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托福考试TPO31阅读真题Savanna Formation

2017-11-17 10:38
摘要:在备考托福的过程中,Tpo是大家的必备材料。那么,在以下内容中100留学就为大家带来托福考试TPO31阅读真题Savanna Formation。

以下就是100留学为大家整理的托福考试TPO31阅读真题Savanna Formation。希望对各位考生的备考有所帮助:


Para.1  Locatedin tropical areas at low altitudes, savannas are stable ecosystems, some wetand 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 isno 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.


Para.2  Savannastypically experience a rather prolonged dry season. One theory behind savannaformation is that wet forest species are unable to withstand the dry season,and thus savanna, rather than rain forest, is favored on the site. Savannasexperience an annual rainfall of between 1,000 and 2,000 millimeters, most ofit falling in a five- to eight-month wet season. Though plenty of rain may fallon a savanna during the year, for at least part of the year little does,creating the drought stress ultimately favoring grasses. Such conditionsprevail throughout much of northern South America and Cuba, but many CentralAmerican savannas as well as coastal areas of Brazil and the island of Trinidaddo not fit this pattern. In these areas, rainfall per month exceeds that in theabove definition, so other factors must contribute to savanna formation.


Para.3  Inmany 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 (soilscontaining no calcium carbonate), with a high acidity and notably lowconcentrations of such minerals as phosphorus, calcium, magnesium, andpotassium, while aluminum levels are high. Some savannas occur on wet,waterlogged soils; others on dry, sandy, well-drained soils. This may seemcontradictory, but it only means that extreme soil conditions, either too wetor too dry for forests, are satisfactory for savannas. More moderate conditionssupport moist forests.


Para.4  Waterloggedsoils occur in areas that are flat or have poor drainage. These soils usuallycontain large amounts of clay and easily become water saturated. Air cannotpenetrate between the soil particles, making the soil oxygen-poor. By contrast,dry soils are sandy and porous, their coarse textures permitting water to drainrapidly. Sandy soils are prone to the leaching of nutrients and minerals and sotend to be nutritionally poor. Though most savannas are found on sites with poorsoils (because of either moisture conditions or nutrient levels of both), poorsoils can and do support lush rain forest.


Para.5  Mostsavannas probably experience mild fires frequently and major burns every twoyears 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 thearrival 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, itprevents competition among plant species from progressing to the point wheresome species exclude others, reducing the overall diversity of the ecosystem.But in experimental areas protected from fire, a few perennial grass specieseventually come to dominate, outcompeting all others. Evidence from otherstudies suggests that exclusion of fire results in markedly decreasedplant-species richness, often with an increase in tree density. There isgenerally little doubt that fire is a significant factor in maintainingsavanna, certainly in most regions.


Para.6  Oncertain sites, particularly in South America, savanna formation seems relatedto frequent cutting and burning of moist forests for pastureland. Increase inpastureland and subsequent overgrazing have resulted in an expansion of savanna.The thin thin upper layer of humus (decayed organic matter) is destroyed bycutting and burning. Humus is necessary for rapid decomposition of leaves bybacteria and fungi and for recycling by surface roots. Once the humus layerdisappears, nutrients cannot be recycled and leach from the soil, convertingsoil from fertile to infertile and making it suitable only for savannavegetation. Forests on white, sandy soil are most susceptible to permanentalteration. 

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Para.1  Locatedin tropical areas at low altitudes, savannas are stable ecosystems, some wetand 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 isno 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.


Para.2  Savannastypically experience a rather prolonged dry season. One theory behind savannaformation is that wet forest species are unable to withstand the dry season,and thus savanna, rather than rain forest, is favored on the site. Savannasexperience an annual rainfall of between 1,000 and 2,000 millimeters, most ofit falling in a five- to eight-month wet season. Though plenty of rain may fallon a savanna during the year, for at least part of the year little does,creating the drought stress ultimately favoring grasses. Such conditionsprevail throughout much of northern South America and Cuba, but many CentralAmerican savannas as well as coastal areas of Brazil and the island of Trinidaddo not fit this pattern. In these areas, rainfall per month exceeds that in theabove definition, so other factors must contribute to savanna formation.



Para.3  Inmany 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 (soilscontaining no calcium carbonate), with a high acidity and notably lowconcentrations of such minerals as phosphorus, calcium, magnesium, andpotassium, while aluminum levels are high. Some savannas occur on wet,waterlogged soils; others on dry, sandy, well-drained soils. This may seemcontradictory, but it only means that extreme soil conditions, either too wetor too dry for forests, are satisfactory for savannas. More moderate conditionssupport moist forests.


Para.4  Waterloggedsoils occur in areas that are flat or have poor drainage. These soils usuallycontain large amounts of clay and easily become water saturated. Air cannotpenetrate between the soil particles, making the soil oxygen-poor. By contrast,dry soils are sandy and porous, their coarse textures permitting water to drainrapidly. Sandy soils are prone to the leaching of nutrients and minerals and sotend to be nutritionally poor. Though most savannas are found on sites withpoor soils (because of either moisture conditions or nutrient levels of both),poor soils can and do support lush rain forest.



