閱讀譯文逐段對應(yīng)tpo-31reading3

1、Savanna Formation薩王納的形成Located in tropical areas at low altitudes, savannas 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 determi

2、nes if a given site will be a savanna, but some factors seem to play important roles in their formation. 薩王納（熱帶草原或熱帶稀樹草原）位于熱帶低海拔地區(qū)，干濕氣候皆有。它稀疏分布著樹木和灌木的廣闊大草原，是一個穩(wěn)定的生態(tài)系統(tǒng)。它的氣候極端，且含蓋了廣泛的土壤種類。對于熱帶草原的形成原因，雖無一個簡單唯一的解釋，但似乎仍有一些因素對他的形成起到了重大作用。Savannas typically experience a rather prolonged dry season. One the

3、ory 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. T

4、hough 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 B

5、razil 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. 熱帶草原通常會經(jīng)歷一個十分漫長的旱季。熱帶草原形成理論之一，就是濕潤的森林物種無法承受這個干燥的季節(jié)，因此這更有利于熱帶草原，而不是熱帶雨林的形成。熱帶草原的年降水量在 1000 到 2000 毫米，絕大多數(shù)降水都集中在一個長達

6、 5 到 8 個月的雨季。雖然每年的降水總量充足，但至少會有幾個月會幾乎無降水，這引起的干旱脅迫最終也只適合草地的生長。這樣的條件廣泛地分布于南美北部和古巴的大部分地區(qū)，但也有例外，比如在中美洲的稀樹大草原，巴西的沿海地區(qū)和特立尼達島氣候的類型就不一樣。在這些地區(qū)，每月的降水量超過了之前定義的降水量范圍，所以，一定還有其他因素影響著熱帶草原的形成。In many characteristics, savanna soils are similar to those of some rain forests, though more extreme. For example, savanna s

7、oils, 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. S

8、ome 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. 雖然有點極端，但在大多數(shù)特點上，熱

9、帶草原和雨林的土壤還是相似的。比如說，和許多熱帶雨林土壤一樣，熱帶草原的土壤通常為含酸量高的氧化土（某些氧化物礦物占主導(dǎo)）和老成土（不含碳酸鈣的土壤）。在這些土壤中，磷、鈣、鎂、鉀這樣的礦物質(zhì)含量非常低，而鋁的含量卻很高。有些熱帶草原的土壤潮濕，呈漬水狀態(tài)；而有些的土壤卻干燥、多沙、易排水。這看起來矛盾，但卻意味著只有像過度濕潤或過度干旱這樣的極端土壤條件，才適合熱帶草原。溫潤的氣候適合濕潤的森林。Waterlogged soils occur in areas that are flat or have poor drainage. These soils usually contain l

10、arge amounts of clay and easily become water-saturated. Air cannot penetrate between the soil particles, making the soil oxygenpoor. 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 a

11、nd 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.漬水土壤出現(xiàn)在地形平坦和排水性差的地方。這些土壤通常含有大量的黏土，這就會使它們很容易處于飽水狀態(tài)。由于空氣不能滲入土壤顆粒中，土壤的含氧量就低了。與之相對，干燥土壤是多沙和多孔滲水

12、的。它們的粗質(zhì)結(jié)構(gòu)可以讓水很快排干。砂質(zhì)土壤更易于營養(yǎng)物和礦物質(zhì)的過濾滲漏，這就導(dǎo)致土壤缺乏營養(yǎng)。雖然大多數(shù)熱帶草原都位于貧瘠土壤區(qū)（無論是因為各自的水分條件還是營養(yǎng)等級），但貧瘠土壤也的確能夠長出繁茂的熱帶雨林。Most savannas probably experience mild fires frequently and major burns every two years or so. Many savanna and dryforest plant species are called pyrophytes, meaning they are adapted in variou

13、s 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 soil dating prior to the arrival of humans suggests that moist forests also occasionally burn. Experiments suggest that if fire di

14、d 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 protec

15、ted 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 plantspecies richness, often with an increase in tree density. There is generally little doubt that fire is a s

16、ignificant factor in maintaining savanna, certainly in most regions. 大多數(shù)熱帶草原都可能經(jīng)歷著頻繁的文火焚燒，而大約兩年就有一次大火燃燒。許多熱帶草原和干燥林的植物種類都被叫做耐火植物，意思是它們善于用各種方法來抵御偶爾發(fā)生的燃燒。雖然追溯到人類初至?xí)r亞馬遜森林土壤中遺留的古木碳表明濕潤林帶也會有不定期的燃燒，但頻繁的大火還是熱帶雨林物種所不能適應(yīng)的因素。實驗表明，如果美國的熱帶草原沒有發(fā)生大火，物種的構(gòu)成就會變得完全不一樣。焚燒的發(fā)生防止了植物之間的競爭，這就避免了因植物間的排擠而減少整個生態(tài)系統(tǒng)的多樣性。但是在防火實驗區(qū)

17、，一些多年生植物物種最終占得主導(dǎo)地位，勝過了其他物種。其他調(diào)查證據(jù)表明，沒有火的焚燒則導(dǎo)致植物物種豐富性的顯著減少，通常還會伴隨著樹的密度增加。這就可以確定，火是維持熱帶草原的重要因素。當(dāng)然，是在大多數(shù)地區(qū)。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 overgrazin

18、g 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 veg