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山坡上从森林到草木不生的苔原地带的过渡常常是非常突兀的。

The transition from forest to treeless tundra on a mountain slope is often a dramatic one.

仅仅在几十米的垂直高度里，森林这种生命形式突然消失，取而代之的是低矮灌木与草丛。

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.

这个快速变化的过渡带被称为上行树带界限或林木线。

This rapid zone of transition is called the upper timberline or tree line.

在很多半干旱地区，还有下行树带界限，在那里，由于缺乏水分，森林会直接变成干草原或沙漠。

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.

上行树带界限，比如雪线，在热带最高，在极地最低。

The upper timberline, like the snow line, is highest in the tropics and lowest in the polar regions.

它从极地的海平面延伸至海拔4500米的干燥亚热带与海拔3500米-4500米的潮湿热带。

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.

树带界限上的树通常都是常青树，这表明，与位于极端环境中的上行树带界限上的落叶树木相比，它们更有优势。

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.

然而，也有一些地区，林木线是由落叶阔叶树组成的。

There are some areas, however, where broadleaf deciduous trees form the timberline.

比如说，部分喜马拉雅山的林木线是由桦树组成的。

Species of birch, for example, may occur at the timberline in parts of the Himalayas.

在上行树带界限之上，树木开始扭曲变形。

At the upper timberline the trees begin to become twisted and deformed.

特别是那些生长在中高纬度的树，它们在山脊上会长的更高。然而，生长在热带地区的树，通常是那些生长在山谷里的树更高。

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.

这是因为中高纬度的树带线受积雪的堆积时间与深度影响较大。

This is because middle- and upper-latitude timberlines are strongly influenced by the duration and depth of the snow cover.

因为山谷里的积雪更厚、堆积时间更长，所以山脊上的树长得更高，尽管山脊上的风速很高，土壤很薄。

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.

在热带，山谷是最有利草木生长的地带，因为这里不容易干旱，霜冻更少，以及土壤更厚。

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.

至今关于为什么上行树带界限以上没有树木生长仍没有统一的解释。

There is still no universally agreed-on explanation for why there should be such a dramatic cessation of tree growth at the upper timberline.

可能各种环境因素在其中起了作用。

Various environmental factors may play a role.

比如，太多的雪会覆盖在树上隔绝了空气，雪崩及雪蠕变会损坏树木。

Too much snow, for example, can smother trees, and avalanches and snow creep can damage or destroy them.

长期积雪降低了生长季的有效性，它会导致种子无法生根发芽。

Late-lying snow reduces the effective growing season to the point where seedlings cannot establish themselves.

风速会随着高度的增加而增大，这会给树带来很大的压力，这从那些位于高处已经扭曲变形的树上就能看出来。

Wind velocity also increases with altitude and may cause serious stress for trees, as is made evident by the deformed shapes at high altitudes.

一些科学家提出，随着高度上升而增强的紫外线照射可能是一个因素，当然草食性动物比如羱羊的啃食也可能是另外一个因素。

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.

可能性最大的环境因素是温度，因为如果气温太低、生长季太短，树芽就会不够成熟，无法在漫长寒冷的冬季中存活下来。

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.

在林木线之上，有个地带通常被称为高山苔原。

Above the tree line there is a zone that is generally called alpine tundra.

高山苔原与树带界限紧紧相邻，冻原上布满低矮的灌木与草丛，再往上去，物种的数量与种类开始减少，直到最后大部分都是裸露的地面，偶尔才有些苔藓地衣与伏地垫状植物。

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.

有些植物甚至能够在雪线以上的有利微环境中生存。

Some plants can even survive in favorable microhabitats above the snow line.

世界上最高的植物生活在喜马拉雅山脉的马卡鲁峰上，海拔约为6100米。

The highest plants in the world occur at around 6,100 meters on Makalu in the Himalayas.

在这个高度上，由于太阳的照耀，岩石的温度得以升高，可以融化小的雪堆。

At this great height, rocks, warmed by the sun, melt small snowdrifts.

高山地带植物的最显著特征是它们长得低矮。

The most striking characteristic of the plants of the alpine zone is their low growth form.

这使得它们一方面能够抵御强度最大的风，另一方面可以利用紧邻地面的较高温度。

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.

在这个低温限制生命延续的地带，近地的额外温度对生命延续是极其重要的。

In an area where low temperatures are limiting to life, the importance of the additional heat near the surface is crucial.

此外，这种低矮的形态还能让植物充分利用因冬雪覆盖而形成的保温层。

The low growth form can also permit the plants to take advantage of the insulation provided by a winter snow cover.

在赤道地带的山脉上，低矮植物则不那么常见。

In the equatorial mountains the low growth form is less prevalent.

Where is the lower timberline mentioned in paragraph 1 likely to be found?

Which of the following can be inferred from paragraph 1 about both the upper and lower timberlines?

Paragraph 2 supports which of the following statements about deciduous trees?

The word “they” in the passage refers to

According to paragraph 3, which of the following is true of trees in the middle and upper latitudes?

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.

In paragraph 4, what is the author’s main purpose in the discussion of the dramatic cessation of tree growth at the upper timberline?

According to paragraph 6, all of the following statements are true of plants in the alpine zone EXCEPT:

Look at the four squares  that indicate where the following sentence could be added to the passage. This explains how, for example, alpine cushion plants have been found growing at an altitude of 6,180 meters.  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.

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.