Forest

A conifer forest in the Swiss Alps (National Park)

 A forest, also referred to as a wood or the woods and less often as a "wold" (or "weald"), "holt", or "frith" (or "firth"), is an area with a high density of trees. There are many definitions of "forest" based on various criteria. These plant communities cover approximately 9.4% 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 biosphere. Although forests are classified primarily by trees, the concept of a forest ecosystem includes additional species (such as smaller plants, fungi , bacteria, and animals) as well as physical and chemical processes such as energy flow and nutrient cycling.

Mixed deciduous forest in Stara Planina, Serbia

A typical forest is composed of the overstory (or upper tree layer of the canopy) and the understory. The understory is further subdivided into the shrub layer, herb layer, and sometimes also a moss layer. In some complex forests, there is also a well-defined lower tree layer. Forests are central to all human life because they provide a diverse range of resources, they store carbon, aid in regulating our climate, purify water and mitigate natural hazards such as floods. Forests also contain roughly 90% of the world terrestrial biodiversity.

Yakushima forest