A forest is an area with a high density of trees. There are many definitions of a forest, based on the various criteria. These plant communities cover approximately 9.4 per cent of the Earth's surface (or 30% of total land area), though they once covered much more (about 50% of total land area), in many different regions and function as habitats for organisms, hydrologic flow modulators, and soil conservers, constituting one of the most important aspects of the Earth's biosphere. Although a forest is classified primarily by trees a forest ecosystem is defined intrinsically with additional species such as fungi. A woodland, with more open space between trees, is ecologically distinct from a forest.