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In physical geography, a tundra is a biome where tree growth is hindered by frigid temperatures and short growing seasons. Tundra vegetation is typically dominated by dwarf shrubs, sedges, grasses, mosses, and lichens, with scattered trees in some regions. The boundary between tundra and forest is called the tree line (timberline). Tundra soils are nutrient-rich (notably in nitrogen and phosphorus) and often contain large stores of biomass in permafrost, where carbon is locked away; this makes tundra soils a carbon sink. Tundra occurs in three main regions: Arctic, Alpine, and Antarctic. Arctic tundra lies in the far Northern Hemisphere north of the taiga belt and is characterized by permanently frozen subsoil (permafrost). It has a stark, cold environment with a short summer when the surface layer thaws, creating soggy ground and many marshes, lakes, bogs, and streams; winters are very cold, dark, and windy. Antarctic tundra occurs on Antarctica and nearby subantarctic islands, where most of the continent is too cold and dry for vegetation; where plants do occur, they are mainly lichens, mosses, liverworts, algae, and a small number of flowering plants on the Antarctic Peninsula. Alpine tundra occurs at high elevations worldwide, where cold climate and mountain soils prevent tree growth; it is generally better drained than arctic soils and transitions to subalpine forests below the tree line. Tundra climates commonly fit the Köppen ET category (tundra climate), with at least one month warm enough to melt snow but no month averaging above 10°C. Despite potential variation in precipitation and seasonality, rainfall and snowfall are generally slight due to low atmospheric water vapor, while evaporation is also extremely low—supporting wet, swampy terrain. A major concern is climate change: warming causes permafrost thaw, which can alter species survival, damage infrastructure, increase wildfire risk, and release greenhouse gases from previously frozen carbon, creating a positive feedback to further warming.
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