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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,ar
2、ainforestisanenvironmentthatreceiveshighrainfallandisdominatedbytalltrees.Awiderangeofecosystemsfallintothiscategory,ofcourse.Butmostofthetimewhenpeopletalkaboutrainforests,theymeanthetropicalrainforestslocatedneartheequator.Theseforestsreceivebetween160and400
3、inchesofrainperyear.Thetotalannualrainfallisspreadprettyevenlythroughouttheyear,andthetemperaturerarelydipsbelow60degreesFahrenheit.Thissteadyclimateisduetothepositionofrainforestsontheglobe.BecauseoftheorientationoftheEarth’saxis,theNorthernandSouthernhemisph
4、ereseachspendpartoftheyeartiltedawayfromthesun.Sincerainforestsareatthemiddleoftheglobe,locatedneartheequator,theyarenotespeciallyaffectedbythischange.Theyreceivenearlythesameamountofsunlight,andthereforeheat,allyear.Consequently,theweatherintheseregionsremain
5、sfairlyconstant.Theconsistentlywet,warmweatherandamplesunlightgiveplantlifeeverythingitneedstothrive.Treeshavetheresourcestogrowtotremendousheights,andtheyliveforhundreds,eventhousands,ofyears.Thesegiants,whichreach60to150ftintheair,formthebasicstructureofther
6、ainforest.Theirtopbranchesspreadwideinordertocapturemaximumsunlight.Thiscreatesathickcanopy（树冠）levelatthetopoftheforest,withthinnergreenerylevelsunderneath.Somelargetreesgrowsotallthattheyeventoweroverthecanopylayer.Asyougolower,downintotherainforest,youfindle
7、ssandlessgreenery.Theforestfloorismadeupofmoss,fungi,anddecayingplantmatterthathasfallenfromtheupperlayers.Thereasonforthisdecreaseingreeneryisverysimple:Theoverabundanceofplantsgatheringsunlightatthetopoftheforestblocksmostsunlightfromreachingthebottomofthefo
8、rest,makingitdifficultforrobustplantstothrive.TheForestfortheTreesTheamplesunlightandextremelywetclimateofmanytropicalareasencouragethegrowthoftoweringtreeswithwidecanopies.Thisthi
