韋恩食記

在做這篇題目前，先來了解什麼是林木線。
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The transition from forest to treeless tundra on a mountain slope is often a dramatic one.
從山坡上的森林過渡到沒有樹的苔原通常是一種非常戲劇化的過程。
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.
在幾十公尺的垂直距離內，樹木這種生命形式就消失了，並被低矮的灌木、草本植物和牧草所取代。
This rapid zone of transition is called the upper timberline or tree line.
這種快速過渡的區域被稱為上行樹帶界線或林木線。
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.
在許多半乾旱地區存在著下行樹帶界線，在這裡由於缺乏水分森林變成乾草原，甚至在最下端會出現沙漠。

The upper timberline, like the snow line, is highest in the tropics and lowest in the Polar Regions.
和雪線一樣，上行樹帶界線在熱帶最高，在極地最低。
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.
範圍從極地的海平面到乾亞熱帶的海拔4500公尺處及濕熱帶的海拔3500公尺至4500公尺處。
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.
林木樹通常是常綠樹，這表示它們和位於上行樹帶界線處極端惡劣環境中生長的落葉樹木相比，具有一些優勢。
There are some areas, however, where broadleaf deciduous trees form the timberline.
然而，在部分地區也有由落葉闊葉林組成的樹帶界線。
Species of birch, for example, may occur at the timberline in parts of the Himalayas.
例如，在喜馬拉雅的部分地區的林線中可能會出現樺樹。

At the upper timberline the trees begin to become twisted and deformed.
上行樹帶界線的樹木開始扭曲和變形。
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.
尤其在中高緯度地區的樹木，這些地區的樹木往往會在山脊上長得更高，而在熱帶地區的樹木則在山谷里長得更高。
This is because middle and upper latitude timberlines are strongly influenced by the duration and depth of the snow cover.
因為中高緯度地區樹帶界線受積雪覆蓋時間和深度的影響很大。
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.
由於山谷中積雪覆蓋較厚且持續時間很長，樹木即便是生長在大風和貧瘠的土地裡，也往往會在山脊上長得更高。
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.
在熱帶地區山谷裡更有利於生長，因為不易乾涸、很少結霜，並且有更深的土壤。

There is still no universally agreed-on explanation for why there should be such a dramatic cessation of tree growth at the upper timberline.
目前還沒有一個普遍認同的解釋來說明為什麼會在上樹帶界線上出現樹木停止生長這種戲劇化的現象。
Various environmental factors may play a role.
多種環境因素都可能起作用。
Too much snow, for example, can smother trees, and avalanches and snow creep can damage or destroy them.
例如，過多的積雪會使讓樹木窒息，而雪崩和雪移會摧毀樹木。
Late-lying snow reduces the effective growing season to the point where seedlings cannot establish themselves.
長時間積雪縮短了有效生長季節的時間，樹苗無法生長。
Wind velocity also increases with altitude and may cause serious stress for trees, as is made evident by the deformed shapes at high altitudes.
風速也會隨著海拔的升高而增加，並可能對樹木造成嚴重的壓力，很明顯，正是這種現象導致樹木在高緯度地區發生變形。
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.
一些科學家提出，隨著海拔的上升而不斷增強的紫外線、野生山羊等動物的放養，都是導致林木線形成的因素。
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.
或許最重要的環境因素是溫度，因為如果生長季節太短並且氣溫太低，樹芽和樹苗都無法充分地成長而存活過冬季。

Above the tree line there is a zone that is generally called alpine tundra.
在林木線上方有一個通常稱為高山凍原的區域。
☐ 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.
由於緊鄰林木線，苔原上都是矮灌木、草本植物和牧草，隨著海拔的增加，物種的數量和多樣性會逐漸減少，直到出現大量空地伴著零星的苔蘚和青苔這樣的伏地墊狀植物。
☐ Some plants can even survive in favorable microhabitats above the snow line.
有些植物甚至可以在雪線以上的有利微環境中生存。
The highest plants in the world occur at around 6,100 meters on Makalu in the Himalayas.
世界上海拔最高的植物是出現在喜馬拉雅山上6100公尺的馬卡魯峰。
☐ At this great height, rocks, warmed by the sun, melt small snowdrifts. ☐
在這個高度上，被陽光溫暖過的岩石融化了小雪堆。

The most striking characteristic of the plants of the alpine zone is their low growth form.
高山植物最突出的特點是其低矮的生長形態。
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.
這種特點使他們能夠避開大風最強勢的勢頭，並且有助於他們利用緊鄰地面相對較高的溫度。
In an area where low temperatures are limiting to life, the importance of the additional heat near the surface is crucial.
在這樣一個低溫限制生命的地區，地表提供的額外溫度是至關重要的。
The low growth form can also permit the plants to take advantage of the insulation provided by a winter snow cover.
低矮的生長形態也可以幫助植物充分利用冬季積雪所提供的保溫環境。
In the equatorial mountains the low growth form is less prevalent.
在赤道區的山脈上低矮的生長形態並不常見。