Biomes are groupings of ecologically similar organisms shaped by the environment in which they are found. The classification of biomes is most often and easily applied to terrestrial systems. Ecologists classify terrestrial biomes principally by the growth forms of their dominant plants, which reflect the evolution of those plants under annual patterns of temperature and precipitation.

The same classic types of biomes may be widely separated, occurring on different continents, depending in large part on the presence of suitable climatic conditions (Figure 53.13). For example, the desert biome occurs in such distant locations as Arizona in the North American Southwest and the Namib Desert in Africa; both locations are extremely dry and dominated by succulent plants such as cacti and by drought-tolerant shrubs and grasses. Why use plants as a way of classifying biomes? Because plants are immobile, they must adapt to environmental conditions in order to be successful over long periods of time. Thus *the growth forms of plants strongly reflect their environment in ways that can be compared around the world. Furthermore, by providing three-dimensional structure, by modifying physical conditions near the ground, and by providing food, dominant plants strongly influence the organisms living there.

1153

Question

The northern hemisphere has more biome types. This is likely a consequence of the northern hemisphere having much more land area than the southern hemisphere, creating the opportunity for a diversity of temperature and rainfall regimes.

In the following pages we briefly describe seven terrestrial biomes (a subset of those shown in Figure 53.13). For each, a plot of the seasonal patterns of temperature and precipitation—known as a Walter climate diagram—is included. Devised by the German biogeographers Heinrich Walter and Helmut Lieth, this graphic technique plots temperature and precipitation data in a simple way that visualizes a “growing season”—those months when average temperatures are above freezing (0°C) and when average precipitation is sufficient for plant growth. Walter climate diagrams are predicated on the “rule of thumb” that plant growth requires temperatures above 0°C and at least 2 mm of precipitation for each 1°C rise in temperature above 0°C. They have two y-axis scales, one for temperature and one for precipitation; these axes align 0 mm of precipitation with 0°C of temperature. The x axis shows 12 months, with the summer solstice placed in the center of the axis.

TROPICAL RAINFOREST The tropical rainforest biome is found in equatorial regions where rainfall and temperatures are high year round. With no season unsuitable for growth, it is the most productive and species-rich of all biomes. In some areas, there are up to 500 species of trees per square kilometer. Although historically these forests covered 12 percent of Earth’s surface, and are now reduced through deforestation, they are home to more than half of all known species. These forests are home to many epiphytes—plants that grow on other plants, deriving their nutrients and moisture from air and water rather than soil.

Tropical rainforests provide humans with a range of products, including fruits, nuts, medicines, fuel, pulp, and furniture wood. Rainforests, however, are currently being cut down or converted to agriculture at a rate of almost 1 percent per year. In some cases, rainforests are recovering but the soils are often nutrient-poor, hindering regrowth.

1154

DESERT The desert biome is concentrated in two belts, centered around 30°N and 30°S latitude (where warm, dry air sinks under high atmospheric pressure; see Figures 53.4 and 53.5). The driest of these regions, where rains rarely fall, are far from the oceans, as in the center of Australia and the middle of the Sahara in Africa.

Desert plants have several structural and physiological adaptations that help them conserve water, as described in Key Concept 38.3. Small desert animals are inactive during the hottest part of the day, remaining in underground burrows. Desert mammals have physiological adaptations for conserving water, including a reduced number of sweat glands and kidneys that produce highly concentrated urine. Many desert animals require no water beyond what they can extract from the carbohydrates in their food.

Humans have used deserts for livestock grazing and agriculture for centuries. Deserts can be irrigated from deep wells or distant mountains, but such efforts typically fail as a result of salinization, the buildup of salts from the evaporation of irrigation water.

TEMPERATE GRASSLAND Temperate grasslands are found in many parts of the world, all of which are relatively dry for much of the year. Most grasslands, such as the pampas of Argentina, the veldt of South Africa, and the Great Plains of North America, have hot summers and relatively cold winters. In some grasslands, most of the precipitation falls in winter (as in California grasslands); in others, the majority falls in summer (as in the Great Plains and the Russian steppe).

