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托福阅读史上最全原文分析—TPO1
山上树带界线的植被-智课教育旗下智课教育
本文和大家分享的是托福阅读史上最全原文分析—TPO1 Timberline Vegetation on
Mountains，正在备考托福阅读的小伙伴赶紧看过来吧！
TPO是托福备考圣经，但是并不是有“圣经”就能完全把握托福考试。还要看大家怎么去利用这份资料。关于TPO阅读，托福小编希望最大限度地利用这些ETS提供的阅读文章。在文章中，到那些生词，长难句；分析文章结构。真正把它作为一份备考资料，而不是做题工具。今天小编分享给大家的是对TPO1 Timberline Vegetation on Mountains的文章详细分析。一起学习吧！
Timberline Vegetation on Mountains
山上树带界线的植被
1.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.
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。
上行树带界线的定义。
2.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.
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。
树带界限生长的树木类型。
3.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 tr
opics 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.
上行树带界线的树木开始扭曲和变形，尤其在中高纬度地区的树木，这些地区的树木往往会在山脊上长得更高，而在热带地区的树木则在山谷里长得更高；因为中高纬度地区树带界线受积雪覆盖时间和深度的影响很大。由于山谷中积雪覆盖较厚且持续时间很长，树木即便是生长在大风和贫瘠的土地里，也往往会在山脊上长得更高。在热带地区山谷里更有利于生长，因为山谷不易干涸、很少结霜，并且有更深的土壤。

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