Tundra

Tundra is treeless vegetation found at high elevation in mountains and in many landscapes of the Arctic. Collectively, ecosystems with tundra vegetation are grouped into the tundra biome. A distinction is usually made between Arctic tundra, which exists beyond the northern limit of tree growth, and alpine tundra, which exists above the elevational limit of tree growth in mountains. In mountains of the far north this distinction becomes blurred as tree lines descend to meet the northern limits of trees. Tundra is dominated by low-growing, perennial angiosperms and by mosses and lichens. Larger plants include grasses, sedges, herbs, and dwarf shrubs, but it is the lack of trees that most characterizes tundra. Plant species commonly found in tundra include sedges; grasses, including many of the genus Poa; willows, blueberries, dwarf birch, and other deciduous and evergreen shrubs or other low-growing woody plants; a host of herbaceous perennials, including many species of the genus Saxifragra, many members of the buttercup family, and several members of the rose family.

Climates of tundra regions are generally cold, and temperatures are commonly below freezing for much of the year, limiting the period of plant growth to a briefly thawed period during summer. Annual precipitation in tundra regions includes snow, although the amount of snow and total precipitation varies tremendously among different areas of tundra. Tundra regions are snow-covered much of the year, but the depth and duration of snow cover differs between locations. The tundra of the Sierra Nevada of California is characterized by heavy winter snow and little summer precipitation, while tundra in the Rocky Mountains generally has less snow but dependable summer rains. Precipitation in tundra regions of the Arctic is generally extremely low, often less than that found in many desert regions, but most soils nonetheless remain moist and may be waterlogged.

It may seem paradoxical that Arctic regions may have less precipitation than many deserts, yet they are covered by moist or wet tundra with numerous ponds and lakes. Low temperatures explain this apparent contradiction, limiting effects of evaporation and contributing to the formation of frost. Soils and materials beneath tundra are considered to be in a permafrost (perennially frozen) condition when they remain frozen for periods of two or more years. Permafrost is a condition generally characteristic of Arctic tundra soils but is also descriptive of isolated portions of the soils of alpine tundra regions. The top of the permafrost layer occurs a few inches to several feet below the surface and can extend downward for many feet. Permafrost soils drain poorly since the frozen soil is as impermeable as rock, and because much of the Arctic landscape is flat. A small amount of precipitation in the Arctic may be held in the thawed soil near the tundra surface, creating a moist landscape dotted with ponds and lakes. The flat Arctic plain is also patterned with irregularly shaped ridges and depressions, called polygons.

Soils of tundra regions are slow to develop due largely to low temperatures and limited periods of thaw. A variety of soil types may develop with time, depending primarily upon moisture or the degree of saturation. Wet tundra, with poor aeration and slow decomposition of plant roots, mosses, and other organic matter, produces highly organic soils, while well-drained tundra is characterized by mineral soils. The soil supply of nitrogen, phosphorus, or other elements needed by plants is often low and limits plant growth, and thus tundra regions generally support fewer animals than grasslands and other biomes. Despite low net primary productivity (plant growth potentially available to grazing animals), tundra regions support a variety of mammals, birds, and insects. Arctic tundra herbivores (plant eaters) include caribou, musk ox, lemmings, insects, hares, ground squirrels, and ptarmigan. Carnivores (eaters of insects or other animals) include many birds, especially waterfowl and shorebirds (that appear only in summer), snowy owls, jaegers, and ravens. Other important Arctic carnivores include Arctic fox, wolves, brown bears, and mosquitoes. The fauna of alpine tundra is variable but commonly includes various species of mountain sheep and/or mountain goats, voles and other rodents, bears, eagles, insects, and a variety of animals characteristic of the adjacent forests that sporadically use tundra habitats.

Within both Arctic and alpine regions exist extremes of vegetation considered atypical of tundra. Many areas within the Arctic are true desert; vegetation scientists classify these areas as polar desert. Tropical high mountains exhibit treeless zones at high elevations, sharing many similarities with the alpine tundra of temperate regions, but the lack of seasons, the large diurnal temperature variations, and the presence of distinct plant growth forms are clear differences. Tropical alpine vegetation commonly includes one plant life form not found in other tundra regions: the tall columnar rosette. In the tropical alpine of Africa this life form is represented by members of the genus Lobelia, in the Andes of South America by members of the genus Espeletia, and in high mountains of Java it is represented by tree ferns. The absence of this life form in Arctic and temperate alpine tundra probably reflects the importance of wind in shaping plants of these ecosystems. Floristic similarities between tropical alpine and temperate alpine or even Arctic tundra regions include genera or species of mosses, lichens, and occasionally vascular plants that are held in common.

During glacial periods of the past, much of the area now covered by the Bering Sea was a land mass. This Bering land bridge, connecting North America and Eurasia, allowed plant and animal populations to migrate between Northern Hemisphere continents of Eurasia and North America. Today many tundra genera, including both plants and animals, have circumpolar distributions (surrounding the northern parts of the world). Since species migrations were made possible during the cold periods of the Pleistocene with lowered sea levels due to great masses of ice on land, tundra-like vegetation formed over great expanses in the Northern Hemisphere. Today isolated alpine regions show remarkable similarities in flora and fauna to those of the Arctic echoing a common tundra heritage.