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1、托托网托福模拟考试软件2.0配套资料托福TPO1真题全阅读文本附答案译文下载地址http://www.tuotuowang.comTIMBERLINEVEGETATIONONMOUNTAINS(TPO1-1)Thetransitionfromforesttotreelesstundraonamountainslopeisoftenadramaticone.Withinaverticaldistanceofjustafewtensofmeters,treesdisappearasalife-formandarereplacedbylowshr
2、ubs,herbs,andgrasses.Thisrapidzoneoftransitioniscalledtheuppertimberlineortreeline.Inmanysemiaridareasthereisalsoalowertimberlinewheretheforestpassesintosteppeordesertatitsloweredge,usuallybecauseofalackofmoisture.Theuppertimberline,likethesnowline,ishighestinthetropicsand
3、lowestinthePolarRegions.ItrangesfromsealevelinthePolarRegionsto4,500metersinthedrysubtropicsand3,500-4,500metersinthemoisttropics.Timberlinetreesarenormallyevergreens,suggestingthatthesehavesomeadvantageoverdeciduoustrees(thosethatlosetheirleaves)intheextremeenvironmentsof
4、theuppertimberline.Therearesomeareas,however,wherebroadleafdeciduoustreesformthetimberline.Speciesofbirch,forexample,mayoccuratthetimberlineinpartsoftheHimalayas.Attheuppertimberlinethetreesbegintobecometwistedanddeformed.Thisisparticularlytruefortreesinthemiddleandupperla
5、titudes,whichtendtoattaingreaterheightsonridges,whereasinthetropicsthetreesreachtheirgreaterheightsinthevalleys.Thisisbecausemiddle-andupper-latitudetimberlinesarestronglyinfluencedbythedurationanddepthofthesnowcover.Asthesnowisdeeperandlastslongerinthevalleys,treestendtoa
6、ttaingreaterheightsontheridges,eventhoughtheyaremoreexposedtohigh-velocitywindsandpoor,thinsoilsthere.Inthetropics,thevalleysappeartobemorefavorablebecausetheyarelesspronetodryout,theyhavelessfrost,andtheyhavedeepersoils.Thereisstillnouniversallyagreed-onexplanationforwhyt
7、hereshouldbesuchadramaticcessationoftreegrowthattheuppertimberline.Variousenvironmentalfactorsmayplayarole.Toomuchsnow,forexample,cansmothertrees,andavalanchesandsnowcreepcandamageordestroythem.Late-lyingsnowreducestheeffectivegrowingseasontothepointwhereseedlingscannotest
8、ablishthemselves.Windvelocityalsoincreaseswithaltitudeandmaycauseseriousstressfortrees,as
