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英['lɪs(ə)n] 美['lɪsn]
vi. 听,倾听;听从,听信
n. 听,倾听

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草莓小菇凉:说的非常好,十分有道理,棒棒棒!

06-08 15:44:55

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TPO-01-Timberline Vegetation on Mountains

The transition from forest to treeless tundra on a mountain slope is often a dramatic one. Within a vertical distance of just a few tens of meters, trees disappear as a life-form and are replaced by low shrubs, herbs, and grasses. This rapid zone of transition is called the upper timberline or tree line. In many semiarid areas there is also a lower timberline where the forest passes into steppe or desert at its lower edge, usually because of a lack of moisture.

The upper timberline, like the snow line, is highest in the tropics and lowest in the Polar Regions. It ranges from sea level in the Polar Regions to 4,500 meters in the dry subtropics and 3,500-4,500 meters in the moist tropics. Timberline trees are normally evergreens, suggesting that these have some advantage over deciduous trees (those that lose their leaves) in the extreme environments of the upper timberline. There are some areas, however, where broadleaf deciduous trees form the timberline. Species of birch, for example, may occur at the timberline in parts of the Himalayas.

At the upper timberline the trees begin to become twisted and deformed. This is particularly true for trees in the middle and upper latitudes, which tend to attain greater heights on ridges, whereas in the tropics the trees reach their greater heights in the valleys. This is because middle- and upper- latitude timberlines are strongly influenced by the duration and depth of the snow cover. As the snow is deeper and lasts longer in the valleys, trees tend to attain greater heights on the ridges, even though they are more exposed to high-velocity winds and poor, thin soils there. 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.

There is still no universally agreed-on explanation for why there should be such a dramatic cessation of tree growth at the upper timberline. Various environmental factors may play a role. Too much snow, for example, can smother trees, and avalanches and snow creep can damage or destroy them. Late-lying snow reduces the effective growing season to the point where seedlings cannot establish themselves. Wind velocity also increases with altitude and may cause serious stress for trees, as is made evident by the deformed shapes at high altitudes. Some scientists have proposed that the presence of increasing levels of ultraviolet light with elevation may play a role, while browsing and grazing animals like the ibex may be another contributing factor. Probably the most important environmental factor is temperature, for if the growing season is too short and temperatures are too low, tree shoots and buds cannot mature sufficiently to survive the winter months.

Above the tree line there is a zone that is generally called alpine tundra. Immediately adjacent to the timberline, the tundra consists of a fairly complete cover of low-lying shrubs, herbs, and grasses, while higher up the number and diversity of species decrease until there is much bare ground with occasional mosses and lichens and some prostrate cushion plants. Some plants can even survive in favorable microhabitats above the snow line. The highest plants in the world occur at around 6,100 meters on Makalu in the Himalayas. At this great height, rocks, warmed by the sun, melt small snowdrifts.

The most striking characteristic of the plants of the alpine zone is their low growth form. This enables them to avoid the worst rigors of high winds and permits them to make use of the higher temperatures immediately adjacent to the ground surface. In an area where low temperatures are limiting to life, the importance of the additional heat near the surface is crucial. The low growth form can also permit the plants to take advantage of the insulation provided by a winter snow cover. In the equatorial mountains the low growth form is less prevalent.

在山坡上从森林到无树苔原的过渡通常是一个戏剧性的。在几十米的垂直距离内,树木消失成为一种生命形式,被低矮的灌木,草药和草所取代。这个快速的过渡地带被称为上部林线或树线。在许多半干旱地区,还有一条较低的林线,森林通常由于缺水而进入草原或沙漠的下缘。

 

上部的林线,如雪线,在热带地区最高,在极地地区最低。其范围从极地海域到干热带亚热带海域4500米,湿热带海域3500米到4500米。林线树木通常是常绿树种,这表明这些树木在上部林线的极端环境中比落叶树(失去叶子的树木)具有一些优势。然而,在一些地区,阔叶落叶乔木形成了林线。例如,可能在喜马拉雅山的部分地区的林线上出现桦树种。

 

在上部林线,树木开始变形和变形。对于中高纬度的树木来说,尤其如此,树木在山脊上的高度往往更高,而在热带地区,树木在山谷中达到更高的高度。这是因为中高纬度的林线受到积雪的持续时间和深度的强烈影响。由于雪越深,山谷越深,树木往往会在山脊上达到更高的高度,尽管它们更容易暴露在高速风和贫瘠薄弱的土壤中。在热带地区,山谷似乎更加有利,因为它们不易干燥,霜冻较少,土壤较深。

 

对于为什么在上部林线应该如此戏剧性地停止树木生长,目前还没有普遍认同的解释。各种环境因素可能起作用。例如,太多的积雪会扼杀树木,雪崩和积雪可能会损坏或摧毁它们。傍晚的积雪把有效的生长季节降到了幼苗不能建立的地步。风速也随着高度的增加而增加,并且可能对树木造成严重的压力,高海拔地区的变形形状就可以看出这一点。一些科学家提出,随着海拔的升高紫外线的存在可能起到一定的作用,而浏览和放牧像北极熊这样的动物可能是另一个促成因素。可能最重要的环境因素是温度,因为如果生长季节太短而温度太低,树梢和花蕾就不能充分地成熟以在冬季生存。

 

树线上方有一个通常称为高山冻原的地带。在紧邻林线的地带,苔原由一种相当完整的低矮灌木,草本植物和草类组成,而物种的数量和多样性则越来越高,直到有很多裸地和偶尔的苔藓和地衣以及一些前躯垫植物。有些植物甚至可以在雪线以上的有利微生境中生存。世界上最高的植物发生在喜马拉雅山脉马卡鲁峰海拔约6100米处。在这个高度,岩石,被太阳升起,融化小雪堆。

 

高山地区植物最显着的特征是低生长型。这使他们能够避免最恶劣的强风,并允许他们利用紧邻地面的较高温度。在低温限制生命的地区,靠近地表的附加热量的重要性至关重要。低增长形式也可以使植物利用冬季积雪提供的隔热。在赤道山脉中,低增长形式不太普遍。

中文翻译

The word “they” in the passage refers to

  • As the snow is deeper and lasts longer in the valleys, trees tend to attain greater heights on the ridges, even though they are more exposed to high-velocity winds and poor, thin soils there. In the tropics, the valleys appear to be more favorable because A valleys B trees C heights D ridge they
正确答案: B

网友解析

  • 用户头像

    十三个  发表于 2018-10-21 21:07:58

    题目解析:"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。"

  • 用户头像

    joannazz  发表于 2017-12-05 15:26:53

    代题看前文,且这是并列句找主干,前句trees是主语,所以they指代trees

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