(mechanized agriculture)


Wet tropical and subtropical forests


Average for the biosphere


"Recall that 1000 g corresponds to about 4100 kcal. Source: Odum (1987).

"Recall that 1000 g corresponds to about 4100 kcal. Source: Odum (1987).

summer. Below this is the deep, permanently frozen soil called permafrost. Tundra is notoriously sensitive to disturbance and slow to recover. A footprint is said to remain visible for years.

The taiga, also called boreal (northern) forest, occupies vast areas of Canada and Russia and other areas of similar latitude. The dominant plants are conifers, including

Figure 15.1 Temperature-moisture regimes of terrestrial biomes.

spruce, fir, and pine. Dense shade inhibits other plants, but the trees support small seed-eating animals. Larger animals are less common, as they require broadleafed plants for food. A form of coniferous forest called the temperate rain forest exists in humid coastal regions of North America from northern California to Alaska. The tree needles degrade slowly, building into a thick detritus mat on the forest floor.

The temperate deciduous forest occupies milder climate zones with ample rainfall. The canopy cover is less impenetrable, and smaller trees and shrubs (the understory) can grow. A large number of subdivisions exist, based on the type of dominant hardwoods that are present. Nuts, berries, and broadleaf plants support a well-developed food pyramid. The rate at which litter falls to the forest floor increases toward the equator, up to 1100 g/m2 per year.

Where rainfall is insufficient to support the deciduous forest, a grassland will be found instead. A variety of types occur, depending on rainfall, as this type of ecosystem grades into desert. Moister regions support tall grasses and sod-forming grasses. More arid areas are characterized by short bunchgrasses. Grasses produce a large amount of litter and therefore organically rich soil. Most of the biomass is below ground. Large grazing animals are typically present, such as bison, gazelles, and kangaroos.

Grassland ecosystems are often maintained by fire, without which they would be succeeded by shrubland or forest. Forest can give way to grassland if fire follows an extended drought. Once the grasses are established in the resulting opening, fires become more frequent, due to the rapid production of leaf litter. This makes it difficult for trees to reestablish themselves.

Fire occurs naturally and can be a force in preserving an ecosystem. Even in forests, periodic fires confined to the understory (surface fires) eliminate deadwood, which otherwise could fuel a much more devastating crown fire. A crown fire destroys the mature trees as well as seeds and organic matter in the soil. A surface fire can release nutrients and stimulate germination of tree seeds. Pines tend to be more resistant to fire than hardwoods. Some mature coastal pine forests are called a fire climax ecosystem.

Woodlands tend to give way to shrublands under maritime influence. The chaparral is a shrubland that dominates in warm areas that have plenty of moisture in winter but are very dry in summer.

The desert includes regions with less than about 25 cm (10 in.) of rain per year, depending on the temperature. Primary productivity is proportional to rainfall, the limiting factor for this ecosystem. The dominant plants are either rapid-growing annuals, cacti, or specially adapted desert shrubs. The creosote bush dominates the hot deserts of southeastern North America. This plant exhibits allelopathy, in which it inhibits nearby competition chemically. In cooler northwestern deserts, sagebrush dominates. Desert animals are specially adapted to conserve water. Lizards are suited to the desert because they excrete dry uric acid instead of urea with water as mammals do. Nevertheless, some small rodents survive by excreting a very concentrated urine and by nocturnal life-styles. Irrigation of deserts can make them productive for agriculture. However, since the evaporation rate will be high, salts can slowly accumulate in the soil unless extra water is provided to remove the salts by runoff and infiltration. (In the Central Valley of California, selenium from irrigation and natural groundwater sources is accumulating in the soil. It is thought that the productive life of this important agricultural region may be limited to a matter of decades by this problem.)

Seasonal tropical forests occur in places such as south Asia, which has monsoon rains and a pronounced dry season, during which the trees lose their leaves. Tropical rain forests occur where there is more than 200 cm of rain per year, distributed throughout the year. They are typically found at low elevations near the equator. They tend to be very rich in both plant and animal species. Some tracts of a few acres can have more species of trees than are found in all of Europe. A 6-square mile area of Panama was found to have at least 20,000 species of insects, compared to several hundred in all of France. Ants, termites, moths, and butterflies are important. A larger proportion of both plants and animals live in the upper layers of the forest, compared to temperate forests. In one forest, 50% of the mammals live in the trees. Plants that grow supported by other plants, with no roots reaching the ground, are called epiphytes. Many animals have formed symbiotic relationships with epiphytes. An important feature of tropical rain forests is the rapid cycling of nutrients within the ecosystem. Litter is decomposed rapidly, and the nutrients soon reabsorbed by the plants. Therefore, most of the nutrients are stored in the biomass, and the soil remains poor in nutrients. The rich growth of the rain forests made the early settlers think the soil must be rich. Many attempts at agriculture failed for a lack of this understanding.

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