Thetransitionfrom forest to treelesstundraon a mountainslopeis often adramaticone. Withinaverticaldistance of just a few tens of meters, trees disappear as a life-form and are replaced by low shrubs, herbs, and grasses. This rapidzoneoftransitionis called the uppertimberlineor treeline. In manysemiaridareas there is also alowertimberlinewhere the forest passes intosteppeordesertat itsloweredge, usually because of alackofmoisture.

The uppertimberline, like the snowline, is highest in the tropics and lowest in the Polar Regions. It ranges from sealevelin the Polar Regions to 4,500 meters in the dry subtropics and 3,500-4,500 meters in themoisttropics. Timberlinetrees arenormallyevergreens, suggesting that these have someadvantageoverdeciduoustrees (those that lose their leaves) in theextremeenvironments of the uppertimberline. There are some areas, however, wherebroadleafdeciduoustrees form thetimberline. Speciesofbirch, for example, mayoccurat thetimberlinein parts of the Himalayas.

At the uppertimberlinethe trees begin to become twisted and deformed. This is particularly true for trees in the middle and upper latitudes, whichtendtoattaingreater heights on ridges, whereas in the tropics the treesreachtheir greater heights in the valleys. This is because middle- and upper-latitudetimberlines are strongly influenced by thedurationand depth of the snow cover. As the snow is deeper and lasts longer in the valleys, treestendtoattaingreater heights on the ridges,eventhough they are moreexposedto high-velocity winds and poor, thin soils there. In the tropics, the valleysappearto be morefavorablebecause they are lessproneto dry out, they have lessfrost, and they have deeper soils.

There is still no universally agreed-on explanation for why there should be such adramaticcessation of tree growth at the uppertimberline. Variousenvironmentalfactors may play a role. Too much snow, for example, cansmothertrees, and avalanches and snowcreepcan damage or destroy them. Late-lying snow reduces theeffectivegrowingseasonto the point where seedlings cannotestablishthemselves. Windvelocityalso increases withaltitudeand may cause seriousstressfor trees, as is madeevidentby the deformed shapes at high altitudes. Some scientists have proposed that the presence of increasing levels ofultravioletlight withelevationmay play a role, while browsing andgrazinganimals like the ibex may be another contributingfactor. Probably the most importantenvironmentalfactoristemperature, for if the growingseasonis too short and temperatures are too low, tree shoots and buds cannotmaturesufficientlytosurvivethe winter months.

Above the treelinethere is azonethat is generally calledalpinetundra.

Immediatelyadjacentto thetimberline, thetundraconsists of afairlycomplete cover oflow-lyingshrubs, herbs,

and grasses, while higher up the number anddiversityofspeciesdecreaseuntil there is muchbareground withoccasionalmosses and lichens and some prostratecushionplants. Some plants canevensurviveinfavorablemicrohabitats above the snowline.

The highest plants in the worldoccurat around 6,100 meters on Makalu in the Himalayas. At this great height, rocks, warmed by the sun,meltsmall snowdrifts.

The moststrikingcharacteristicof the plants of thealpinezoneis their low growth form. This enables them toavoidthe worst rigors of high winds and permits them to make use of the higher temperatures immediatelyadjacentto the ground surface. In an area where low temperatures are limiting to life, the importance of theadditionalheat near the surface iscrucial. The low growth form can alsopermitthe plants to takeadvantageof theinsulationprovided by a winter snow cover. In theequatorialmountains the low growth form is lessprevalent.

Questions:
question_stem: The word “dramatic” in the passage is closest in meaning to
choices: tag: A
text: gradual

tag: B
text: complex

tag: C
text: visible

tag: D
text: striking
answer_key: D
explanation: "dramatic: 剧烈的，戏剧化的，就单词本身能够想到 drama 戏剧，所以这个应该是戏剧的形容词。A 是逐渐的；B 是复杂的；C 是可见的；D 是显著的，突出的，惊人的。根据词意，正确答案是 D。

question_stem: Where is the lower timberline mentioned in paragraph 1 likely to be found?
choices: tag: A
text: In an area that has little water

tag: B
text: In an area that has little sunlight

tag: C
text: Above a transition area

tag: D
text: On a mountain that has on upper timberline.
answer_key: A
explanation: 以 lower timberline 做关键词定位至本段最后一句，说有 lower timberline 是因为 a lack of moisture，缺乏湿度，等于 A 选项中的没有水。

question_stem: Which of the following can be inferred from paragraph 1 about both the upper and lower timberlines?
choices: tag: A
text: Both are treeless zones.

tag: B
text: Both mark forest boundaries.

tag: C
text: Both are surrounded by desert areas.

tag: D
text: Both suffer from a lack of moisture.
answer_key: B
explanation: "分别以 upper timberline 和 lower timberline 为关键词定位至本段倒数两句，不管是 upper timberline 还是 lower timberline 都是快速的过渡带，树还是有的，只是在向草原等等过渡，所以 A 错，既然是过渡，也就是边界了，B 对，C 没说，只有 lower timberline 缺水，upper timberline 并没有说缺水，所以 D 错。"

question_stem: Paragraph 2 supports which of the following statements about deciduous trees?
choices: tag: A
text: They cannot grow in cold climates.

tag: B
text: They do not exist at the upper timberline.

tag: C
text: They are less likely than evergreens to survive at the upper timberline.

