The Types and Distribution of Terrestrial Biomes
From space, terrestrial biomes appear as broad latitudinal bands (Figure 7-8). Biomes are distinguished by their vegetation and are associated with particular climates. Rain forests tend to be found near the equator where it is warm and rainy, while deserts are located 30° north and south of the equator, where there is relatively little precipitation. The use of satellites to track biomes is described in Case Study: Keeping Track of Terrestrial Biomes—The Use of Satellite Remote Sensing.
|FIGURE 7-8. Vegetation on Earth An image of the Earth taken by a satellite designed to measure terrestrial net primary production (NPP).The darkest green areas are the lushest in vegetation, while the pale colors are sparse in vegetation cover either due to snow, drought, rock, or urban areas. Credit: NASA/NOAA|
The distribution of terrestrial biomes is determined primarily by three factors: temperature, water, and sunlight (Figure 7-9). Temperature affects the length of the growing season, which is the number of consecutive days during which the temperature remains above 0°C (32°F). During this period plants must generate enough energy to reproduce or store enough energy to survive until the next growing season. The distribution of biomes is also determined by the availability of water. Plants get water from soil. The amount of water in soil is determined by the rates of precipitation and potential evaporation. Potential evaporation refers to the amount of water that would evaporate if water were available (that is, if the soil never dried out). When precipitation exceeds potential evaporation (hereafter referred to simply as evaporation), water accumulates in the soil and is available to plants. When evaporation exceeds precipitation, soil water supplies dwindle. Under these conditions, plants slow the rate at which they lose water by closing their stomata. But closing their stomata prevents the plants from taking in fresh supplies of carbon dioxide, and this slows their photosynthesis.
FIGURE 7-9. Limiting Factors Geographical differences in the factors that limit net primary production. Notice that water limits net primary production in deserts, such as the Sahara and the Australian outback. Temperature limits net primary production in forested areas of northern North America and eastern Russia. Finally, clouds reduce the amount of light that reaches the surface near the equator, which limits net primary production by tropical rain forests, such as the Amazon. (Redrawn from Nemani et al., “Climate-Driven Increases in Global Terrestrial Net Primary Production from 1982 to 1999,” Science 300: 1560–1563; Figure from Kaufmann, Robert K. and Cleveland, Cutler J. 2007. Environmental Science (McGraw-Hill, Dubuque, IA)).
Finally, the availability of light may limit plant growth. The amount of light available is determined by latitude and local climate. Because of the tilt of Earth’s axis, the greatest amount of sunlight reaches Earth’s atmosphere between 23.5° north and 23.5° south (see Chapter 4). However, sunlight and plant growth in this region are often limited by clouds. Remember from Chapter 4 that high rates of convection generate many clouds, which reduce the amount of sunlight that reaches the ground.
In any given area, one of these factors may limit net primary production in much the same way that a single nutrient may limit plant growth. Geographical differences in the limiting factor create challenges for which local plants and animals have evolved adaptations. For plants, natural selection has generated different leaf shapes. For animals, local climate may limit the time that they can be active and influence their allocation of energy. In the following sections we use these limiting factors and the relevant adaptations to describe nine of the terrestrial biomes that are shown in Figure 7-10.
FIGURE 7-10. Terrestrial Biomes The location terrestrial biomes. Source: NASA GSFC.