Home > Environment > Major Ecosystems of the World > MAJOR TERRESTRIAL BIOMES

MAJOR TERRESTRIAL BIOMES

A biome is a large, relatively distinct ecosystem that is characterized by particular climate, soil, plants, and animals, regardless of where it occurs on Earth. Examples of biomes include deserts, tropical rain forests, and tundra. A biome's boundaries are determined by climate more than any other factor. Because the northernmost biome, the tundra, is colder and has shorter growing sea­sons, for example, it has fewer kinds of vegetation than warmer biomes; few plants can tolerate its extreme conditions. Moving from the poles toward the equator, precipitation becomes a very important climatic factor, producing the temperate communities of forest, grassland, and desert, in decreasing order of precipitation. Thus, a characteristic biome develops in each major kind of climate.

Tundra: Cold Boggy Plains of the Far North

The tundra occurs in the extreme northern lati­tudes wherever the snow melts seasonally. (The Southern Hemisphere has no equivalent of the arctic tundra because it has no land in the corresponding latitudes.) Tundra is exposed to long, harsh winters and very short summers. Although its growing season, with warmer tempera­tures, is short (from 50 to 160 days depending on location), the days are long. In many places the sun does not set at all for a considerable number of days in midsummer, although the amount of light at midnight is one-tenth that at noon. Over much of the tundra, little precipitation (10 to 25 cm [4 to 10 inches] per year) occurs, with most of it falling dur­ing the summer months.

Tundra soils tend to be geologically young, most of them having been formed since the last ice age. They are usually nutrient-poor and have little organic litter. Although the soil's surface melts dur­ing the summer, tundra has a layer of permanently frozen ground called permafrost, varying in depth and thickness, that interferes with drainage and prevents the roots of larger plants from becoming established. The limited precipitation in combina­tion with low temperatures, flat topography (sur­face features), and the permafrost layer produces a landscape of broad shallow lakes, sluggish streams, and bogs.

The few species found in the tundra tend to exist in great numbers. Tundra is dominated by mosses, lichens (such as reindeer moss), grasses, and grasslike sedges. There are no readily recogniz­able trees or shrubs except in very sheltered locali­ties, although dwarf willows and other dwarf trees are common—tundra plants seldom grow taller than ,30 cm (12 inches).

The year-round animal life of the tundra in­cludes weasels, arctic foxes, snowshoe hares, ptarmigan, snowy owls, and hawks. In the summer large herbivores such as musk-oxen and caribou migrate north to the tundra to graze on sedges, grasses, and dwarf willow. There are no reptiles or amphibians. Insects such as mosquitoes, blackflies, and deerflies survive the winter as eggs or pupae and appear in great numbers during summer weeks. Tundra regenerates very slowly after it has been disturbed. Even casual use by hikers can injure it. Damage that is likely to persist for hundreds of years has been done to large portions of the arctic tundra by oil exploration and military use.

Taiga: Evergreen Forests of the North

Just south of the tundra is the taiga, or boreal for­est, which stretches across North America and Eur­asia, covering approximately 11 percent of the Earth's land (A biome comparable to the taiga is not found in the Southern Hemi­sphere.) Winters are extremely cold and severe, al­though not as harsh as in the tundra. The growing season of the boreal forest is somewhat longer than that of the tundra.

Deciduous trees such as aspen and birch, which shed their leaves in autumn, may form striking stands in the taiga, bur overall, spruce, fir, and other conifers (cone-bearing evergreens) clearly dominate. Conifers have many drought-resistant adaptations, such as needlelike leaves with minimal surface area for water loss that enable them to withstand the "drought" of the northern winter months (plant roots cannot absorb water when tta ground is frozen).

The animal life of the boreal forest consists of some larger species such as caribou (which migrate from the tundra to the taiga in winter), wolves, bears, and moose. However, most of the animal life is medium-sized to small, including rodents, rabbits, and fur-bearing predators such as lynx, sable, and mink. Most species of birds are abundant in the summer but migrate to warmer climates in the winter. Insects are abundant, but there are few amphib­ians and reptiles except in the southern taiga.

Most of the taiga is not well suited to agricul­ture because of its short growing season and min­eral-poor soil. However, the boreal forest yields vast quantities of lumber and pulpwood (for making paper products), plus furs and other forest products.

Temperate Forests: Various Kinds in Temperate Areas

In temperate latitudes, precipitation varies greatly with location (see Chapter 5). Continental interi­ors tend to be dry for a variety of reasons. Perma­nent high-pressure areas, such as those over the Sahara Desert, may nudge moist air masses aside. Air passing over a large land mass also may dry our without having the opportunity to be recharged with fresh moisture.

