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  3. (1) Located in tropical area at low altitudes, savannas (热带草原) are stable ecosystems, some wet and some dry consisting of vast g

(1) Located in tropical area at low altitudes, savannas (热带草原) are stable ecosystems, some wet and some dry consisting of vast g

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问题     (1) Located in tropical area at low altitudes, savannas (热带草原) are stable ecosystems, some wet and some dry consisting of vast grasslands with scattered trees 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 seem to play important roles in their formation.
    (2) Savannas typically experience a rather prolonged dry season. One theory behind savanna formation is that wet forest species are unable to withstand the dry season, 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 season. 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 grasses. 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 these 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 those of some rain forests, though more extreme. For example, savanna soils, like many rain forest soils, are typically oxisols (氧化土) (dominated by certain oxide minerals) and ultisols (老成土) (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 (溃水) soils; other dry, sandy, well-drained soils. This may seem contradictory, but it only means that extreme soil conditions, either too wet or too dry for forests, are satisfactory for savannas. More moderate conditions support moist forests.
    (4) Waterlogged soils (溃水土壤) occur in areas that are flat or have poor drainage. These soils usually contain large amounts of clay and easily become water-saturated (饱水状态) . Air cannot penetrate between the soil particles, making the soil oxygen-poor. By contrast, dry soils are sandy and porous, their coarse 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 (because of either moisture conditions or nutrient levels of both), poor soils can and do support lush rain forests.
    (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 (耐火植物) , meaning they are adapted in various ways to withstand occasional burning. Frequent fire is a factor to which rain forest species seem 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 decreased plant-species richness, often with an increase in tree density. There is generally little doubt that fire is a significant factor in maintaining savannas, certainly in most regions.
    (6) In addition, humans have contributed to the conditions favoring the formation of savannas. On certain sites, particularly in South America, savanna formation seems related to frequent cutting and burning of moist forests for pastureland. Increase in pastureland and subsequent overgrazing have resulted in an expansion of savanna. The thin upper layer of humus (腐殖质) (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.
The fact that "poor soils can and do support lush rain forests" suggests that________.
选项 A、poor soils alone may not be enough to explain why an area becomes a savanna
B、rain forest vegetation can significantly lower the quality of soils
C、drought stress is the single most important factor in savanna formation
D、minerals are more important than moisture for the growth of trees
答案A
解析 推断题。根据文章第四段最后一句话：Though most savannas ale found on sites with poor soils because of either moisture conditions or nutrient levels of both)，poor soils can and do support lush rain forest. 可知，虽然大多数热带草原都位于贫瘠土壤区(无论是因为水分条件还是营养等级)，但贫瘠土壤也的确能够长出繁茂的热带雨林。因此答案为选项A。
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