Grassland vegetation is structurally simple but rich in species of perennial grasses and forbs (herbaceous plants). Grassland plants support herds of large grazing mammals and are adapted to grazing and to fire. They store much of their energy underground and resprout quickly after being burned or grazed. There are comparatively few trees in temperate grasslands because trees cannot survive the periodic fires or the dry conditions.

1155

The topsoil of grasslands is usually rich and deep, and thus exceptionally well suited to growing crops such as corn and wheat. As a consequence, most of the world’s temperate grasslands have been turned over to agriculture and no longer exist in their natural state.

TEMPERATE DECIDUOUS FOREST The temperate deciduous forest biome is found in eastern North America, eastern Asia, and Europe. Temperatures in these regions fluctuate dramatically between summer and winter, although precipitation is fairly evenly distributed throughout the year. The deciduous trees that dominate these forests lose their leaves during the cold winters and produce new leaves during the warm, moist summers.

The temperate deciduous forests have many more species than boreal forest ecosystems. Those with the highest species richness occur in the southern Appalachian Mountains of the United States and eastern China and Japan—areas that were not covered by glaciers during the Pleistocene. Many plant genera are shared among the three geographically separate regions where this biome is found.

Although many animals are permanent residents of deciduous forests, some (including many birds) migrate to find food resources and escape the winter cold. Others that remain through the winter hibernate (see Key Concept 39.5), often in underground burrows. Many insects pass the winter in a state of diapause (suspended development), the onset of which is triggered by the decreasing hours of daylight—a reliable predictor of winter.

1156

BOREAL FOREST AND TEMPERATE EVERGREEN FOREST The boreal forest biome (also known as taiga) occurs just above 50°N but below Arctic tundra, and at elevations below alpine tundra on temperate-zone mountains. Winters in the boreal forest are long and very cold; summers are short, although often relatively warm. The boreal forests of the Northern Hemisphere are dominated by coniferous trees such as spruces and firs. The short summers favor evergreen leaves, which are ready to photosynthesize as soon as temperatures warm. In winter, downward-drooping limbs allow the trees to shed snow easily.

The dominant mammals of the boreal forest, such as moose and hares, eat leaves, but the seeds in conifer cones support a variety of rodents, birds, and insects. Many small mammals hibernate in winter, but voles, lemmings, and mice remain active under the snowpack, serving as food for predators such as foxes and owls.

The temperate evergreen forest biome occurs along the coasts of continents in both hemispheres at middle to high latitudes, where winters are mild and wet and summers are cool and dry. In the Northern Hemisphere, the dominant trees in temperate evergreen forests are conifers, some of which are the world’s most massive tree species (including the giant sequoia and coast redwood). In the Southern Hemisphere, the dominant trees are southern beeches (Nothofagus), some of which are evergreen.

TUNDRA The tundra biome is found at high latitudes (above 65°) characterized by low temperatures and a short growing season. This biome is underlain by permafrost—soil permeated with permanently frozen water. The top few centimeters of the soil thaw during the short summers, when the sun may be above the horizon 24 hours a day. Thus even though there is little precipitation near the poles, the soil in lowland Arctic tundra is wet because water cannot drain through the permafrost. As a result, trees cannot grow in the tundra biome because their roots cannot penetrate the permafrost. Instead, the tundra biome is characterized by sedges, forbs, grasses, and low-growing shrubs such as heaths, willows, and birches. Lichens and mosses are also important vegetation.

Tundra plants have several structural and physiological adaptations that help them conserve heat, as described in Key Concept 38.3. Most animals are either summer migrants or are dormant for much of the year. Resident birds and mammals, such as the willow ptarmigan (Lagopus lagopus) and Arctic fox (Vulpes lagopus), have thick fur or feathers that may change color with the seasons, from brown in summer to white in winter.

1157