段意及难句解析：
●第一段：
段意：本段提出了上树带界线1（timberline）的概念，并描述了树带界线1附近的植被特征。
难句分析：
1．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．
短语：semiarid areas：半干旱地区
pass into：传递到，延伸到
分句：at its lower edge：its指代“a lower timberline”，lower edge即指该树带界线的下限
释义：在许多半干旱地区有一种较低的树带界线1，该线以内由于缺乏水分森林变为荒原或沙漠。
●第二段：
段意：本段讲述了树带界线1在各自然带高度的不同以及其树木组成。
难句分析：
1．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．
短语：Polar Regions：极地地区
释义：它的高度范围处于极地地区的海平面高度至干燥***带地区的4500米以及湿润热带地区的3500-4500米之间。
2．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．
短语：have advantage over：与…相比更具优势
释义：树带界线1的树木通常是常青的，这表明在上树带界线1处极其恶劣的环境下，常青树比落叶树更有优势。
●第三段：
段意：本段指出中高纬地区上树带界线1的树木会因为长在山脊之上而扭曲变形，而低纬地区的树带界线1的树木长在山谷中，接着文章解释了其原因。
难句分析：
1．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．
短语：middle and upper latitudes：中高纬度地区
reach greater heights：长得更高
释义：这用来形容中高纬度地区的树木最贴切，那里的树木在山脊上长得更高；而在热带地区树木在山谷中长得更高。
2．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．
短语：be exposed to：暴露在…之中
释义：（在中高纬地区）由于雪在山谷中更深并且持续时间更长，树木倾向于在山脊处长得更高，即便在那里它们要承受高风速和更加贫瘠的土壤。
3．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．
短语：be prone to：倾向于…
释义：在热带地区，山谷则更适合树木生长，因为它们不容易干燥，并且有较少冰霜和较丰厚的土壤。
●第四段：
段意：本段讲述了一些导致上树带界线1以内树木停止生长的环境因素，包括雪量，风速，紫外线，放牧和温度等。
难句分析：
1．Late-lying snow reduces the effective growing season to the point where seedlings cannot establish themselves．
短语：Late-lying snow：久积不化的雪
分句：where seedlings cannot establish themselves修饰point,establish意为“幼苗扎根生长”
释义：持续不化的雪会减少树木有效生长的时间以至于幼苗无法扎根。
2．Wind velocity also increases with altitude and may cause serious stress for trees, as is made evident by the deformed shapes at high altitudes．
短语：is made evident：被证明…
主干：Wind velocity also increases…and may cause stress…,as is made evident by…
释义：风速随着海拔升高而提升，并可能给树造成严重的压力，高海拔地区树木的变形已经证明了这一点。
3．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．
短语：increasing levels of ultraviolet light with elevation：指随海拔升高而加强的紫外线强度
释义：一些科学家提出，随海拔升高而加强的紫外线强度可能是原因之一，而放养动物如野山羊等也可能是一个原因。
4．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．
分句：for引导原因从句
释义：也许最为重要的环境因素是气温，因为如果生长季节过短且气温过低，树苗和花蕾便不能足够成熟，也就不能抵过冬天。
●第五段：
段意：本段描述了树带界线1以上的苔原环境。
难句分析：
1．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．
短语：adjacent to：临近…
a fairly complete cover of：完全由…组成的覆盖物
prostrate cushion plants：伏地状垫状植物
主干：the tundra consists of…,while the number and diversity of species decrease until…
释义：苔原紧靠着树带界线1，它完全由低矮灌木，草本植物和草覆盖，而再往上则生物的数量和多样性都会下降，直到只剩下偶尔会有苔藓、地衣和伏地状垫状植物的大量秃地。
●第六段：
段意：本段讲述了苔原地区植物的低矮生长形态（low growth form）如何为植物带来好处。
难句分析：
1．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．
短语：the worst rigors of high winds：高处风的严酷打击
分句：immediately adjacent to the ground surface修饰higher temperatures，指地表附近的相对较高的温度
主干：This enables them to avoid…and permits them to make use of…
释义：这使得它们能够避免经受高处风的酷烈打击并利用地表附近的较高温度。
2．In an area where low temperatures are limiting to life, the importance of the additional heat near the surface is crucial．
短语：where low temperatures are limiting to life：在那些迫近生命极线1的低温环境中，limiting此处可理解为“接近生命极线1的”
释义：在那些迫近生命极限的低温环境中，贴近地表的额外热量是至关重要的。
本篇词汇
1．tundra 苔原
2．slope 斜坡(n)
3．vertical 垂直的(a)
4．semiarid 半干旱的(a)
5．steppe 特指西伯利亚一带的）没有树木的大草原（n．）
6．subtropics ***带地区(n．)
7．deciduous （树木）每年落叶的(adj．)
8．birch 桦树(n．)
9．twist 扭曲(v．)
10．deform 变形(v．)
11．latitude 纬度(n．)
12．ridge 山脊(n．)
13．duration 持续时间(n．)
14．high-velocity 高速的(adj．)
15．prone 倾向于(adj．)
16．frost 霜(n．)
17．cessation 终止(n．)
18．smother 使窒息(v．)
19．avalanche 雪崩(n．)
20．creep 蔓延(n．/v．)
21．ultraviolet 紫外线的(adj．)
22．elevation 海拔(n．)
23．browse 吃草(v．)
24．ibex 野山羊(n．)
25．alpine tundra 高山冻原(n．)
26．adjacent 毗邻的(adj．)
27．moss 苔藓(n．)
28．lichen 地衣(n．)
29．prostrate 伏地状的(adj．)
30．cushion plant 垫状植物(n．)
31．microhabitat 小环境(n．)
32．striking 惊人的(adj．)
33．rigor 酷烈的(adj．)
34．crucial 至关重要的(adj．)
35．insulation 隔绝(n．)
36．equatorial 近赤道的(adj．)
37．prevalent 普遍的(adj．)
本文来源：托福考试辅导 http://www.91toefl.com/ksfd/tfyd/ |
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