Page 687 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

Chapter 20 Terrestrial Biomes 651
down to approximately 18000 km2 (Figure GIA 20.2c). By continent, rainforest losses are estimated at more than 50% in Africa, more than 40% in Asia, and 40% in Central and South America.
Deforestation is a highly charged issue, especially for Brazil’s growing cattle industry, which uses deforested lands for pasture. Cattle herds reached over 60 million head by 2010, generating USD $3 billion in revenue. Among many available websites, see the Tropical Rain- forest Coalition at www.rainforest.org/ or Rainforest Action Network at www.ran.org/.
shrublands. The monsoonal forests have an average height of 15 m with no continuous canopy of leaves, transition- ing into drier areas with open grassy areas or into areas choked by dense undergrowth. In the more-open tracts, a common tree is the acacia, with its flat top and usually thorny stems. Scrub vegetation consists of low shrubs and grasses with some adaptations to semiarid conditions.
Local names are given to these communities: the Caatinga of the Bahia State of northeastern Brazil; the Chaco (or Gran Chaco) of southeastern Brazil, Paraguay, and northern Argentina (Figure 20.8a); the brigalow scrub of Australia; and the dornveld of southern Africa. In Africa, this biome extends west to east from Angola through Zambia to Tanzania and Kenya. Tropical seasonal forests are also present in Southeast Asia and portions of India, from interior Myanmar through north- eastern Thailand; and in parts of Indonesia.
The trees throughout most of this biome make poor lumber, but some, especially teak, may be valuable for fine cabinetry and furniture. In addition, some of the plants with dry-season adaptations produce usable waxes and gums, such as the carnauba wax produced by the Brazilian palm tree. Animal life includes the koalas and cockatoos of Australia and the elephants, large cats, anteaters, rodents, and ground-dwelling birds in other occurrences of this biome. Worldwide, humans use these areas for ranching (Figure 20.8b); in Africa, this biome in- cludes numerous wildlife parks and preserves.
Tropical Savanna
The tropical savanna biome consists of large expanses of grassland, interrupted by scattered trees and shrubs. Tree- less tracts of grassland can also occur in this biome, with grasses in discontinuous clumps separated by bare ground. Tropical savannas receive precipitation during approxi- mately 6 months of the year, when they are influenced by the shifting ITCZ. The rest of the year they are under the drier influence of shifting subtropical high-pressure cells. This is a transitional biome between the tropical seasonal forests and the semiarid tropical steppes and deserts.
Shrubs and trees of the savanna biome are adapted to drought, grazing by large herbivores, and fire. Most species are xerophytic, or drought resistant, with vari- ous adaptations to help them conserve moisture dur- ing the dry season: small, thick leaves; rough bark; or leaf surfaces that are waxy or hairy. Many trees of the savanna woodlands have small leaves to retain mois- ture and are characteristically flat-topped or umbrella shaped in order to capture the maximum amount of sunlight on their small leaf surfaces (Figure 20.9).
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CRITICALthinking 20.2 Tropical Forests: A Global or Local Resource?
given the information presented in this chapter about deforestation in the tropics, and in the previous chapter about declining biodiversity, assess the present contro- versy over rainforest resources. What are the main issues? How do developing countries, which possess most of the world’s rain forests, view rainforest destruction? How do developed countries, with their transnational corporations, view rainforest destruction? How do concerns for plan- etary natural resources balance against the needs of local peoples and sovereign state rights? How is climate change related to these issues? What kind of action plan would you develop to accommodate all parties? •
Tropical Seasonal Forest and Scrub
The area of seasonal changes in precipitation at the mar- gins of the world’s rain forests is the tropical seasonal forest and scrub biome, which occupies regions of lower and more erratic rainfall than occurs in the equatorial regions. The biome encompasses the tropical monsoon and tropical savanna climates, with fewer than 40 rainy days during their four consecutive driest months and heavy monsoon downpours during their summers (see Chapter 6, Figures 6.15 and GIA 6). The shifting inter- tropical convergence zone (ITCZ) affects precipitation regimes, bringing moisture with the high Sun of summer and then a season of dryness with the low Sun of win- ter. This shift produces a seasonal pattern of moisture deficits, which affects vegetation leaf loss and flowering. The term semideciduous applies to many broadleaf trees that lose some of their leaves during the dry season.
Thus, the tropical seasonal forest and scrub is a varied biome that occupies a transitional area from wetter to drier tropical climates. Natural vegetation ranges from monsoon forests to open woodlands to thorn forests to semiarid
Georeport 20.3 Tropical Rain Forests as Nature’s Medicine Cabinet
Nature’s biodiversity is like a full medicine cabinet. Since 1959, 25% of all prescription drugs were originally derived from higher plants. Scientists have identified 3000 plants, many of them in tropical rain forests, as having anticancer properties. The rosy periwinkle (Catharanthus roseus) of Madagascar contains two alkaloids that combat two forms of cancer. yet less than 3% of
flowering plants have been examined for alkaloid content. It defies common sense to throw away the medicine cabinet before we even open the door to see what is inside.