Para.5  Mostsavannas probably experience mild fires frequently and major burns every twoyears or so. Many savanna and dry-forest plant species are called pyrophytes,meaning they are adapted in various ways to withstand occasional burning. Frequentfire is a factor to which rain forest species seem unable to adapt, althoughancient charcoal remains from Amazon forest soils dating prior to the arrivalof humans suggest that moist forests also occasionally burn. Experimentssuggest that if fire did not occur in savannas in the Americas, speciescomposition would change significantly. When burning occurs, it preventscompetition among plant species from progressing to the point where somespecies exclude others, reducing the overall diversity of the ecosystem. But inexperimental areas protected from fire, a few perennial grass specieseventually come to dominate, outcompeting all others. Evidence from otherstudies suggests that exclusion of fire results in markedly decreasedplant-species richness, often with an increase in tree density. There isgenerally little doubt that fire is a significant factor in maintainingsavanna, certainly in most regions.


Para.6  Oncertain sites, particularly in South America, savanna formation seems relatedto frequent cutting and burning of moist forests for pastureland. Increase inpastureland and subsequent overgrazing have resulted in an expansion ofsavanna. The thin upper layer of humus (decayed organic matter) is destroyed bycutting and burning. Humus is necessary for rapid decomposition of leaves bybacteria and fungi and for recycling by surface roots. Once the humus layerdisappears, nutrients cannot be recycled and leach from the soil, convertingsoil from fertile to infertile and making it suitable only for savannavegetation. Forests on white, sandy soil are most susceptible to permanentalteration.

========================


Para.1  Locatedin tropical areas at low altitudes, savannas are stable ecosystems, some wetand 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 isno 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.


Para.2  Savannastypically experience a rather prolonged dry season. One theory behind savannaformation 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 experiencean annual rainfall of between 1,000 and 2,000 millimeters, most of it fallingin a five- to eight-month wet season. Though plenty of rain may fall on asavanna during the year, for at least part of the year little does, creatingthe drought stress ultimately favoring grasses. Such conditions prevailthroughout much of northern South America and Cuba, but many Central Americansavannas as well as coastal areas of Brazil and the island of Trinidad do notfit this pattern. In these areas, rainfall per month exceeds that in the abovedefinition, so other factors must contribute to savanna formation.


1.   Theword “prolonged” in the passage is closest in meaning to:

o    Predictable

o    Destructive

o    Lengthy

o    Unproductive


2.   Inparagraph 2, the author mentions savannas in Central America, Brazil, and theisland of Trinidad in order to:

o    Arguethat these savannas are similar to those in South America and Cuba

o    Pointout exceptions to the pattern of savanna formation in areas with drought stress

o    Providesadditional examples of savannas in areas with five- to eight-month wet seasons

o    Indicateareas where savannas are being gradually replaced by rain forests


Para.3  Inmany 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 (soilscontaining no calcium carbonate), with a high acidity and notably lowconcentrations of such minerals as phosphorus, calcium, magnesium, andpotassium, while aluminum levels are high. Some savannas occur on wet,waterlogged soils; others on dry, sandy, well-drained soils. This may seemcontradictory, but it only means that extreme soil conditions, either too wet ortoo dry for forests, are satisfactory for savannas. More moderate conditionssupport moist forests.


3.   Accordingto paragraph 3, rain forests and savannas differ in that:

o    Thesoils in rain forests contain fewer minerals than savanna soils do

o    Savannasaffect soil conditions more than rain forests do

o    Unlikerain forests, savannas prefer sandy, well-drained soils to soils that are verywet

o    Unlikerain forests, savannas may develop under both very dry and very wet soilconditions


4.   Theword “notably” in the passage is closest in meaning to:

o    Similarly

o    Especially

o    Usually

o    Relatively


5.   Accordingto paragraph 3, all of the following are true of savanna soils EXCEPT:

o    Theyhave high concentrations of potassium.

o    Theycontain high levels of aluminum.

o    Theyare very acidic.

o    Theycontain large amounts of certain oxide minerals.


Para.4  Waterloggedsoils occur in areas that are flat or have poor drainage. These soils usuallycontain large amounts of clay and easily become water saturated. Air cannotpenetrate between the soil particles, making the soil oxygen-poor. By contrast,dry soils are sandy and porous, their coarse textures permitting water to drainrapidly. Sandy soils are prone to the leaching of nutrients and minerals and sotend to be nutritionally poor. Though most savannas are found on sites withpoor soils (because of either moisture conditions or nutrient levels of both),poor soils can and do support lush rain forest.


6.   Accordingto paragraph 4, which of the following is true of waterlogged soils?

o    Theirupper layers are usually sandy and porous.

o    Theycannot support savannas.

o    Theycontain little oxygen.

o    Theyare prone to the leaching of nutrients and minerals.


