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热带稀树草原位于热带地区，海拔较低，是一种稳定的生态系统，有些是湿的，有些是干的，由广阔的草地和分散的树木或灌木组成。

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.

它们广泛存在于各种土壤类型和极端气候中。

They occur on a wide range of soil types and in extremes of climate.

没有简单或单一的因素来决定一个特定的地点是否会成为稀树草原，但一些因素似乎在它们的形成中扮演着重要的角色。

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.

稀树草原通常经历一个相当漫长的旱季。

Savannas typically experience a rather prolonged dry season.

在热带稀树大草原形成背后的一个理论是，潮湿的森林物种无法承受干燥的季节，因此在这个地点，热带稀树大草原比热带雨林更有利。

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.

稀树大草原的年降雨量在1000到2000毫米之间，其中大部分降雨在5到8个月的雨季里。

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.

尽管草原一年中可能会有大量降雨，但至少有一段时间降雨很少，造成最终有利于牧草生长的干旱压力。

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.

这种情况普遍存在于南美洲北部和古巴的大部分地区，但许多中美洲大草原以及巴西沿海地区和特立尼达岛不符合这种模式。

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.

在这些地区，每个月的降雨量超过了上述定义，所以其他因素一定也有助于草原的形成。

In these areas, rainfall per month exceeds that in the above definition, so other factors must contribute to savanna formation.

在许多特征上，热带稀树草原的土壤与一些热带雨林的土壤相似，尽管更为极端。

In many characteristics, savanna soils are similar to those of some rain forests, though more extreme.

例如，稀树草原土壤，像许多雨林土壤一样，是典型的氧化土（由某些氧化物矿物占主导地位）和老成土（不含碳酸钙的土壤），具有高酸性，磷、钙、镁和钾等矿物的浓度很低，而铝的浓度很高。

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.

一些稀树大草原生长在潮湿、涝渍的土壤上；其它则在干燥、多沙、排水良好的土壤上。

Some savannas occur on wet, waterlogged soils; others on dry, sandy, well-drained soils.

这似乎是矛盾的，但这只意味着对森林来说太湿或太干的极端土壤条件，对草原来说是符合要求的。

This may seem contradictory, but it only means that extreme soil conditions, either too wet or too dry for forests, are satisfactory for savannas.

更温和的条件才能支持潮湿的森林。

More moderate conditions support moist forests.

涝渍土壤发生在地势平坦或排水不畅的地区。

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

这些土壤通常含有大量的粘土，很容易被水饱和。

These soils usually contain large amounts of clay and easily become water-saturated.

空气不能穿透土壤颗粒，使土壤缺氧。

Air cannot penetrate between the soil particles, making the soil oxygen-poor.

相比之下，干燥的土壤是沙质和多孔的，其粗糙的质地允许水分迅速流失。

By contrast, dry soils are sandy and porous, their coarse textures permitting water to drain rapidly.

沙质土壤易于养分和矿物质的浸出，因此往往营养不良。

Sandy soils are prone to the leaching of nutrients and minerals and so tend to be nutritionally poor.

尽管大部分的热带草原都是在土壤贫瘠的地方（因为水分条件或两者的营养水平），贫瘠的土壤可以而且确实支持茂盛的雨林。

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.

大多数稀树草原可能经常发生轻微火灾，大约每两年就会发生一次大火灾。

Most savannas probably experience mild fires frequently and major burns every two years or so.

许多热带稀树草原和干森林植物物种被称为耐火植物，这意味着它们适应以各种方式抵御偶尔的燃烧。

Many savanna and dry-forest plant species are called pyrophytes, meaning they are adapted in various ways to withstand occasional burning.

频繁的火灾是雨林物种似乎无法适应的一个因素，尽管来自亚马逊森林土壤的远古木炭遗迹可以追溯到人类到来之前，这表明潮湿的森林偶尔也会燃烧。

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.

实验表明，如果美洲大草原没有发生火灾，物种组成将发生显著变化。

Experiments suggest that if fire did not occur in savannas in the Americas, species composition would change significantly.

当燃烧发生时，它阻止了植物物种之间的竞争发展到某些物种排斥其他物种的程度，从而减少了生态系统的整体多样性。

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.

但在受保护而免于火灾的实验区，一些多年生草本植物最终占据了主导地位，超过了其他所有植物。

But in experimental areas protected from fire, a few perennial grass species eventually come to dominate, outcompeting all others.

来自其他研究的证据表明，火灾的消灭导致植物物种丰富度显著下降，通常伴随着树木密度的增加。

Evidence from other studies suggests that exclusion of fire results in markedly decreased plant-species richness, often with an increase in tree density.

毫无疑问，火灾是维持稀树草原的一个重要因素，在大多数地区都是如此。

There is generally little doubt that fire is a significant factor in maintaining savanna, certainly in most regions.

在某些地方，特别是在南美洲，稀树草原的形成似乎与频繁地砍伐和焚烧潮湿的森林作为牧场有关。

On certain sites, particularly in South America, savanna formation seems related to frequent cutting and burning of moist forests for pastureland.

草场的增加和随之而来的过度放牧导致了稀树草原的扩张。

Increase in pastureland and subsequent overgrazing have resulted in an expansion of savanna.

上层的腐殖质（腐烂的有机物）被切割和燃烧所破坏。

The thin upper layer of humus (decayed organic matter) is destroyed by cutting and burning.

腐殖质对于细菌和真菌对叶片的快速分解和表层根的循环利用是必要的。

Humus is necessary for rapid decomposition of leaves by bacteria and fungi and for recycling by surface roots.

一旦腐殖质层消失，养分就无法从土壤中回收和过滤，使土壤由肥沃变为贫瘠，只适合于热带草原植被。

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.

白色沙质土壤上的森林最容易发生永久性的变化。

Forests on white, sandy soil are most susceptible to permanent alteration.

The word “prolonged” in the passage is closest in meaning to

In paragraph 2, the author mentions savannas in Central America, Brazil, and the island of Trinidad in order to

According to paragraph 3, rain forests and savannas differ in that

According to paragraph 3, all of the following are true of savanna soils EXCEPT

According to paragraph 4, which of the following is true of waterlogged soils?

Which of the sentences below best expresses the essential information in the highlighted sentence in the passage? Incorrect choices change the meaning in important ways or leave out essential information.

Paragraph 5 supports which of the following statements regarding the importance of fires in maintaining savannas?

The word “subsequent” in the passage is closest in meaning to

Look at the four squares  that indicate where the following sentence could be added to the passage. In addition, humans have contributed to the conditions favoring the formation of savannas.  Where would the sentence best fit? Click on a square  to add the sentence to the passage.

Directions: An introductory sentence for a brief summary of the passage is provided below. Complete the summary by selecting the THREE answer choices that express the most important ideas in the passage. Some sentences do not belong in the summary because they express ideas that are not presented in the passage or are minor ideas in the passage. This question is worth 2 points.

Savanna Formation 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 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 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.