Forest

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.

[12][13] 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.

Deforestation occurs when humans remove trees from a forested area by cutting or burning, either to harvest timber or to make way for farming.

The remaining 20 percent is located in more than 34 million patches around the world – the vast majority less than 1,000 hectares (2,500 acres) in size.

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.

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.

[citation needed] By the beginning of the fourteenth century, English texts used the word in all three of its senses: common, legal, and archaic.

[27] Other English words used to denote "an area with a high density of trees" are firth, frith, holt, weald, wold, wood, and woodland.

The first known forests on Earth arose in the Middle Devonian (approximately 390 million years ago), with the evolution of cladoxylopsid plants like Calamophyton.

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 also encompasses shrubs, herbaceous plants, mosses, ferns, lichens, fungi, and a variety of animals.

[34] Forests have intricate three-dimensional structures that increase in complexity with lower levels of disturbance and greater variety of tree species.

[36] 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.

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).

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.

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.

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".

The open canopy allows sufficient light to reach the ground to support an unbroken herbaceous layer that consists primarily of grasses.

Commonly mono-specific, planted with even spacing between the trees, and intensively managed, these forests are generally important as habitat for native biodiversity.

[67] An assessment of European forests found early signs of carbon sink saturation, after decades of increasing strength.

[70] In India, approximately 22 percent of the population belongs to forest-dependent communities, which live in close proximity to forests and practice agroforestry as a principal part of their livelihood.

This change occurs through a few main pathways, including increase in commercial tree plantations, adoption of agroforestry techniques by small farmers, or spontaneous regeneration when former agricultural land is abandoned.

[84] According to the Special Report on Global Warming of 1.5 °C of the Intergovernmental Panel on Climate Change, to avoid temperature rise by more than 1.5 degrees above pre-industrial levels, there will need to be an increase in global forest cover equal to the land area of Canada (10 million square kilometres (3.9 million square miles)) by 2050.

In 2015, a study for Nature Climate Change showed that the trend has recently been reversed, leading to an "overall gain" in global biomass and forests.

On 7 September 2015, the FAO released a new study stating that over the last 25 years the global deforestation rate has decreased by 50% due to improved management of forests and greater government protection.

[91] 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 new clear-cuts, although some older ones grew to 110 square kilometres (42 sq mi) over several years.

[92][93] In the United States, most forests have historically been affected by humans to some degree, though in recent years improved forestry practices have helped regulate or moderate large-scale impacts.