The forest biome consists of close growing trees with leaf canopies that generally overlap. Much sunlight can be prevented from reaching the forest floor if tree limbs and foliage tightly intermingle. The lack of sunlight inhibits the development of undergrowth, creating an open forest structure. If the canopy is open, sunlight can reach the forest floor and promote the development of undergrowth resulting in a closed forest structure. Forests require ample amounts of annual precipitation to support their growth. The shaded conditions of the closed forest keep soils relatively moist.
Forests are found over a wide range of temperature regimes from the hot equatorial regions to the cold subarctic. Forests occupy approximately one third of Earth's land surface, but their areal extent is shrinking as humans cut the forest for material needs and economic gain.