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TPO 31—3 Savanna Formation
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原文：
【1】Located in tropical area at low altitudes, savannas are stable ecosystems, some wet and some dry consisting of vast grasslands with scattered tree or shrubs. They occur on a wide range of soil types and in extremes of climate. There is no simple or single factor that determines if a given site will be a savanna, but some factors em to play important roles in their formation.
【2】Savannas typically experience a rather prolonged dry ason. One theory behind savanna formation is that wet forest species are unable to withstand the dry ason, and thus savanna, rather than rain forest, is favored on the site. Savannas experience an annual rainfall of between 1000 and 2000 millimeters, most of it falling in a five-to-eight month wet ason. Though plenty of rain may fall on a savanna during the year, for at least part of the year little does, creating the drought stress ultimately favoring grass. Such conditions prevail throughout much of northern South America and Cuba, but many Central American savannas as well as coastal areas of Brazil and the island of Trinidad do not fit this pattern. In the areas, rainfall per month exceeds that in the above definition, so other factors must contribute to savanna formation.
【3】In many characteristics, savanna soils are similar to tho of some rain forests, though more extreme. For example, savanna soils, like many rain forest soils, are typically oxisols (dominated by certain oxide minerals) and ultisols (soils containing no calcium carbonate), with a high acidity notably low concentrations of such
minerals as phosphorus, calcium, magnesium, and potassium, while aluminum levels are high. Some savannas occur on wet, waterlogged soils; other dry, sandy, well-drained soils. This many em contradictory, but it only means that extreme soil conditions, either too wet or too dry for forest
s, are satisfactory for savannas. More moderate conditions support moist forests.
【4】Waterlogged soils occur in areas that are flat or have poor drainage. The soils usually contain large amounts of clay and easily become water-saturated. Air cannot penetrate between the soil particles, making the soil oxygen-poor. By contrast, dry soils are sandy and porous, their coar textures permitting water to drain rapidly. Sandy soils are prone to the leaching of nutrients and minerals and so tend to be nutritionally poor. Though most savannas are found on sites with poor soils (becau of either moisture conditions or nutrient levels of both), poor soils can and do support lush rain forest.
分处【5】Most savannas probably experience mild fires frequently and major burns every two years or so. Many savanna and dry-forest plant species are called pyrophytes, meaning they are adapted in various ways to withstand occasional burning. Frequent fire is a factor to which rain forest species em unable to adapt, although ancient charcoal remains from Amazon forest soils dating prior to the arrival of humans suggest that moist forests also occasionally burn. Experiments suggest that if fire did not occur in savannas in the Americas, species composition would change significantly. When burning occurs, it prevents competition among plant species from progressing to the point where some species exclude others, reducing the
overall diversity of the ecosystem. But in experimental areas protected from fire, a few perennial grass species eventually come to dominate, outcompeting all others. Evidence from other studies suggests that exclusion of fire results in markedly decread plant-species richness, often with an increa in tree density. There is generally little doubt that fire is a significant factor in maintaining savanna, certainly in most regions.
【6】On certain sites, particularly in South America, savanna formation ems related to frequent cutting and burning of moist forests for pastureland. Increa in pastureland and subquent overgrazing have resulted in an expansion of savanna. The thin upper layer of humus (decayed organic matter) is destroyed by cutting and burning. Humus is necessary for rapid decomposition of leaves by bacteria and fungi and for recycling by surface roots. Once the humus layer disappears, nutrients cannot be recycled and leach from the soil, converting soil from fertile to infertile and making it suitable only for savanna vegetation. Forests on white, sandy soil are most susceptible to permanent alteration.佛印绝类弥勒
题目：
1.The word "prolonged" in the passage is clost in meaning to
A.predictable.
B.destructive.
C.lengthy.
D.unproductive.
2.In paragraph 2，the author mentions savannas in Central America, Brazil, and the island of Trinidad in order to
大班表演游戏A.argue that the savannas are similar to tho in South America and Cuba.
B.point out exceptions to the pattern of savanna formation in areas with drought stress.
C.provide additional examples of savannas in areas with five- to eight-month wet asons.
D.indicate areas where savannas are being gradually replaced by rain forests.
3.According to paragraph 3, rain forests and savannas differ in that
西装领带怎么打A.the soils in rain forests contain fewer minerals than savanna soils do.
B.savannas affect soil conditions more than rain forests do.
C.unlike rain forests, savannas prefer sandy, well-drained soils to soils that are very wet.
巴比伦酒店D.unlike rain forests, savannas may develop under both very dry and very wet soil conditions.
4.The word "notably" in the passage is clost in meaning to
A.similarly.
B.especially.
C.usually.
5.According to paragraph 3, all of the following are true of savanna soils EXCEPT
A.They have high concentrations of potassium.
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