A forest, also referred to as a wood or the woods, is an area witha high density of trees. As with cities, depending on variouscultural definitions, what is considered a forest may varysignificantly in size and have different classifications accordingto how and of what the forest is composed.[1] A forest is usuallyan area filled with trees but any tall densely packed area ofvegetation may be considered a forest, even underwater vegetationsuch as kelp forests, or non-vegetation such as fungi,[2] andbacteria. Tree forests cover approximately 9.4 percent of theEarth's surface (or 30 percent of total land area), though theyonce covered much more (about 50 percent of total land area). Theyfunction as habitats for organisms, hydrologic flow modulators, andsoil conservers, constituting one of the most important aspects ofthe biosphere. A typical tree forest is composed of the overstory(canopy or upper tree layer) and the understory. The understory isfurther subdivided into the shrub layer, herb layer, and also themoss layer and soil microbes. In some complex forests, there isalso a well-defined lower tree layer. Forests are central to allhuman life because they provide a diverse range of resources: theystore carbon, aid in regulating the planetary climate, purify waterand mitigate natural hazards such as floods. Forests also containroughly 90 percent of the world's terrestrial biodiversity.[3]