Forest In 4k

[Vennie Fireman]

A forest is an ecosystem characterized by a dense community of trees.[1] 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.[2][3][4] 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."[5] Using this definition, Global Forest Resources Assessment 2020 (FRA 2020) found that forests covered 4.06 billion hectares (10.0 billion acres; 40.6 million square kilometres; 15.7 million square miles), or approximately 31 percent of the world's land area in 2020.[6]

Forests are the largest terrestrial ecosystems of Earth by area, and are found around the globe.[7] 45 percent of forest land is in the tropical latitudes. The next largest share of forests are found in subarctic climates, followed by temperate, and subtropical zones.[8]

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.[7]

Forests form distinctly different biomes at different latitudes and elevations, and with different precipitation and evapotranspiration rates.[9] These biomes include boreal forests in subarctic climates, tropical moist forests and tropical dry forests around the Equator, and temperate forests at the middle latitudes. Forests form in areas of the Earth with high rainfall, while drier conditions produce a transition to savanna. However, in areas with intermediate rainfall levels, forest transitions to savanna rapidly when the percentage of land that is covered by trees drops below 40 to 45 percent.[10] Research conducted in the Amazon rainforest shows that trees can alter rainfall rates across a region, releasing water from their leaves in anticipation of seasonal rains to trigger the wet season early. Because of this, seasonal rainfall in the Amazon begins two to three months earlier than the climate would otherwise allow.[11][12] Deforestation in the Amazon and anthropogenic climate change hold the potential to interfere with this process, causing the forest to pass a threshold where it transitions into savanna.[13]

Human society and forests can affect one another positively or negatively.[17] 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.[18]

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.[4] 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,[19] or was legally designated as a forest regardless of vegetation type.[20][21]

There are three broad categories of definitions of forest in use: administrative, land use, and land cover.[20] Administrative definitions are legal designations, and may not reflect the type of vegetation that grows upon the land; an area can be legally designated "forest" even if no trees grow on it.[20] Land-use definitions are based on the primary purpose the land is used for. Under a land-use definition, any area used primarily for harvesting timber, including areas that have been cleared by harvesting, disease, fire, or for the construction of roads and infrastructure, are still defined as forests, even if they contain no trees. Land-cover definitions define forests based upon the density of trees, area of tree canopy cover, or area of the land occupied by the cross-section of tree trunks (basal area) meeting a particular threshold.[20] This type of definition depends upon the presence of trees sufficient to meet the threshold, or at least of immature trees that are expected to meet the threshold once they mature.[20]

Under land-cover 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%,[22] 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%.[19]

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

Uses of forest in English to denote any uninhabited and unenclosed area are presently considered archaic.[26] The Norman rulers of England introduced the word as a legal term, as seen in Latin texts such as Magna Carta, to denote uncultivated land that was legally designated for hunting by feudal nobility (see Royal Forest).[26][27]

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.[citation needed] By the beginning of the fourteenth century, English texts used the word in all three of its senses: common, legal, and archaic.[26] 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.[28] 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.[29] The resorting to sylva in English indicates more precisely the denotation that the use of forest intends.

The first known forests on Earth arose in the Middle Devonian (approximately 390 million years ago), with the evolution of cladoxylopsid plants like Calamophyton.[30] Appeared in the Late Devonian, Archaeopteris was both tree-like and fern-like plant, growing to 20 metres (66 ft) in height or more.[31] It quickly spread throughout the world, from the equator to subpolar latitudes.[31] It is 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 early forest.[31] The shed organic matter altered the freshwater environment, slowing its flow and providing food. This promoted freshwater fish.[31]

Forests account for 75% of the gross primary productivity of the Earth's biosphere, and contain 80% of the Earth's plant biomass.[7] 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 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.[32]

Forest ecosystems broadly differ based on climate; latitudes 10 north and south of the equator are mostly covered in tropical rainforest, and the latitudes between 53N and 67N 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. The trees that form the principal structural and defining component of a forest may be of a great variety of species (as in tropical rainforests and temperate deciduous forests), or relatively few species over large areas (e.g., taiga and arid montane coniferous forests). The biodiversity of forests also encompasses shrubs, herbaceous plants, mosses, ferns, lichens, fungi, and a variety of animals.

Trees rising up to 35 meters (115 ft) in height add a vertical dimension to the area of land that can support plant and animal species, opening up numerous ecological niches for arboreal animal species, epiphytes, and various species that thrive under the regulated microclimate created under the canopy.[33] Forests have intricate three-dimensional structures that increase in complexity with lower levels of disturbance and greater variety of tree species.[34]

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.[36]

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