In physical geography, tundra is an area where the tree growth is hindered by low temperatures and short growing seasons. There are three types of tundra: Arctic tundra, Antarctic tundra, and alpine tundra. In all of these types, the dominant vegetation is grasses, mosses, and lichens. Trees grow in some of the tundra. The ecotone (or ecological boundary region) between the tundra and the forest is known as the tree-line or timberline. The word "tundra" usually refers only to the areas where the subsoil is permafrost, or permanently frozen soil. The arctic tundra is a vast area of stark landscape, which is frozen for much of the year. The soil there is frozen from 25-90 cm (9.8-35.4 inches) down, and it is impossible for trees to grow. Instead, bare and sometimes rocky land can only support low growing plants such as moss, heath, and lichen. There are two main seasons, winter and summer, in the polar Tundra areas. The biodiversity of the tundras is low: 1,700 species of flora and only 48 land mammals can be found, although thousands of insects and birds migrate there each year for the marshes. There are few species with large populations. Notable animals in the arctic tundra include caribou (reindeer), musk ox, arctic hare, arctic fox, snowy owl, lemmings, and polar bears (only the extreme north).