7.   Thefact that “poor soils can and do support lush rain forest” suggests that:

o    Poorsoils alone may not be enough to explain why an area becomes a savanna

o    Rainforest vegetation can significantly lower the quality of soils

o    Droughtstress is the single most important factor in savanna formation

o    Mineralsare more important than moisture for the growth of trees



Para.5  Mostsavannas probably experience mild fires frequently and major burns every twoyears 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 thearrival 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, itprevents competition among plant species from progressing to the point wheresome species exclude others, reducing the overall diversity of the ecosystem.But in experimental areas protected from fire, a few perennial grass specieseventually come to dominate, outcompeting all others. Evidence from otherstudies suggests that exclusion of fire results in markedly decreasedplant-species richness, often with an increase in tree density. There isgenerally little doubt that fire is a significant factor in maintainingsavanna, certainly in most regions.


8.   Whichof the sentence below best expresses the essential information in thehighlighted sentence in the passage? Incorrect choices change the meaning inimportant ways or leave out essential information.

o    Rainforest species seem unable to adapt to fires created by humans.

o    Ancientcharcoal remains suggest that, prior to the arrival of humans, fires occurredfrequently in rain forests.

o    Ancientcharcoal remains in Amazon forests suggest that rain forest species adapted tothe area long before the arrival of humans.

o    Rainforests species appear unable to adapt to frequent fires, but evidence from thepast suggests that rain forests sometimes do burn.


9.   Theword “markedly” in the passage is closest in meaning to:

o    Dangerously

o    Noticeably

o    Rapidly

o    Gradually


10. Paragraph5 supports which of the following statements regarding the importance of firesin maintaining savannas?

o    Firesprevent the growth of pyrophytes.

o    Fireseliminate some species and thus reduce the overall diversity of the ecosystem.

o    Firesthat occur once every two years are unlikely to help maintain savannas.

o    Firesprevent some species from eliminating other species with which they compete.


Para.6  Oncertain sites, particularly in South America, savanna formation seems relatedto frequent cutting and burning of moist forests for pastureland. Increase inpastureland and subsequent overgrazing have resulted in an expansion ofsavanna. The thin upper layer of humus (decayed organic matter) is destroyed bycutting and burning. Humus is necessary for rapid decomposition of leaves bybacteria and fungi and for recycling by surface roots. Once the humus layerdisappears, nutrients cannot be recycled and leach from the soil, convertingsoil from fertile to infertile and making it suitable only for savannavegetation. Forests on white, sandy soil are most susceptible to permanentalteration.


11. Theword “subsequent” in the passage is closest in meaning to:

o    Expandedin area

o    Harmful

o    Followingin time

o    Repeated


12. Accordingto paragraph 6, human activity affects soils in all of the following waysEXCEPT:

o    Decompositionof leaves occurs too fast for surface roots to obtain nutrients.

o    Nutrientsare not recycled.

o    Humusis destroyed.

o    Certainsoils become unable to support vegetation other than savanna vegetation.


13. Inaddition, humans have contributed to the conditions favoring the formation ofsavannas.

Where would the sentence best fit?


Para.7  Mostsavannas probably experience mild fires frequently and major burns every twoyears 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 thearrival 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, itprevents competition among plant species from progressing to the point wheresome species exclude others, reducing the overall diversity of the ecosystem.But in experimental areas protected from fire, a few perennial grass specieseventually come to dominate, outcompeting all others. 1Evidence from other studies suggests that exclusion of fire resultsin markedly decreased plant-species richness, often with an increase in treedensity. 2There is generallylittle doubt that fire is a significant factor in maintaining savanna, certainlyin most regions.


Para.8  3On certain sites, particularly in SouthAmerica, savanna formation seems related to frequent cutting and burning ofmoist forests for pastureland. 4Increase in pastureland and subsequent overgrazing have resulted inan expansion of savanna. The thin upper layer of humus (decayed organic matter)is destroyed by cutting and burning. Humus is necessary for rapid decompositionof leaves by bacteria and fungi and for recycling by surface roots. Once thehumus layer disappears, nutrients cannot be recycled and leach from the soil,converting soil from fertile to infertile and making it suitable only forsavanna vegetation. Forests on white, sandy soil are most susceptible topermanent alteration.


14. ProseSummary

Several factors seem to play importantroles in savanna formation.

Answer Choices

A.   Savannascan form in areas with a five- to eight-month wet season, but they morecommonly have a longer wet season.    

D.Drought stress affects trees and shrubs in savannas far less than it affectssavanna grasses.

B.   Soilstress caused by drought, extreme moisture, or low nutrient levels favors theformation of savannas.

E.Frequent fires is a major factor contributing to the formation and maintenanceof savannas.

C.   Studiesconducted in various regions indicate that an upper layer of white, sandy, soilis present in most permanent savannas.     

F.In some areas, human cutting and burning is associated with savanna formation,and increase in pastureland has led to savanna expansion.

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