Biomes: definitions and determinants

Categories: Biomes; ecology; ecosystems

One who travels latitudinally from the equator to the Arctic will cross tropical forests, deserts, grasslands, temperate forests, coniferous forest, tundra, and ice fields. Those major types of natural vegetation at regional scales are called biomes. A biome occurs wherever a particular set of climatic and edaphic (soil-related) conditions prevail with similar physiognomy. For example, prairies and other grasslands in the North American Middle West and West form a biome of temperate grasslands, where moderately dry climate prevails. Tropical rain forests in the humid tropical areas of South and Central America, Africa, and Southeast Asia create a biome where rainfall is abundant and well-distributed through the year.

In general, biomes are delineated by both physiognomy and environment. There are six major physiognomic types on land: forest, grassland, woodland, shrubland, semidesert scrub, and desert. Each of the six types occurs in a wide range of environments. Therefore, more than one biome may be defined within each physiognomic type according to major differences in climate. Tropical forests, temperate deciduous forests, and coniferous forests are, for example, separate biomes, although forests dominate all of them. On the other hand, some biome types, such as the tundra, are dominated by a range of physiognomic types and are in one prevailing environmental region.

Classification of Biomes

There are many ways to classify biomes. One system, which designates a small number of broadly defined biomes, divides global vegetation into nine major terrestrial biomes: tundra, taiga, temperate forest, temperate rain forest, tropical rain forest, savanna, temperate grasslands, chaparral, and desert. Other systems more narrowly define biomes, designating a larger total number. In those cases, some of the broadly defined biomes are divided into two or more biomes. For example, the biome called temperate forest in a broad classification may be separated into temperature deciduous forest and temperate evergreen forest in a fine classification. The biome of desert in the broad classification may be broken into warm semidesert, cool semidesert, Arctic-alpine semidesert, Arctic-pine desert, and true desert in the fine classification.

Description of Biome Distributions

Naturalists, geographers, and ecologists have tried to correlate world major types of biomes to climatic patterns in both descriptive and quantitative approaches. For example, in northern North America, the tundra and boreal forests are two broad belts of vegetation that stretch from east to west. The distribution of the two biomes is primarily influenced by temperature. South of those two belts are biome types that are mostly controlled by precipitation and evaporation. From east to west in North America, available moisture decreases, influencing biome distribution. Humid regions along the East Coast support forest biomes, including temperate coniferous forests and temperate deciduous forest. West of the eastern forests is a biome type of grasslands, including tall-grass prairie and short-grass steppe. In this zone, there is less precipitation than evaporation. The ratio of precipitation to evaporation is about 0.6 to 0.8 in the land that supports a tall-grass prairie and 0.2 to 0.4 farther west, where a short-grass steppe is supported. Beyond the short-grass steppe are shrubland and the deserts of the West. Western North America is a mountainous country in which vegetation zones reflect climatic changes on an altitudinal gradient. The vegetation in the lowlands is characteristic of the regions (short-grass steppe on the east side of Rocky Mountains, sagebrush cold semideserts in the Great Basin between the Rocky Mountains and the Sierra Nevada, and grasslands in California’s Central Valley west of the Sierra Nevada). Above the base regions, the vegetation changes from shrub, woodland, or deciduous forest to montane coniferous forest or alpine tundra. In Central America, from Mexico to Panama where precipitation becomes ample and temperatures are high, tropical rain forests and tropical seasonal forests occur.

Similar distributions of biomes along latitude and altitude can be found in South America, Africa, and Eurasia. In general, the climate-induced patterns of vegetation are influenced by latitude; the location of regions within a continent, which affects the amount of moisture they receive; and altitude, in which mountains modify the climate patterns. In addition, other factors, such as fire and human disturbance, may influence distributions of biomes. For example, most grasslands require periodic fires for maintenance, renewal, and elimination of incoming woody growth. Grasslands at one time covered about 42 percent of the land surface of the world. Humans have converted much of that area into croplands.

Quantitative Relationships

Descriptive relationships can provide pictures of world vegetation distributions along latitudinal and altitudinal gradients of temperature and moisture. Ecologists in the past several decades have also sought quantitative relationships between distributions of biomes and environmental factors. For example, when R. H. Whittaker plotted various types of biomes on gradients of mean annual temperature and mean annual precipitation in 1975, a global pattern emerged relating biomes to climatic variables. It was shown that tropical rain-forest biomes are distributed in regions with annual mean precipitation of 2,500 to 4,500 millimeters and annual mean temperatures of 20 to 30 degrees Celsius. Tropical seasonal forest and savannas also occur in warm regions with precipitation of 1,500-2,500 millimeters and 500-1,500 millimeters per year, respectively. Temperate forests occupy regions with annual temperature of 5 to 20 degrees Celsius and precipitation exceeding 1,000 millimeters per year. This thermal zone can support temperate rain forest when annual precipitation is more than 2,500 millimeters and temperate grassland when annual precipitation is below 750 millimeters. Temperate woodland occurs between temperate forests and grasslands. Tundra and taiga are distributed in regions with an annual mean temperature below 3 degrees Celsius, whereas deserts occupy areas with annual precipitation below 250 millimeters.

These relationships between climatic variables and biomes provide a reasonable approximation of global vegetation patterns. Many types of biomes intergrade with one another. Soil, exposure to fire, and regional climate can influence distributions of biomes in a given area.

Bibliography

Archibold, O. W. Ecology of World Vegetation. London: Chapman & Hall, 1995. An advanced book of world major biomes.

Smith, R. L., and T. M. Smith. Ecology and Field Biology . 6th ed. San Francisco: Benjamin Cummings, 2001. An introductory ecology book with general descriptions and great illustrations of biomes.

Whittaker, R. H. Communities and Ecosystems . 2d ed. New York: Macmillan, 1975. A comprehensive book on community and ecosystem ecology with excellent discussions on biomes.

Biomes and Their Features