The term "tundra" has many definitions derived from geographic location, climate, soils and vegetation composition. Several definitions refer to tundra as areas with low growing vegetation that persists over permafrost, while others suggest it includes areas with mean annual air temperatures below 0 °C. For this website, we consider tundra and alpine to be treeless areas of land beyond the climatic limit for growth of trees. Tundra is land that is treeless due to its location within or just below the Arctic Circle and alpine is land that is treeless due to high altitudes. In alpine systems it is important to note that the climatic tree line varies strongly with latitude (i.e., the closer to the equator one goes, the higher in elevation the tree line reaches).