首页
外语
计算机
考研
公务员
职业资格
财经
工程
司法
医学
专升本
自考
实用职业技能
登录
考研
The transition from forest to treeless tundra on a mountain slope is often a dramatic one. Within a vertical distance of just a
The transition from forest to treeless tundra on a mountain slope is often a dramatic one. Within a vertical distance of just a
admin
2010-12-21
32
问题
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.
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.
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. 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.
There is still no universally agreed-on explanation for why there should be such a dramatic cessation of tree growth at the upper timberline. 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. Late-lying snow reduces the effective growing season to the point where seedlings cannot establish themselves. Wind velocity also increases with altitude and may cause serious stress for trees, as is made evident by the deformed shapes at high altitudes. 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. 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.
Above the tree line there is a zone that is generally called alpine tundra. 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. Some plants can even survive in favorable microhabitats above the snow line. 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.
The most striking characteristic of the plants of the alpine zone is their low growth form. 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. In an area where low temperatures are limiting to life, the importance of the additional heat near the surface is crucial. 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.
What can we learn about characteristic of plants in the alpine zone?
选项
答案
Because they are low, they are less exposed to strong winds and the winter snow cover gives them more protection from the extreme cold. Moreover, their low growth form keeps them closer to the ground, where there is more heat than further up.
解析
转载请注明原文地址:https://jikaoti.com/ti/YKwUFFFM
本试题收录于:
翻译硕士(翻译硕士英语)题库专业硕士分类
0
翻译硕士(翻译硕士英语)
专业硕士
相关试题推荐
Itisaresearchthatisguaranteedtodelightmen—andinfuriatethewomenintheirlives.Acontroversialnewstudyhasclaime
Americanurbanizationwentforwardinthesameclimateoftheunexpectedthathascharacterizedurbanizationeverywhere.Townsme
trustcompany
MartinLutherKing,Jr.ayoungblackclergyman,becameanationalleaderofthe________Movement.
中国国际进口博览会(ChinaInternationalImportExpo,简称CIIE)。2017年5月,习近平主席在“一带一路”国际合作【R31】________上宣布,中国将从2018年起举办中国国际进口博览会。2018年
PopstarstodayenjoyastyleoflivingwhichwasoncetheprerogativeonlyofRoyalty.Wherevertheygo,peopleturnoutinthe
Thevalueofheatforthepreservationoffoodhasbeenknownforthousandsofyears,butitwasnotrealizeduntilthenineteen
Moststudentsarriveatcollegeusing"discrete,concreteandabsolutecategoriestounderstandpeople,knowledge,andvalues."
Since2011,whenStanfordUniversitylauncheditsfirst"massiveopenonlinecourses",thesefree,internet-enabledprogrammes
Sheisalwayscomplaining________something.It________mynerves.
随机试题
单位时间内照射量的增量称为
迎香穴位于()
土地使用权的转让方式有()。
总量分析侧重于分析经济运行的相对静止状态,结构分析侧重于分析经济现象的动态过程。()
甲公司的分公司在其经营范围内以自己的名义对外签订一份货物买卖合同。根据《公司法》的规定,下列关于该合同的效力及其责任承担的表述中,正确的是()。
①小李到高铁站取了票②小李乘上了去北京的高铁③小李在网上订了一张周末去北京的高铁票④单位领导吩咐小李周末去北京出差⑤小李打电话给朋友取消周末的约会下列对上述5个事件排序最合理的是:
梅花和桃花同样香、同样艳,怎么理解?
圣库制度
一般来说,MIS从职能结构上进行横向划分时,町分成高层战略层、中层()和基层执行层。
软件设计包括软件的结构、数据接口和过程设计,其中软件的过程设计是指______。
最新回复
(
0
)