段意及難句解析：
●第一段：
段意：
本段提出了上樹帶界線1（timberline）的概念，并描述了樹帶界線1附近的植被特征。
難句分析：
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.
短語：semiarid areas：半干旱地區(qū)
pass into：傳遞到，延伸到
分句：at its lower edge：its指代“a lower timberline”，lower edge即指該樹帶界線的下限
釋義：在許多半干旱地區(qū)有一種較低的樹帶界線1，該線以內(nèi)由于缺乏水分森林變?yōu)榛脑蛏衬?BR> ●第二段：
段意：
本段講述了樹帶界線1在各自然帶高度的不同以及其樹木組成。
難句分析：
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.
短語：Polar Regions：極地地區(qū)
釋義：它的高度范圍處于極地地區(qū)的海平面高度至干燥亞熱帶地區(qū)的4500米以及濕潤熱帶地區(qū)的3500-4500米之間。
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.
短語：have advantage over：與…相比更具優(yōu)勢
釋義：樹帶界線1的樹木通常是常青的，這表明在上樹帶界線1處極其惡劣的環(huán)境下，常青樹比落葉樹更有優(yōu)勢。
●第三段：
段意：
本段指出中高緯地區(qū)上樹帶界線1的樹木會因?yàn)殚L在山脊之上而扭曲變形，而低緯地區(qū)的樹帶界線1的樹木長在山谷中，接著文章解釋了其原因。
難句分析：
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.
短語：middle and upper latitudes：中高緯度地區(qū)
reach greater heights：長得更高
釋義：這用來形容中高緯度地區(qū)的樹木最貼切，那里的樹木在山脊上長得更高；而在熱帶地區(qū)樹木在山谷中長得更高。
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.
短語：be exposed to：暴露在…之中
釋義：（在中高緯地區(qū)）由于雪在山谷中更深并且持續(xù)時(shí)間更長，樹木傾向于在山脊處長得更高，即便在那里它們要承受高風(fēng)速和更加貧瘠的土壤。
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.
短語：be prone to：傾向于…
釋義：在熱帶地區(qū)，山谷則更適合樹木生長，因?yàn)樗鼈儾蝗菀赘稍?，并且有較少冰霜和較豐厚的土壤。
●第四段：
段意：
本段講述了一些導(dǎo)致上樹帶界線1以內(nèi)樹木停止生長的環(huán)境因素，包括雪量，風(fēng)速，紫外線，放牧和溫度等。
難句分析：
1. Late-lying snow reduces the effective growing season to the point where seedlings cannot establish themselves.
短語：Late-lying snow：久積不化的雪
分句：where seedlings cannot establish themselves修飾point,establish意為“幼苗扎根生長”
釋義：持續(xù)不化的雪會減少樹木有效生長的時(shí)間以至于幼苗無法扎根。
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.
短語：is made evident：被證明…
主干：Wind velocity also increases…and may cause stress…,as is made evident by…
釋義：風(fēng)速隨著海拔升高而提升，并可能給樹造成嚴(yán)重的壓力，高海拔地區(qū)樹木的變形已經(jīng)證明了這一點(diǎn)。
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.
短語：increasing levels of ultraviolet light with elevation：指隨海拔升高而加強(qiáng)的紫外線強(qiáng)度
釋義：一些科學(xué)家提出，隨海拔升高而加強(qiáng)的紫外線強(qiáng)度可能是原因之一，而放養(yǎng)動物如野山羊等也可能是一個(gè)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引導(dǎo)原因從句
釋義：也許最為重要的環(huán)境因素是氣溫，因?yàn)槿绻L季節(jié)過短且氣溫過低，樹苗和花蕾便不能足夠成熟，也就不能抵過冬天。
●第五段：
段意：
本段描述了樹帶界線1以上的苔原環(huán)境。
難句分析：
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.
短語：adjacent to：臨近…
a fairly complete cover of：完全由…組成的覆蓋物
prostrate cushion plants：伏地狀墊狀植物
主干：the tundra consists of…,while the number and diversity of species decrease until…
釋義：苔原緊靠著樹帶界線1，它完全由低矮灌木，草本植物和草覆蓋，而再往上則生物的數(shù)量和多樣性都會下降，直到只剩下偶爾會有苔蘚、地衣和伏地狀墊狀植物的大量禿地。
●第六段：
段意：
本段講述了苔原地區(qū)植物的低矮生長形態(tài)（low growth form）如何為植物帶來好處。
難句分析：
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.
