托福阅读真题：TPO 1(Timberline Vegetation on Mountains)原文及答案

　　文章逐句翻译

　　第一段：

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

　　山体斜坡上由森林至苔原的转变是非常戏剧化的。

　　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.

　　在仅数十米的垂直距离内，树木逐渐消失，取而代之的是低矮灌木、草本植物和草地。

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

　　这一植被迅速转变的区域被称为上行树带界线或林木线。

　　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.

　　这一植被迅速转变的区域被称为上行树带界线或林木线。在许多半干旱地区，林木线通常较低，该线以内的植物经常由于缺乏水分而转变为荒原或沙漠。

　　第二段：

　　The upper timberline, like the snow line, is highest in the tropics and lowest in the Polar Regions. 上行树带界线，如同雪线一样，通常在热带地区海拔位置最高而在极地地区海拔位置最低。

　　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.

　　极地地区树带界线通常与海平面持平，而在干燥的亚热带地区其海拔可至4500米，在湿润的热带地区树带界线1则位于3500至4500米之间。

　　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.

　　树带界线内的树木通常是长青植物，这或许是因为它们与落叶树木相比更适合生存在极其恶劣的环境中。

　　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.

　　然而，在部分地区，阔叶落叶林也能形成树带界线。例如，在喜马拉雅山的部分地区，桦树就可能存在于树带界线内。

　　第三段：

　　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.

　　上行树带界线内的树木通常会扭曲变形，特别是中高纬度地区，那里的树木往往在山脊处长得更高，而热带地区的树木则在山谷里长得更高。

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

　　这是因为中高纬度树带界线受积雪覆盖的时长和深度的影响很大。

　　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.

　　由于山谷中积雪埋藏较深并持续时间很长，树木往往在山脊上长得更高，即便在那里它们要承受更高速的风和更贫瘠的土地。

　　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.

　　而在热带地区，山谷更有利于植物生长，因为那里更不易干涸，很少结霜，并且土层更深厚。

　　第四段：

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

　　为什么树带界线处树木会停止生长?关于这一戏剧性变化尚未有广为接受的解释。

　　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.

　　多种环境因素很可能起到一定作用，例如，积雪过多可以导致树木窒息死亡，雪崩和积雪蔓延同样能破坏或摧毁树木。

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

　　久积不化的雪缩短了植物的有效生长时间，导致树苗无法存活。

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

　　另外，风速会随着海拔的升高而增加，这很可能给树木施加巨大压力，高海拔树木的变形已很明确地证实了这一点。

　　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.

　　一些科学家还提出，随着海拔的上升，紫外线强度也会不断增加，而野生山羊等动物的放养也可能对植物生长产生不利影响。

　　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.

　　或许最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，树芽和花蕊都无法发育成熟，因而无法在冬季存活。

　　第五段：

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

　　树带界线之上的区域被称为高山苔原。

　　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. 该区域紧挨林木线，被低矮灌木，草本植物和草地完全覆盖。随着海拔继续升高，物种数量和多样性逐渐降低，直到转变为裸露空地，偶尔伴有苔藓、地衣和倒伏植物的生长。

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

　　有些植物在雪线以上仍能生长，这得益于一些有益生存的小环境。

　　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.

　　世界上海拔位置最高的植物生长在喜马拉雅山脉Makalu山上6100米处。在这一地区，受阳光热化的岩石可以将小雪堆融化，(从而为植物提供水分)。

　　第六段：

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

　　高山苔原植物均以低矮形态生长，这是它们最显著的特征。

　　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.

　　这种特点使它们得以免受恶劣强风的侵袭，并有助于它们利用地面附近的较高温度。

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

　　在那些迫近生命极限的低温环境中，贴近地表的额外热量对植物生长而言是至关重要的。

　　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.

　　低矮生长形态也可以帮助植物充分利用冬季积雪提供的隔离环境。当然，在赤道地区的山脉上这种低矮生长形态就并不常见了。