The climate of the North American continent, especially the West, is dominated by rain shadows cast by mountain ranges. As prevailing westerly winds push against the bases of the Cascade Range in the Pacific Northwest, masses of moist air from the Pacific Ocean are forced upward to higher alti­tudes, where they cool and precipitate much of their moisture. Thus, the western slopes of the mountains are so well watered that a temperate rain forest has developed. Considerable precipitation also falls in the upper reaches of the eastern slopes, but by the time the air has sunk back to lower alti­tudes, most of its available moisture has already been released.

Temperate Rain Forest

A coniferous temperate rain forest occurs on the northwest coast of North America; similar vegetation exists in southeastern Australia and in southern South America. Annual precipitation in this biome is high, from 200 to 380cm (80 to 152 inches), and is augmented by condensation of water from dense coastal fogs. The proximity of temperate rain forest to the coastline moderates the temperature so that it has only a nar­row seasonal fluctuation—winters are mild and summers are cool. Temperate rain forest has rela­tively nutrient-poor soil, although its organic con­tent may he high.

The dominant vegetation in the North Ameri­can temperate rain forest is large evergreen trees such as western hemlock, Douglas fir, Sitka spruce, and western arborvitae. Like tropical rain forest, temperate rain forest is rich in epiphytic vegeta­tion—smaller plants that grow nonparasitically on large trees. The epi­phytes in temperate rain forest are mainly mosses, club mosses, lichens, and ferns. Squirrels, deer, and numerous bird species are among the animals found in the temperate rain forest.

    Temperate rain forest is one of the richest wood producers in the world, supplying us with lumber and pulpwood. It is also one of the most complex ecosystems on Earth. Care must be taken to avoid over-harvesting the original (never logged) old-growth forest, however, because such an ecosystem takes hundreds of years to develop. The logging industry typically harvests old-growth forest and replants the area with trees of a single species that will be harvested in 100 years or less. Thus, the old-growth forest ecosystem never has h chance to redevelop.

decay, mineral ions arc released. If the ions are not immediately absorbed by the roots of the living trees, they leach into the clay, where they may be retained.

Temperate deciduous forest originally con­tained a variety of large mammals including puma, wolves, deer, bison, bears, and other species now extinct, plus many small mammals and birds. Both reptiles and amphibians abounded, together with a denser and more varied insect life than exists today. Much of the original temperate deciduous forest was removed by logging and land clearing. Where it has been allowed to regenerate, temperate decid-

Chaparral vegetation looks strikingly similar around the world, even though the individual spe­cies of different areas are quite distinct. Chaparral is usually dominated by a dense growth of evergreen shrubs but may contain drought-resistant pine or scrub oak trees. During the rainy sea­son the habitat may be lush and green, but during the hot, dry summer the plants lie dormant. Chap­arral trees and shrubs often have sclerophyllous

leaves hard, small, leathery leaves that resist water loss. Many plants are also specifically fire-adapted and actually grow best in the months fol­lowing a fire. When the aboveground parts of these plants burn, their minerals are released.

    Human population growth and industrializa­tion in tropical countries may spell the end of most or all tropical rain forests by the end of this century. Scientists think many rain forest organisms will become extinct before they have even been discovered

and scientifically described. The ecological impacts of tropical rain forest destruction are dis­cussed extensively throughout this text.

The Distribution of Vegetation on Mountains

Hiking up a mountain is similar to traveling toward the North Pole with respect to the major life zones encountered. This is because, as one climbs a mountain, the temperature drops, just as it does when one travels north, and the type of plants growing on the mountain change with the Temperature.

The base of the mountain in Colorado, for example, might be covered by deciduous trees, which shed their leaves every autumn. Above that altitude, where the climate is colder   and more severe, one might find a coniferous forest called subalpine forest, which resembles the northern taiga. Higher still, where the climate is very cold, a kind of tundra occurs, with vegetation composed of grasses, sedges, and small tufted plants; it is called alpine tundra to distinguish it from arctic tundra. At the very top of the moun­tain, a permanent ice or snow cap might be found, similar to the nearly lifeless polar land areas.

There are important environmental differences between high altitudes and high lati­tudes; however that affect the types of or­ganisms found in each place. Alpine tundra typically lacks permafrost and has more pre­cipitation than arctic tundra. Also, high elevations of Temperature Mountains do not have the extremes in day length, associated with the changing seasons that occur in high-latitude biomes. Furthermore, the intensity of solar radiation is greater at high elevations than at high latitudes. For example, at high elevations the sun's rays pass through less atmosphere, which result in a greater amount of ultraviolet (UV) radiation {less UV is filtered out by the atmosphere) than at high latitudes.