短語：the worst rigors of high winds：高處風(fēng)的嚴(yán)酷打擊
分句：immediately adjacent to the ground surface修飾higher temperatures，指地表附近的相對較高的溫度
主干：This enables them to avoid…and permits them to make use of…
釋義：這使得它們能夠避免經(jīng)受高處風(fēng)的酷烈打擊并利用地表附近的較高溫度。
2. In an area where low temperatures are limiting to life, the importance of the additional heat near the surface is crucial.
短語：where low temperatures are limiting to life：在那些迫近生命極線1的低溫環(huán)境中，limiting此處可理解為“接近生命極線1的”
釋義：在那些迫近生命極限的低溫環(huán)境中，貼近地表的額外熱量是至關(guān)重要的。
本篇詞匯
1.tundra 苔原
2.slope 斜坡(n)
3.vertical 垂直的(a)
4.semiarid 半干旱的(a)
5.steppe 特指西伯利亞一帶的）沒有樹木的大草原（n.）
6.subtropics 亞熱帶地區(qū)(n.)
7.deciduous （樹木）每年落葉的(adj.)
8.birch樺樹(n.)
9.twist扭曲(v.)
10.deform 變形(v.)
11.latitude 緯度(n.)
12.ridge 山脊(n.)
13.duration 持續(xù)時(shí)間(n.)
14.high-velocity 高速的(adj.)
15.prone 傾向于(adj.)
16.frost 霜(n.)
17.cessation 終止(n.)
18.smother 使窒息(v.)
19.avalanche 雪崩(n.)
20.creep 蔓延(n./v.)
21.ultraviolet 紫外線的(adj.)
22.elevation 海拔(n.)
23.browse 吃草(v.)
24.ibex野山羊(n.)
25.alpine tundra 高山凍原(n.)
26.adjacent 毗鄰的(adj.)
27.moss苔蘚(n.)
28.lichen 地衣(n.)
29.prostrate 伏地狀的(adj.)
30.cushion plant 墊狀植物(n.)
31.microhabitat小環(huán)境(n.)
32.striking 驚人的(adj.)
33.rigor 酷烈的(adj.)
34.crucial 至關(guān)重要的(adj.)
35.insulation 隔絕(n.)
36.equatorial 近赤道的(adj.)
37.prevalent 普遍的(adj.)
●第一段：
段意：
本段提出了上樹帶界線1（timberline）的概念，并描述了樹帶界線1附近的植被特征。
難句分析：
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.
短語：semiarid areas：半干旱地區(qū)
pass into：傳遞到，延伸到
分句：at its lower edge：its指代“a lower timberline”，lower edge即指該樹帶界線的下限
釋義：在許多半干旱地區(qū)有一種較低的樹帶界線1，該線以內(nèi)由于缺乏水分森林變?yōu)榛脑蛏衬?BR> ●第二段：
段意：
本段講述了樹帶界線1在各自然帶高度的不同以及其樹木組成。
難句分析：
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.
短語：Polar Regions：極地地區(qū)
釋義：它的高度范圍處于極地地區(qū)的海平面高度至干燥亞熱帶地區(qū)的4500米以及濕潤熱帶地區(qū)的3500-4500米之間。
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.
短語：have advantage over：與…相比更具優(yōu)勢
釋義：樹帶界線1的樹木通常是常青的，這表明在上樹帶界線1處極其惡劣的環(huán)境下，常青樹比落葉樹更有優(yōu)勢。
●第三段：
段意：
本段指出中高緯地區(qū)上樹帶界線1的樹木會因?yàn)殚L在山脊之上而扭曲變形，而低緯地區(qū)的樹帶界線1的樹木長在山谷中，接著文章解釋了其原因。
難句分析：
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.
短語：middle and upper latitudes：中高緯度地區(qū)
reach greater heights：長得更高
釋義：這用來形容中高緯度地區(qū)的樹木最貼切，那里的樹木在山脊上長得更高；而在熱帶地區(qū)樹木在山谷中長得更高。
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.
短語：be exposed to：暴露在…之中
釋義：（在中高緯地區(qū)）由于雪在山谷中更深并且持續(xù)時(shí)間更長，樹木傾向于在山脊處長得更高，即便在那里它們要承受高風(fēng)速和更加貧瘠的土壤。
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.
