Tundra is a type of biome in the arctic and antarctic regions and on mountain tops that is mostly treeless due to the short growing seasons, low temperatures, low rainfall, typically strong winds, and the limiting of root growth by the permafrost. Biodiversity is low, and vegetation consists mostly of dwarf shrubs, grasses, sedges, mosses and lichens. The boundary (ecotone) between tundra and forests is referred to as the tree line or timberline.
Tundra soil is rich in nitrogen and phosphorus. It also contains vast amounts of biomass and decomposed biomass that has been stored in the permafrost in the form of methane and carbon dioxide, making the tundra soil an important carbon sink. As global warming heats the permafrost and causes its thawing, the permafrost carbon cycle accelerates and increasingly releases its stored greenhouse gases into the atmosphere, creating a feedback cycle that further increases global warming.