Tundra is a generic term for a type of low-growing ecosystem found in climatically stressed environments. Latitudinal tundra occurs in the Arctic and Antarctic, environments with cool and short growing seasons. Altitudinal tundra occurs at the top of mountains, where the growing season is cool, although it can be long. A major environmental factor affecting tundra communities is the availability of moisture. Wet meadows are dominated by hydric sedges and grasses, while mesic sites are dominated by dwarf shrubs and herbaceous species and dry sites by cushion plants and lichens.