短語：be prone to：傾向于…
釋義：在熱帶地區(qū)，山谷則更適合樹木生長，因?yàn)樗鼈儾蝗菀赘稍?，并且有較少冰霜和較豐厚的土壤。
●第四段：
段意：
本段講述了一些導(dǎo)致上樹帶界線1以內(nèi)樹木停止生長的環(huán)境因素，包括雪量，風(fēng)速，紫外線，放牧和溫度等。
難句分析：
1. Late-lying snow reduces the effective growing season to the point where seedlings cannot establish themselves.
短語：Late-lying snow：久積不化的雪
分句：where seedlings cannot establish themselves修飾point,establish意為“幼苗扎根生長”
釋義：持續(xù)不化的雪會減少樹木有效生長的時(shí)間以至于幼苗無法扎根。
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.
短語：is made evident：被證明…
主干：Wind velocity also increases…and may cause stress…,as is made evident by…
釋義：風(fēng)速隨著海拔升高而提升，并可能給樹造成嚴(yán)重的壓力，高海拔地區(qū)樹木的變形已經(jīng)證明了這一點(diǎn)。
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.
短語：increasing levels of ultraviolet light with elevation：指隨海拔升高而加強(qiáng)的紫外線強(qiáng)度
釋義：一些科學(xué)家提出，隨海拔升高而加強(qiáng)的紫外線強(qiáng)度可能是原因之一，而放養(yǎng)動物如野山羊等也可能是一個(gè)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引導(dǎo)原因從句
釋義：也許最為重要的環(huán)境因素是氣溫，因?yàn)槿绻L季節(jié)過短且氣溫過低，樹苗和花蕾便不能足夠成熟，也就不能抵過冬天。
●第五段：
段意：
本段描述了樹帶界線1以上的苔原環(huán)境。
難句分析：
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.
短語：adjacent to：臨近…
a fairly complete cover of：完全由…組成的覆蓋物
prostrate cushion plants：伏地狀墊狀植物
主干：the tundra consists of…,while the number and diversity of species decrease until…
釋義：苔原緊靠著樹帶界線1，它完全由低矮灌木，草本植物和草覆蓋，而再往上則生物的數(shù)量和多樣性都會下降，直到只剩下偶爾會有苔蘚、地衣和伏地狀墊狀植物的大量禿地。
●第六段：
段意：
本段講述了苔原地區(qū)植物的低矮生長形態(tài)（low growth form）如何為植物帶來好處。
難句分析：
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.
短語：the worst rigors of high winds：高處風(fēng)的嚴(yán)酷打擊
分句：immediately adjacent to the ground surface修飾higher temperatures，指地表附近的相對較高的溫度
主干：This enables them to avoid…and permits them to make use of…
釋義：這使得它們能夠避免經(jīng)受高處風(fēng)的酷烈打擊并利用地表附近的較高溫度。
2. In an area where low temperatures are limiting to life, the importance of the additional heat near the surface is crucial.
短語：where low temperatures are limiting to life：在那些迫近生命極線1的低溫環(huán)境中，limiting此處可理解為“接近生命極線1的”
釋義：在那些迫近生命極限的低溫環(huán)境中，貼近地表的額外熱量是至關(guān)重要的。
本篇詞匯
1.tundra 苔原
2.slope 斜坡(n)
3.vertical 垂直的(a)
4.semiarid 半干旱的(a)
5.steppe 特指西伯利亞一帶的）沒有樹木的大草原（n.）
6.subtropics 亞熱帶地區(qū)(n.)
7.deciduous （樹木）每年落葉的(adj.)
8.birch樺樹(n.)
9.twist扭曲(v.)
10.deform 變形(v.)
11.latitude 緯度(n.)
12.ridge 山脊(n.)
13.duration 持續(xù)時(shí)間(n.)
14.high-velocity 高速的(adj.)
15.prone 傾向于(adj.)
16.frost 霜(n.)
17.cessation 終止(n.)
18.smother 使窒息(v.)
19.avalanche 雪崩(n.)
20.creep 蔓延(n./v.)
21.ultraviolet 紫外線的(adj.)
22.elevation 海拔(n.)
23.browse 吃草(v.)
24.ibex野山羊(n.)
25.alpine tundra 高山凍原(n.)
26.adjacent 毗鄰的(adj.)
27.moss苔蘚(n.)
28.lichen 地衣(n.)
29.prostrate 伏地狀的(adj.)
30.cushion plant 墊狀植物(n.)
31.microhabitat小環(huán)境(n.)
32.striking 驚人的(adj.)
33.rigor 酷烈的(adj.)
34.crucial 至關(guān)重要的(adj.)
35.insulation 隔絕(n.)
36.equatorial 近赤道的(adj.)
37.prevalent 普遍的(adj.)