托福 新托福 31 - Savanna Formation
题目
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1.In paragraph 2, the author mentions savannas in Central America, Brazil, and the island of Trinidad in order to
  • A.Argue that these savannas are similar to those in South America and Cuba
  • B.Point out exceptions to the pattern of savanna formation in areas with drought stress
  • C.Provides additional examples of savannas in areas with five- to eight-month wet seasons
  • D.Indicate areas where savannas are being gradually replaced by rain forests
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    Located in tropical areas at low altitudes, savannas are stable ecosystems, some wet and some dry consisting of vast grasslands with scattered trees and 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.

    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 1,000 and 2,000 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.

    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 (dominated by certain oxide minerals) and ultisols (soils containing no calcium carbonate), with a high acidity and notably low concentrations of such minerals as phosphorus, calcium, magnesium, and potassium, while aluminum levels are high. Some savannas occur on wet, waterlogged soils; others on 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.

    Waterlogged soils occur in areas that are flat or have poor drainage. These 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 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 forest.

    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 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 savanna, certainly in most regions.

    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 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.

    坐落于低纬度的热带地区,稀树大草原是稳定的生态系统,一些潮湿,而另一些则干旱。它们由大面积的草地和零星的灌木或乔木组成。它们适应极宽泛的土壤类型和各种极端气候条件。决定一个地区是稀树草原的因素既不简单也非单一,但有一些因素在其形成过程中扮有重要角色。 稀树草原通常有很长的一段旱季。一种稀树草原形成的理论认为潮湿森林不能忍受这种旱季,因此在该地区就形成了稀树草原而不是雨林。稀树草原每年降雨量介于1000mm到2000mm,绝大部分降落在5-8个月的雨季。尽管一年降雨充沛,但至少有一半的时间很少下雨,干旱的压力更倾向于形成草原。这种情况发生在北美和古巴的大部分地区。但是美洲中部、巴西海岸地区和特立尼达和多巴哥共和国的岛屿上的稀树草原则不是这种情况。这些地区每个月的降雨都超过了上述定义,因此是其他因素导致稀树草原的形成。 稀树草原土壤的很多特征和雨林相似,只是更加极端。例如,稀树草原土壤,与雨林土壤相似,都是氧化物(大部分是某种氧化物矿物)和极育土(没有碳酸钙的土壤),酸性很强,但像磷、钙、镁和钾等矿物含量很少,而铝的含量很高。一些稀树草原土壤潮湿,而另一些则干燥、砂质、排水性好。这可能相互矛盾,但都是很极端的土壤条件,要么太潮湿要么太干燥,因而不能形成雨林而是稀树草原。潮湿的雨林需要更加温和的条件。 浸满水的土壤一般位于平坦或排水极差的地区。这种土壤粘土含量一般很高,很容易饱和水。空气不能从土壤颗粒间穿过,因此土壤变成缺氧环境。相反,干旱的土壤都是砂质和多孔的,他们粗糙的性质使得水分很快就排出了。砂质土壤更容易遗失营养和矿物质,因此比较贫瘠。虽然绝大部分稀树草原土壤贫瘠(要么是水分少,要么是营养水平低),但是贫瘠土壤可以也已经支撑过茂密的雨林。 大部分稀树草原很可能经常有较小的野火,并大约每隔一年都会有一场大火。很多稀树草原和干旱森林的物种都称作防火物种,因为他们进化出各种方式以忍受间歇的野火。频繁的野火也是一个雨林物种无法适应的因素,尽管人类到达前亚马逊雨林的古老木炭遗迹表明潮湿的森林也时常有火灾。实验表明如果美国的稀树草原没有野火,则其物种组成将发生剧烈变化。燃烧可阻止植物物种间的竞争以防止某一物种将其他物种灭绝,进而减少了整个生态系统的多样性。其他研究的证据表明如果没有野火,植物物种的丰富程度将降低,乔木的密度将增加。很少有人怀疑野火是保持绝大部分稀树草原的一个重要因素。 在特定区域,尤其是南美,稀树草原的形成可能跟频繁的砍伐和燃烧森林以便形成牧场有关。牧场和随后的过度放牧的增加可导致稀树草原的扩张。腐殖质的表层(腐烂的有机物)被砍伐和燃烧破坏了。腐殖质是细菌或真菌分解树叶以便根系再度吸收的必要条件。一旦腐殖质被破坏,营养物质就不能循环利用,而从土壤流失了,因此土壤将变得贫瘠,只能适应稀树草原了。白色砂质土壤上的森林最容易遭受永久的改变。  

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    最新提问
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