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A forest is a large area of land covered with trees or other woody vegetation. Hundreds of more precise definitions of forest are used throughout the world, incorporating factors such as tree density, tree height, land use, legal standing and ecological function. According to the widely-used United Nations Food and Agriculture Organization definition, forests covered an area of four billion hectares (15 million square miles) or approximately 30 percent of the world's land area in 2006.

Forests are the dominant terrestrial ecosystem on Earth, and are distributed across the globe. Forests account for 75% of the gross primary productivity of the Earth's biosphere, and contain 80% of the Earth's plant biomass.

Forests at different latitudes form distinctly different ecozones: boreal forests near the poles tend to consist of evergreens, while tropical forests near the equator tend to be distinct from the temperate forests at mid-latitude. The amount of precipitation and the elevation of the forest also affects forest composition.

Human society and forests influence each other in both positive and negative ways. Forests provide ecosystem services to humans, but also impose economic, environmental, health and aesthetic costs. Human interactions with the forest, including harvesting forest resources, affect the forest ecosystem.

Definition
Although forest is a term of common parlance, 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 forest definitions in use: administrative, land use, and land cover. Administrative definitions are based primarily upon the legal designations of land, and commonly bear no relationship to the vegetation growing on the land: land that is legally designated as a forest is defined as a forest even if no trees are growing on it. Land Use definitions are based upon the primary purpose that the land serves. For example, a forest may 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 within the region 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, and area of land only be defined as forest if it is growing trees. Areas that fail to meet the Land Cover definition may be still included under while immature trees are establishing if they 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, forests require very high levels of tree canopy cover, from 60% to 100%, excluding savannas and woodlands in which trees have a lower canopy cover. Other definitions consider savannas to be a type of forest, and include all areas with tree canopies over 10%.

Etymology


The word forest comes from Middle English, from Old French forest (also forès) "forest, vast expanse covered by trees"; first introduced in English as the word for wild land set aside for hunting without the necessity in definition for the existence of trees. Possibly a borrowing (probably via Frankish or Old High German) of the Medieval Latin word foresta "open wood", foresta was first used by Carolingian scribes in the Capitularies of Charlemagne to refer specifically to the king's royal hunting grounds. The term was not endemic to Romance languages (e.g. native words for "forest" in the Romance languages evolved out of the Latin word silva "forest, wood" (English sylvan); cf. Italian, Spanish, Portuguese selva; Romanian silvă; Old French selve); and cognates in Romance languages, such as Italian foresta, Spanish and Portuguese floresta, etc. are all ultimately borrowings of the French word.

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

Uses of the word "forest" in English to denote any uninhabited area of non-enclosure are now considered archaic. The word was introduced by the Norman rulers of England as a legal term (appearing in Latin texts like the Magna Carta) denoting an uncultivated area legally set aside for hunting by feudal nobility (see Royal Forest). These hunting forests were not necessarily wooded much, if at all. However, as hunting forests did often include considerable areas of woodland, the word "forest" eventually came to mean wooded land more generally. By the start of the fourteenth century the word appeared in English texts, indicating all three senses: the most common one, the legal term and the archaic usage.

Other terms used to mean "an area with a high density of trees" are wood, woodland, wold, weald, holt, frith and firth. Unlike forest, these are all derived from Old English and were not borrowed from another language. Some classifications now reserve the term woodland for an area with more open space between trees and distinguish among woodlands, open forests, and closed forests based on crown cover.

Forest distribution
The distribution of forests is strongly influenced by climate. This can be most clearly seen by the distribution of forest types by latitudinal, with major forests types having a distribution that clearly corresponds to climate as influenced by latitude. For example, evergreen broadleaf forests are primarily distributed in a band across the equator, while evergreen needleleaf forests are distributed in a band adjacent to the poles. This correspondence of major forest types with latitude reflects the adaptation of the plants within these forest types to the temperature and rainfall patterns experienced at different latitudes: for example the trees of tropical broadleaf forests are adapted to the year-round rainfall and temperatures of equatorial regions that permit a continuous growing season while the trees of the evergreen needleleaf forests have evolved to cope with the short growing season and long cold winters of the high latitudes.

While latitude is the controls the distribution of the major forest types, factors such as soil type, topography and disturbance control the distribution of forest types at a more local scale. Mountains play a major role in the local distribution of forest types by influencing the amount of rainfall through rain shadows, modifying temperature and altering wind circulation. Soil type also plays a major role in determining the distribution of forests on a local scale. Soils with limited water holding capacity will favour more drought resistant forest types, while high nutrient soils will favour denser tree growth.

Precipitation also plays a major role in controlling the distribution of forests. Trees require wetter soil then shrubs or grasses, and where precipitation is so low that the soil water potential cannot support tree growth forest are replaced by shrublands and grasslands. The strong correlation between forest type and climate has permitted the development of schemes that classify forest types, and other biomes, based upon climate. Such classification schemes include the Holdridge life zones system and the WWF ecological land classification system. Differences in the weighting given to various climate factors and the interaction between those factors results in different schemes producing different forest types. Such classification schemes predict the potential distribution of forest types based upon environmental variables however they do not represent the actual distribution of forest types, which is influenced by various disturbance factors that may impede or promote tree growth.

