The study of the distribution of plants is the science of plant geography. The units of vegetation the plant geographers study vary in size from the basic community, to groups of communities and their environment— ecosystems—to assemblages of ecosystems with distinctive vegetation and growth forms that extend over large geographic areas— biomes. Although these three terms each delineate portions of vegetation that occupy space, the terms imply no size per se. A plant community can be the lichens and mosses growing on the bark of a single tree trunk, an acre woodlot, or the plant communities of Yellowstone National Park. So, too, the size of ecosystems varies, although in this case energy flow and nutrient cycling dynamics determine the boundaries of the ecosystem; that is, more reactions occur within the system than between systems. Biomes are the largest units and are identified on continental, hemispheric, or worldwide scale. They are composed of similar growth forms and occur in broadly similar environments. The following paragraphs describe some commonly recognized terrestrial biomes.
Arctic tundra occurs north of the tree line and principally north of the Arctic Circle in an area of low precipitation and little snow with cold average temperatures. Vegetation consists of small perennial herbs, low shrubs, creeping willows, and a variety of grasses, sedges, mosses and lichens.
Taiga is the Russian name for the coniferous forest that forms a wide belt between the tundra of the north and the temperate deciduous forest to the south. It is composed primarily of species of pine, spruce, and fir with an understory of ericaceous shrubs (heaths), mosses, and lichens. Over 65 percent is underlain by permafrost and thick peat deposits. Lakes and ponds are common and soils are poor.
Temperate deciduous forest is a mixture of broad‐leaved deciduous trees (such as beech, maples, and oaks) together with species of perennial herbs. Seasons are pronounced with precipitation distributed evenly throughout the year. In North America this forest extends from the Atlantic coast westward to about 100° longitude.
Temperate moist evergreen forest, sometimes called “temperate rainforest,” occurs in areas of heavy precipitation both north and south of the equator. In North America it is the forest of the northwestern Pacific Coast. Western hemlock, white cedar, coast redwood, spruce, and other trees attain large size with mosses and lichens festooning their branches and the whole forming a luxuriant, dense forest.
Temperate grasslands are the typical vegetation of the interior of continents where they cover thousands of acres. Although grasses are the dominant life form, trees occur along streams in riparian woodlands. Most of the highly fertile grasslands have been plowed and cropped and a majority of the remaining grasslands are managed as rangeland.
Warm deserts have hot summer temperatures with great diurnal temperature variations. Precipitation is slight and irregular. Shrubs, succulents, and annuals are common life forms in deserts worldwide. Productivity is low and limited by lack of moisture. Some deserts may have no rainfall for 10 to 15 years and plants survive on dew.
Cold deserts (and polar deserts) are dominated by shrubs. The growing season is condensed between cold winters and dry summers. In contrast to the grasslands, most of the photosynthate here goes to wood production rather than to digestible foods. Polar deserts lie closer to the pole in the high arctic and are colder and drier than the surrounding tundra and have only scattered patches of vegetation in protected spots.
Mediterranean (broad sclerophyll) scrub consists of shrubs and small trees with broad, hard, evergreen leaves called sclerophylls. Many of the shrubs produce secondary metabolites toxic to other plants, an example of allelopathy. Five areas occur worldwide on the southwestern coasts of continents in addition to the namesake area of the Mediterranean Basin. In California the type is called chaparral. All the areas have wet winters and warm, dry summers Tropical savannas are grasslands with scattered trees and with three growing seasons, warm and rainy, cool and dry, hot and dry. There is no cold season. They are highly productive and in Africa, for example, support large populations of grazing and browsing hoofed herbivores and large carnivores.
Tropical rainforests have little climatic variation—no seasons and no cold nor dry period–and are either tradewind type (with steady, almost daily rains) or equatorial (with frequent, heavy thunderstorms). Trees are broad‐leaved evergreens and are covered with lianas and epiphytes, which forms dense jungles. These are highly productive ecosystems with huge numbers of decomposers.