Page 683 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

Chapter 20 Terrestrial Biomes 647
▲Figure 20.6 Invasive purple loosestrife in southern Ontario, Canada. [gaertner/Alamy.]
In Canada and the United States, humans are per- petuating a new type of terrestrial plant community in developed areas. This new community, a mix of native and non-native species used for landscaping, is some- where between a grassland and a forest. Investments of water, energy, and capital are required to sustain the new species. Additionally, on rangelands and in agri- cultural areas, humans alter natural biomes by grazing non-native animals and planting crops from other re- gions (we discuss anthropogenic biomes further at the end of the chapter). Whether the land would return to its natural vegetation if human influences were removed is unknown.
Earth’s Terrestrial Biomes
Given that extensive transition zones separate many of Earth’s biomes, the classification of biomes according to distinct vegetation associations is difficult and some- what arbitrary. The result is a number of classification systems—similar in concept but different in detail— used in biology, ecology, and geography textbooks. In Geosystems, we describe 10 biomes that are common to most classification systems: tropical rain forest, tropical seasonal forest and scrub, tropical savanna, midlatitude broadleaf forest, boreal and montane forest, temper- ate rain forest, Mediterranean shrubland, midlatitude grassland, desert, and arctic and alpine tundra.
The global distribution of these biomes is portrayed in Figure 20.7 and summarized in Table 20.1, which also includes pertinent information regarding climate, soils, and water availability. The following pages pro- vide descriptions of each biome, synthesizing all we have learned in previous chapters about the interactions of atmosphere, hydrosphere, lithosphere, and biosphere. Because plant distributions respond to environmental conditions and reflect variations in climate and soil, the world climate map in Chapter 10, Figure 10.2, is a helpful reference for this discussion.
Tropical Rain Forest
The lush biome covering Earth’s equatorial regions is the tropical rain forest. In the tropical climates of these forests, with consistent year-round daylength (12 hours), high insolation, average annual temperatures around 25°C, and plentiful moisture, plant and animal popula- tions have responded with the most diverse expressions of life on the planet. Rainforest species evolved during the long-term residence of the continental plates near equatorial latitudes. Although this biome is stable in its natural state, undisturbed tracts of rain forest are be- coming increasingly rare; deforestation is perhaps the most pervasive human impact.
The largest tract of tropical rain forest occurs in the Amazon region, where it is called the selva. Tropical rain forests also cover the equatorial regions of Africa, parts of Indonesia, the margins of Madagascar and Southeast Asia, the Pacific coast of Ecuador and Colombia, and the east coast of Central America, with small discontinuous patches elsewhere. The cloud forests of western Venezuela are high-elevation tropical rain forests, perpetuated by high humidity and cloud cover. Rain forests occupy about 7% of the world’s total land area but represent ap- proximately 50% of Earth’s species and about half of its remaining forests.
Rainforest Flora and Fauna The rainforest canopy forms three levels (Figure GIA 20.1). The upper level, called the overstory, is not continuous, but features emer- gent tall trees whose high crowns rise above the middle canopy, which is continuous. Biomass in a rain forest is concentrated in the dense mass of overhead leaves in these two areas of the canopy. The lower level is the understory, where broad leaves block much of the light so that the forest floor receives only about 1% of the sun- light arriving at the canopy. The lower level of vegetation is composed of seedlings, ferns, and bamboo, leaving the litter-strewn ground surface in deep shade and fairly open. The constant moisture, odours of mold and rotting vegetation, strings of thin roots and vines dropping down from above, windless air, and echoing sounds of life in the trees together create a unique environment.
Infertile soils support these biologically rich forests. Rainforest trees have adapted to the poor soils with root systems able to capture nutrients from litter decay at the soil surface.
CRITICALthinking 20.1 Reality Check
Using the map in Figure 20.7, the information in Table 20.1, and the discussion in this chapter, describe the biome in which you are located. What changes in the natural veg- etation do you see, as we all live in altered environments brought on by human activities? Consider the climate clas- sification information in Chapter 10, as you evaluate your biome. What generalizations can you make? •