A forest is an area with a high density of trees (or, historically, a wooded area set aside for hunting). These plant communities cover large areas of the globe and function as animal habitats, hydrologic flow modulators, and soil conservers, constituting one of the most important aspects of the Earth's biosphere. Although often thought of as as carbon dioxide sinks, mature forests are approximately carbon neutral with only disturbed and young forests acting as carbon sinks. Nonetheless mature forests do play an important role in the global carbon cycle as stable carbon pools, and clearance of forests leads to an increase of atmospheric carbon dioxide levels.