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1、智课网托福备考资料托福阅读――事实否定题(八)TimberlineVegetationonMountainsThetransitionfromforesttotreelesstundraonamountainslopeisoftenadramaticone.Withinaverticaldistanceofjustafewtensofmeters,treesdisappearasalife-formandarereplacedbylowshrubs,herbs,andgrasses.Thisrapidzoneoftransitioniscalledtheuppertimberlineor
2、treeline.Inmanysemiaridareasthereisalsoalowertimberlinewheretheforestpassesintosteppeordesertatitsloweredge,usuallybecauseofalackofmoisture.Theuppertimberline,likethesnowline,ishighestinthetropicsandlowestinthePolarRegions.ItrangesfromsealevelinthePolarRegionsto4,500metersinthedrysubtropicsand3,5
3、00-4,500metersinthemoisttropics.Timberlinetreesarenormallyevergreens,suggestingthatthesehavesomeadvantageoverdeciduoustrees(thosethatlosetheirleaves)intheextremeenvironmentsoftheuppertimberline.Therearesomeareas,however,wherebroadleafdeciduoustreesformthetimberline.Speciesofbirch,forexample,mayoc
4、curatthetimberlineinpartsoftheHimalayas.Attheuppertimberlinethetreesbegintobecometwistedanddeformed.Thisisparticularlytruefortreesinthemiddleandupperlatitudes,whichtendtoattaingreaterheightsonridges,whereasinthetropicsthetreesreachtheirgreaterheightsinthevalleys.Thisisbecausemiddle-andupper-latit
5、udetimberlinesarestronglyinfluencedbythedurationanddepthofthesnowcover.Asthesnowisdeeperandlastslongerinthevalleys,treestendtoattaingreaterheightsontheridges,eventhoughtheyaremoreexposedtohigh-velocitywindsandpoor,thinsoilsthere.Inthetropics,thevalleysappeartobemorefavorablebecausetheyarelesspron
6、etodryout,theyhavelessfrost,andtheyhavedeepersoils.Thereisstillnouniversallyagreed-onexplanationforwhythereshouldbesuchadramaticcessationoftreegrowthattheuppertimberline.Variousenvironmentalfactorsmayplayarole.Toomuchsnow,forexample,cansmothertrees,andavalanchesandsnowcreepcandamageordestroythem.
7、Late-lyingsnowreducestheeffectivegrowingseasontothepointwhereseedlingscannotestablishthemselves.Windvelocityalsoincreaseswithaltitudeandmaycauseseriousstressfortrees,asismadeevidentbythedeformedshapesathighaltitudes.So