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1、阅读原文:Thetransitionfromforesttotreelesstundraonamountainslopeisoftenadramaticone.Withinaverticaldistanceofjustafewtensofmeters,treesdisappearasalife-formandarereplacedbylowshrubs,herbs,andgrasses.Thisrapidzoneoftransitioniscalledtheuppertimberlineortreeline.Inmanysemiaridareasthere
2、isalsoalowertimberlinewheretheforestpassesintosteppeordesertatitsloweredge,usuallybecauseofalackofmoisture.通常从山坡上的森林到没有树的苔原是一种非常戏剧化的转变。在一个垂直距离只有几十米的地方,树木这种生命形式就消失了,取而代之的是低矮的灌木、草本植物和牧草。这种快速过渡的区域被称为上行树带界线或林木线。在许多干旱的地区存在着下行树带界线,在这里由于缺乏水分森林变成干草原,甚至在最下端会出现沙漠。Theuppertimberline,likethes
3、nowline,ishighestinthetropicsandlowestinthePolarRegions.ItrangesfromsealevelinthePolarRegionsto4,500metersinthedrysubtropicsand3,500-4,500metersinthemoisttropics.Timberlinetreesarenormallyevergreens,suggestingthatthesehavesomeadvantageoverdeciduoustrees(thosethatlosetheirleaves)in
4、theextremeenvironmentsoftheuppertimberline.Therearesomeareas,however,wherebroadleafdeciduoustreesformthetimberline.Speciesofbirch,forexample,mayoccuratthetimberlineinpartsoftheHimalayas.上行树带界线,和雪线一样,在热带最高,在极地最低。从极地地区的海平面到干燥的亚热带地区的海拔4500米处以及潮湿的热带地区海拔3500米至4500米处都有上行树带界线。树带界线内通常是常绿树
5、,它们和处于上行树带界线处极端恶劣环境中生长的落叶树木相比,具有一定的优势。然而,在部分地区也有由落叶阔叶林组成的树带界线。例如,在喜马拉雅的部分地区,桦树就在树带界线上。Attheuppertimberlinethetreesbegintobecometwistedanddeformed.Thisisparticularlytruefortreesinthemiddleandupperlatitudes,whichtendtoattaingreaterheightsonridges,whereasinthetropicsthetreesreachthei
6、rgreaterheightsinthevalleys.Thisisbecausemiddle-andupper-latitudetimberlinesarestronglyinfluencedbythedurationanddepthofthesnowcover.Asthesnowisdeeperandlastslongerinthevalleys,treestendtoattaingreaterheightsontheridges,eventhoughtheyaremoreexposedtohigh-velocitywindsandpoor,thins
7、oilsthere.Inthetropics,thevalleysappeartobemorefavorablebecausetheyarelesspronetodryout,theyhavelessfrost,andtheyhavedeepersoils.上行树带界线的树木开始扭曲和变形,尤其在中高纬度地区的树木,这些地区的树木往往会在山脊上长得更高,而在热带地区的树木则在山谷里长得更高;因为中高纬度地区树带界线受积雪覆盖时间和深度的影响很大。由于山谷中积雪覆盖较厚且持续时间很长,树木即便是生长在大风和贫瘠的土地里,也往往会在山脊上长得更高。在热带地区山
8、谷里更有利于生长,因为山谷不易干涸、很少结霜,并且有更深的土壤。T