tpo1阅读原文（精选5篇）
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    Timberline Vegetation on Mountains
    The transition from forest to treeless tundra on a mountain slope is often a dramatic  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  This rapid zone of transition is called the upper timberline or tree  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 
    The upper timberline, like the snow line, is highest in the tropics and lowest in the Polar  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  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  There are some areas, however, where broadleaf deciduous trees form the  Species of birch, for example, may occur at the timberline in parts of the 
    At the upper timberline the trees begin to become twisted and  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  This is because middle- and upper- latitude timberlines are strongly influenced by the duration and depth of the snow  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  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 
    There is still no universally agreed-on explanation for why there should be such a dramatic cessation of tree growth at the upper  Various environmental factors may play a  Too much snow, for example, can smother trees, and avalanches and snow creep can damage or destroy  Late-lying snow reduces the effective growing season to the point where seedlings cannot establish  Wind velocity also increases with altitude and may cause serious stress for trees, as is made evident by the deformed shapes at high  Some scientists have proposed that the presence of increasing levels of ultraviolet light with eleva
tion may play a role, while browsing and grazing animals like the ibex may be another contributing  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 
    Above the tree line there is a zone that is generally called alpine  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  Some plants can even survive in favorable microhabitats above the snow  The highest plants in the world occur at around 6,101 meters on Makalu in the  At this great height, rocks, warmed by the sun, melt small 
    The most striking characteristic of the plants of the alpine zone is their low growth  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  In an area where low temperature
s are limiting to life, the importance of the additional heat near the surface is  The low growth form can also permit the plants to take advantage of the insulation provided by a winter snow  In the equatorial mountains the low growth form is less 
    Paragraph 1: The transition from forest to treeless tundra on a mountain slope is often adramatic  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  This rapid zone of transition is called the upper timberline or tree  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 
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    山上树带界线的植被
vertical怎么读
    通常从山坡上的森林到没有树的苔原是一种非常戏剧化的转变。在一个垂直距离只有几十米的地方，树木这种生命形式就消失了，取而代之的是低矮的灌木、草本植物和牧草。这种快速过渡的区域被称为上行树带界线或林木线。在许多干旱的地区存在着下行树带界线，在这里由于缺乏水分森林变成干草原，甚至在最下端会出现沙漠。
    上行树带界线，和雪线一样，在热带最高，在极地最低。从极地地区的海平面到干燥的亚热带地区的海拔4 500米处以及潮湿的热带地区海拔3 500米至4 500米处都有上行树带界线。树带界线内通常是常绿树，它们和处于上行树带界线处极端恶劣环境中生长的落叶树木相比，具有一定的优势。然而，在部分地区也有由落叶阔叶林组成的树带界线。例如，在喜马拉雅的部分地区，桦树就在树带界线上。