Forest disturbance
Many factors are disturb forest, including human activities such as logging and deforestation, fire and introduced plant and animal pests. Fires also experience natural disturbances such as fires, cyclones and natural pest outbreaks. According to a recent global assessment, more than 60% of the world’s 4 billion ha of forest are recovering from a past disturbance, and 3% of the world’s forests are disturbed annually by logging, fire, pests, or weather. Such disturbance can have a profound effect on forest distribution. For example, savannas commonly exist in areas that receive sufficient rainfall to permit tree growth, yet forest are prevented from establishing due to disturbance from fire and browsing animals. Direct human induced disturbance in the form of logging or conversion of forests to agricultural land can have a similarly dramatic impact.

Human forest interactions
Forests may provide a diversity of ecosystem services including recycling carbon dioxide into oxygen, acting as a carbon sink, aiding in regulating climate, purify water, mitigating natural hazards such as floods, and serving as a genetic reserve. Forests also serve as a source of lumber and as recreational areas.

It is often forgotten that forests do not only provide benefits, but can also incur large and significant costs to humans. Forests may impose a considerable economic burden, diminish the enjoyment of natural areas, cause a dramatic decline in the food producing capacity of grazing land and cultivated land, reduce biodiversity  remove available water for humans and wildlife,  harbour dangerous and destructive wildlife, and act as reservoirs of human and livestock disease.

Humans have attempted to control both the positive and negative effects of forests throughout history.

Current
Causes, rate and effects

Other disturbance
Pests, disease, fire, climate change

The management of forests is often referred to as forestry. Forest management has changed considerably over the last few centuries, with rapid changes from the 1980s onwards culminating in a practice now referred to as sustainable forest management. Forest ecologists concentrate on forest patterns and processes, usually with the aim of elucidating cause-and-effect relationships. Foresters who practice sustainable forest management focus on the integration of ecological, social, and economic values, often in consultation with local communities and other stakeholders.

Humans have generally decreased the amount of forest worldwide. Anthropogenic factors that can affect forests include logging, urban sprawl, human-caused forest fires, acid rain, invasive species, and the slash and burn practices of swidden agriculture or shifting cultivation. The loss and re-growth of forest leads to a distinction between two broad types of forest, primary or old-growth forest and secondary forest. There are also many natural factors that can cause changes in forests over time including forest fires, insects, diseases, weather, competition between species, etc. In 1997, the World Resources Institute recorded that only 20% of the world's original forests remained in large intact tracts of undisturbed forest. More than 75% of these intact forests lie in three countries—the Boreal forests of Russia and Canada and the rainforest of Brazil.

In 2010, the Food and Agriculture Organization of the United Nations reported that world deforestation, mainly the conversion of tropical forests to agricultural land, had decreased over the past ten years but still continues at a high rate in many countries. Globally, around 13 million hectares of forests were converted to other uses or lost through natural causes each year between 2000 and 2010 as compared to around 16 million hectares per year during the 1990s. The study covered 233 countries and areas. Brazil and Indonesia, which had the highest loss of forests in the 1990s, have significantly reduced their deforestation rates. China instituted a ban on logging, beginning in 1998, due to the erosion and flooding that it caused. In addition, ambitious tree planting programmes in countries such as China, India, the United States and Vietnam - combined with natural expansion of forests in some regions - have added more than seven million hectares of new forests annually. As a result the net loss of forest area was reduced to 5.2 million hectares per year between 2000 and 2010, down from 8.3 million hectares annually in the 1990s.

Smaller areas of woodland in cities may be managed as Urban forestry, sometimes within public parks. These are often created for human benefits; Attention Restoration Theory argues that spending time in nature reduces stress and improves health, while forest schools and kindergartens help young people to develop social as well as scientific skills in forests. These typically need to be close to where the children live, for practical logistics.

Canada
Canada has about 4020000 km2 of forest land. More than 90% of forest land is publicly owned and about 50% of the total forest area is allocated for harvesting. These allocated areas are managed using the principles of sustainable forest management, which includes extensive consultation with local stakeholders. About eight percent of Canada’s forest is legally protected from resource development. Much more forest land—about 40 percent of the total forest land base—is subject to varying degrees of protection through processes such as integrated land use planning or defined management areas such as certified forests.

By December 2006, over 1,237,000 square kilometers of forest land in Canada (about half the global total) had been certified as being sustainably managed. Clearcutting, first used in the latter half of the 20th century, is less expensive, but devastating to the environment, and companies are required by law to ensure that harvested areas are adequately regenerated. Most Canadian provinces have regulations limiting the size of clearcuts, although some older clearcuts can range upwards of 110 km2 in size which were cut over several years.

United States
In the United States, most forests have historically been affected by humans to some degree, though in recent years improved forestry practices has helped regulate or moderate large scale or severe impacts. However, the United States Forest Service estimates a net loss of about 2 million hectares (4,942,000 acres) between 1997 and 2020; this estimate includes conversion of forest land to other uses, including urban and suburban development, as well as afforestation and natural reversion of abandoned crop and pasture land to forest. However, in many areas of the United States, the area of forest is stable or increasing, particularly in many northern states. The opposite problem from flooding has plagued national forests, with loggers complaining that a lack of thinning and proper forest management has resulted in large forest fires.