tag: D
text: They do not require as much moisture as evergreens do.
answer_key: C
explanation: 以 deciduous trees 做关键词定位至原文的第三句和第四句，根据第四句说有的地方的 timberline 是由落叶树构成的，所以 timberline 上还是有落叶树的，所以选项 A 和 B 说反，D 项 moisture 原文没说，第三句说 timberline 通常是常绿树构成的，第四句说有的时候也出现落叶树，所以常绿树比落叶树出现的概率大，所以 C 正确。

question_stem: The word “attain” in the passage is closest in meaning to
choices: tag: A
text: require

tag: B
text: resist

tag: C
text: achieve

tag: D
text: endure
answer_key: C
explanation: "attain: 获得。联想到相似的 obtain（获得）和 retain（保留）。A 是要求；B 是抵抗；C 是获得；D 是忍耐。根据词意，C 是正确答案。

question_stem: The word “they” in the passage refers to
choices: tag: A
text: valleys

tag: B
text: trees

tag: C
text: heights

tag: D
text: ridges
answer_key: B
explanation: "并列句，往前找，找主语，trees 是正确答案，此外，被暴露在大风之下的应该是树，山谷山脊和高度都不靠谱。"

question_stem: The word “prone” in the passage is closest in meaning to
choices: tag: A
text: adapted

tag: B
text: likely

tag: C
text: difficult

tag: D
text: resistant
answer_key: B
explanation: prone: 有…的倾向。A 是适应；B 是很可能；C 是困难的；D 是抵抗的。根据词意，B 是正确答案。对应原文：In the tropics, the valleys appear to be more favorable because they are less prone to dry out, they have less frost, and they have deeper soils.原文说热带地区山谷是更有利于生长的地方因为那里怎么样干涸，比较有利生长当然不容易干涸，C 和 D 的意思都是能干涸，所以都不对，A 适应不靠谱，所以 B 正确。

question_stem: According to paragraph 3, which of the following is true of trees in the middle and upper latitudes?
choices: tag: A
text: Tree growth is negatively affected by the snow cover in valleys

tag: B
text: Tree growth is greater in valleys than on ridges.

tag: C
text: Tree growth on ridges is not affected by high-velocity winds.

tag: D
text: Tree growth lasts longer in those latitudes than it does in the tropics.
answer_key: A
explanation: 以 middle and upper latitudes 做关键词定位至原文第三四两句，说中高纬度地区树木在很大程度上受到积雪覆盖的时长和深度的影响，山谷地区积雪深，时间长，所以树长得不好，所以是 negatively affected，选项 B 和 C 与原文相反，D 所做比较原文没有提到。

question_stem: Which of the sentences below best express the essential information in the highlighted sentence in the passage? In correct choices change the meaning in important ways or leave out essential information.
choices: tag: A
text: Because of their deformed shapes at high altitudes, trees are not likely to be seriously harmed by the strong winds typical of those altitudes.

tag: B
text: As altitude increases, the velocity of winds increase, leading to a serious decrease in the number of trees found at high altitudes.

tag: C
text: The deformed shapes of trees at high altitudes show that wind velocity, which increase with altitude, can cause serious hardship for trees.

tag: D
text: Increased wind velocity at high altitudes deforms the shapes of trees, and this may cause serious stress for trees.
answer_key: C
explanation: "原句说风速增加而且会给树木很大压力，已经被 blablabla 证实。C 是原句的忠实改写，所以 C 正确。A 的因果关系在原文不存在，错；B 数目减少没说到，错；D 把原文非主要成分变成主要成分，改变了原文的结构，错。"

question_stem: In paragraph 4, what is the author’s main purpose in the discussion of the dramatic cessation of tree growth at the upper timberline?
choices: tag: A
text: To argue that none of several environment factors that are believed to contribute to that phenomenon do in fact play a role in causing it.

tag: B
text: To argue in support of one particular explanation of that phenomenon against several competing explanations

tag: C
text: To explain why the primary environmental factor responsible for that phenomenon has not yet been identified

tag: D
text: To present several environmental factors that may contribute to a satisfactory explanation of that phenomenon
answer_key: D
explanation: 以 dramatic cessation of tree growth at the timberline 为关键词定位至本段第一句，说对于这个 cessation 没有统一的解释，接着后面就说了有好多种解释，作者没说他认为哪个对，也没解释为什么没有一致意见，所以答案是 D，提供了多种解释，A 说所有都不对，B 说支持其中一个，C 说解释为什么没有一致意见都错了。

question_stem: The word “prevalent” in the passage is closest in meaning to
choices: tag: A
text: predictable

tag: B
text: widespread

tag: C
text: successful

tag: D
text: developed
answer_key: B
explanation: "prevalent: 普遍的，常见的。A 是可预测的；B 是普遍的；C 是成功的；D 是发达的，成熟的。根据词意，B 正确。

question_stem: According to paragraph 6, all of the following statements are true of plants in the alpine zone EXCEPT:
choices: tag: A
text: Because they are low, they are less exposed to strong winds.

tag: B
text: Because they are low, the winter snow cover gives them more protection from the extreme cold.

tag: C
text: In the equatorial mountains, they tend to be lower than in mountains elsewhere.

tag: D
text: Their low growth form keeps them closer to the ground, where there is more heat than further up.
answer_key: C
explanation: EXCEPT 题排除法，以 wind 做关键词定位至本段第二句，A 对，不选；以 winter snow cover 做关键词定位至倒数第二句，B 对，不选；以 equatorial mountains 做关键词定位至最后一句，原文说 less prevalent，不那么广泛分布，不是选项 C 所说的低，所以 C 错，选 C；D 选项与 A 选项在同一句，注意 heat 和 higher temperature 的替换，不选。