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1、大学英语六级样题RainforestsTropicalrainforestsarethemostdiverseecosystem(生态系统)onEarth,andalsotheoldest.Today,tropicalrainforestscoveronly6percentoftheEarth’sgroundsurface,buttheyarehometooverhalfoftheplanet’splantandanimalspecies.WhatIsaRainforest?Generallyspeaking,arainforestisanen
2、vironmentthatreceiveshighrainfallandisdominatedbytalltrees.Awiderangeofecosystemsfallintothiscategory,ofcourse.Butmostofthetimewhenpeopletalkaboutrainforests,theymeanthetropicalrainforestslocatedneartheequator.Theseforestsreceivebetween160and400inchesofrainperyear.Thetotalan
3、nualrainfallisspreadprettyevenlythroughouttheyear,andthetemperaturerarelydipsbelow60degreesFahrenheit.Thissteadyclimateisduetothepositionofrainforestsontheglobe.BecauseoftheorientationoftheEarth’saxis,theNorthernandSouthernhemisphereseachspendpartoftheyeartiltedawayfromthesu
4、n.Sincerainforestsareatthemiddleoftheglobe,locatedneartheequator,theyarenotespeciallyaffectedbythischange.Theyreceivenearlythesameamountofsunlight,andthereforeheat,allyear.Consequently,theweatherintheseregionsremainsfairlyconstant.Theconsistentlywet,warmweatherandamplesunlig
5、htgiveplantlifeeverythingitneedstothrive.Treeshavetheresourcestogrowtotremendousheights,andtheyliveforhundreds,eventhousands,ofyears.Thesegiants,whichreach60to150ftintheair,formthebasicstructureoftherainforest.Theirtopbranchesspreadwideinordertocapturemaximumsunlight.Thiscre
6、atesathickcanopy（树冠）levelatthetopoftheforest,withthinnergreenerylevelsunderneath.Somelargetreesgrowsotallthattheyeventoweroverthecanopylayer.Asyougolower,downintotherainforest,youfindlessandlessgreenery.Theforestfloorismadeupofmoss,fungi,anddecayingplantmatterthathasfallenfr
7、omtheupperlayers.Thereasonforthisdecreaseingreeneryisverysimple:Theoverabundanceofplantsgatheringsunlightatthetopoftheforestblocksmostsunlightfromreachingthebottomoftheforest,makingitdifficultforrobustplantstothrive.TheForestfortheTreesTheamplesunlightandextremelywetclimateo
8、fmanytropicalareasencouragethegrowthoftoweringtreeswithwidecanopies.Thisthi
