托福TPO閱讀真題及譯文：Savanna Formation

第一段：

托福TPO閱讀原文：
　　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 seem to play important roles in their formation.

托福TPO閱讀翻譯：
　　熱帶草原或熱帶稀樹(shù)草原位于熱帶低海拔地區(qū)，干濕氣候皆有。稀疏分布著樹(shù)木和灌木的廣闊大草原，其為一個(gè)穩(wěn)定的生態(tài)系統(tǒng)。氣候極端，涵蓋了廣泛的土壤種類。對(duì)于熱帶草原的形成原因，雖無(wú)單一的解釋，但似乎仍有一些因素對(duì)其形成起到了重大作用。

第二段：

托福TPO閱讀原文：
　　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.

托福TPO閱讀翻譯：
　　熱帶草原通常會(huì)經(jīng)歷十分漫長(zhǎng)的旱季。熱帶草原形成理論之一，就是濕潤(rùn)的森木物種無(wú)法承受干燥的季節(jié)——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)降水都集中在長(zhǎng)達(dá)5到8個(gè)月的雨季中。雖然每年的降水總量充足，但至少會(huì)有幾個(gè)月幾乎零降水，由此引起的干旱使得該地區(qū)最終只適合草地生長(zhǎng)。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(shù)大草原，巴西的沿海地區(qū)和特立尼達(dá)島氣候的類型就不一樣。在這些地區(qū)每月的降水量超過(guò)了之前定義的降水量范圍，因此還有其他因素影響著熱帶草原的形成。

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第三段：

托福TPO閱讀原文：
　　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 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 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.

托福TPO閱讀翻譯：
　　雖然極端，但在大多數(shù)特點(diǎn)上，熱帶草原和雨林的土壤還是相似的。比如說(shuō)，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（這些土壤中不含碳酸鈣，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，呈潰水狀態(tài)；而有些的土壤卻干燥、多沙、易排水。這看起來(lái)矛盾，但卻意味著只有像過(guò)度濕潤(rùn)或過(guò)度干旱那樣的極端土壤條件，才適合熱帶草原，溫潤(rùn)的氣候適合濕潤(rùn)的森林。

第四段：

托福TPO閱讀原文：
　　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.

托福TPO閱讀翻譯：
　　潰水土壤出現(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(huì)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。與之相對(duì)，干燥土壤是多沙和多孔滲水的。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(yíng)養(yǎng)物和礦物質(zhì)的過(guò)濾滲漏，這就導(dǎo)致土壤缺乏營(yíng)養(yǎng)。雖然大多數(shù)熱帶草原都位于貧瘠土壤區(qū)（無(wú)論是因?yàn)楦髯缘乃�分條件還是營(yíng)養(yǎng)等級(jí)），但貧瘠土壤也的確能夠長(zhǎng)出繁茂的熱帶雨林。

第五段：

托福TPO閱讀原文：
　　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.

托福TPO閱讀翻譯：
　　大多數(shù)熱帶草原都可能經(jīng)歷著頻繁的焚燒，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被叫做耐火植物，意思是其善于用各種方法來(lái)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(rùn)林帶也會(huì)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(shí)驗(yàn)表明，如果美國(guó)的熱帶草原沒(méi)有發(fā)生大火，物種的構(gòu)成就會(huì)變得完全不一樣。焚燒的防止了植物之間的競(jìng)爭(zhēng)，這就避免了因植物間的排擠而使整個(gè)生態(tài)系統(tǒng)的多樣性發(fā)生變化。但是在防火實(shí)驗(yàn)區(qū)，一些多年生植物物種最終占得主導(dǎo)地位，勝過(guò)了其他物種。其他調(diào)査證據(jù)表明，沒(méi)有焚燒則導(dǎo)致植物物種豐富性的顯著減少，通常還會(huì)伴隨著樹(shù)的密度増加。這就可以確定，火是維持熱帶草原的重要因素。當(dāng)然，這指的是在大多數(shù)地區(qū)。

第六段：

托福TPO閱讀原文：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 (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.

托福TPO閱讀翻譯：
　　在某些地區(qū)，特別是在南美洲，熱帶草原的形成可能和為了建造牧場(chǎng)而對(duì)濕潤(rùn)林地進(jìn)行頻繁的砍伐和焚燒有關(guān)。牧場(chǎng)的増加和隨之而來(lái)的過(guò)度放牧導(dǎo)致了熱帶草原的擴(kuò)張。稀薄的腐殖質(zhì)（腐爛的有機(jī)物）上層被砍伐和焚燒行為所破壞。腐殖質(zhì)是樹(shù)葉被細(xì)菌和真菌迅速分解和根部表面循環(huán)的必需品。一旦腐面層消失了，營(yíng)養(yǎng)物質(zhì)便不能循環(huán)并滲入土壤中，土壤由肥沃變得貧瘠，這就使它只適合熱帶草原植被的生長(zhǎng)。白土和砂土上的森林結(jié)構(gòu)容易發(fā)生永久性的改變。