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Forest Land Use

Forests differ from other biomes on the planet because of the large number of trees and shrubs that are found there. The Food and Agriculture Organization of the United Nations (FAO) considers any area with greater than 10% tree canopy cover as a forest. According to this definition, forests cover approximately 44 million km2 (square kilometers) of Earth's surface, around 30% of the planet's land area, including a large range of forest types under different types of disturbance regimes. Three main forest types are found in the world: tropical forest, temperate forest, and boreal forest. They differ from each other in terms of tree species, physiology, and the architecture of individual trees. In general, trees are adapted to different climate regimes, but their growth is mostly limited by rainfall. The global spatial distribution of forests (composition and structure) is determined by latitude, altitude, and patterns of dispersion. Locally, forest types are determined by the microclimate, the underlying soils, and the relationship between tree species (e.g., competition).

Forest Types and Spatial Distribution

Tropical rain forests cover approximately 6% of the total surface of the planet (one fifth of the total forest area) and are located across the equatorial line. Tropical forests are considered the most diverse ecosystems in the world: They contain around one half of Earth's plant and animal species. Temperate forests are located in temperate regions of the Northern Hemisphere and are characterized by a combination of deciduous and evergreen trees. Temperate forests grow mostly in the northern latitudes, wherever there is sufficient water and it is not too cold to support the growth of large trees. Boreal forests (called taigas or swamp forests) are dominated by only a few evergreen species (e.g., spruce, fir, and larch), and they occur in a broad band across the northern regions of North America, Europe, and Asia. Boreal forests occupy regions with a cold climate and adequate moisture. They are also present across the mountainous regions of Western North America, all the way to Southern Mexico.

Forests can also be categorized according to the degree of disturbance. There are three types of forests, characterized by different disturbance regimes. Primary forests are composed mostly of native tree species, where there are no clear indications of human activities and the ecological processes (e.g., carbon sequestration, nutrient cycles) are not significantly disturbed. Modified natural forests show some presence of human activities without these forces changing the underlying function of the ecosystem. Forest plantations and seminatural forests are characterized by native tree species that have been planted or by assisted natural regeneration; they comprise very few species that have been mostly introduced and are evenly spaced.

Environmental Services

Forests are critical elements in the climate system and other global and local ecological processes, such as the carbon, oxygen, nitrogen, and water cycles. Forests influence the climate system largely by affecting the amount of carbon dioxide in the atmosphere. When forests grow, carbon is taken from the atmosphere and absorbed in wood, leaves, and soil. Because forests can absorb and store carbon over an extended period of time, they are considered “carbon sinks.” This carbon remains stored in the forest ecosystem but can be released into the atmosphere when forests are burned. Forests are critical to humans not only as providers of ecosystem services (e.g., provision of clean water and air) but also as a critical resource for human livelihoods since forests are also home to other organisms that provide important resources to humans.

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