grazing animals are plentiful. On very poor soils, and especially where fire or herbivory are recurrent phenomena, savannas develop.

Sparse trees and savanna

Sparse trees and savanna are forests with sparse tree- canopy cover. They occur principally in areas of transition from forested to non-forested landscapes. The two major zones in which these ecosystems occur are in the boreal region and in the seasonally dry tropics. At high latitudes, north of the main zone of boreal forestland, growing conditions are not adequate to maintain a continuously closed forest cover, so tree cover is both sparse and discontinuous. This vegetation is variously called open taiga, open lichen

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