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1、托福作文写作范文详解ReadingPopulationsoftheyellowcedar,aspeciesoftreethatiscommoninnorthwesternNorthAmerica,havebeensteadilydecliningformorethanacenturynowzsinceabout1880.Scientistshaveadvancedseveralhypothesesexplainthisdecline.Onehypothesisisthattheyellowcedardeclinemaybecausedbyinsectparasites,specifically
2、thecedarbarkbeetle.Thisbeetleisknowntoattackcedartrees;thebeetlelarvaeeatthewood.Therehavebeenrecordedinstancesofsustainedbeetleattacksoverwhelmingandkillingyellowcedarszsothisinsectisagoodcandidateforthecauseofthetree,sdecline.Asecondhypothesisattributesthedeclinetobrownbears.Bearssometimesclawatth
3、ecedarsinordertoeatthetreebarkzwhichhasahighsugarcontent.Infact,thecedarbarkcancontainasmuchsugarasthewildberriesthatareastapleofthebearszdiet.Althoughthebearsclawingisunlikelytodestroytreesbyitselfztheiraggressivefeedinghabitsmaycriticallyweakenenoughtreestoberesponsibleforthedecline.Thethirdhypoth
4、esisstatesthatgradualchangesofclimatemaybetoblame.Overthelasthundredyears,thepatternsofseasonalaswellasday-to-daytemperatureshavechangedinnorthwesternNorthAmerica.Thesechangeshaveaffectedtherootsystemsoftheyellowcedartrees:thefinesurfacerootsnowstartgrowinginthelatewinterratherthanintheearlyspring.T
5、hechangeinthetimingofrootgrowthmayhavesignificantconsequences.Growingrootsaresensitiveandarethereforelikelytosufferdamagefrompartialfreezingoncoldwinternights.Thisfrozenrootdamagemaybecapableofunderminingthehealthofthewholetreezeventuallykillingit.ListeningUnfortunately,westilldonrtknowwhatskillingt
6、heyellowcedarznoneoftheexplanationsdiscussedinthereadingisadequate.First,thecedarbarkbeetle.Well,theproblemwiththisexplanationisthathealthyyellowcedarsaregenerallymuchmoreresistanttoinsectinfestationthanothertreespecies.Forexample,thebarkandleavesofyellowcedararesaturatedwithpowerfulchemicalsthatare
7、poisonoustoinsects.So,healthycedarsareunlikelytosufferfrominsectdamage.Solhowcanweexplainthosedeadcedarsthatwereinfestedwithbeetles.Inthosecases,thebeetlesattacktreesthatwerealreadydamagedorsickzandwouldzveprobablydiedanyway.So,thebeetlesarenotthefundamentalcauseresponsibleforthedeclineofyellowcedar
8、s.Second,althoughbearsdamagesometreeszthere7renotthecauseoftheoverallpopulationdecline.Yellowcedarpopulation;sbeendecliningallacrossthenorthwesterncoastofNorthAmericabothonthemainlandandonislandsjustoffthecoast.Therewerenobearsontheislands,yettheislandscedarsarestillindecline.Sincethedeclineoccurswi
9、thandwithoutbears,thebearscannotberesponsible.Andfinally,thetheoryaboutrootssufferingfromfrostdamage,well,thereadingpassageforgottotakeonefactorintoaccount.Manymoretreesaredyingatlowerelevationswhereitiswarmthanathigherelevationswhereitiscold.Iffreezingdamagewereresponsibleforthedecline,wecouldexpec
10、ttoseemoretreesdyinginthecoldweatherofhigherelevations.Instead,moretreesaredyingintherelativewarmthofthelowerelevations.So,althoughtheclimatechangemayhavemadethecedarrootsmoresensitivethanitusedtobezthisisntwhatskillingthem.一、阅读听力要点概括ReadingPassageMainpoints:Scientisthavesomehypothesestoexplainthedr
11、opofyellowcedarpopulation.Subpoint1:Declinecausedbyinsectparasites.Subpoint2:Declinecausedbybrownbearsforfood.Subpoint3:Declinecausedbycoldwinter.LectureAttitude:Against.Noneexplanationsisadequate.SubPoint1:Healthyyellowcedarscanresistinsectinfestation.(Poison.Treealreadysick)SubPoint2:Bearsarenotth
12、ecauseofoveralldecline.(Declineonnobearisland)SubPoint3:Coldweatherisnotresponsibleforthedecline.(Dieatlowerwarmelevations)二、范文逐段贯析Paragraph1Thelecturerholdsthatthemainreasoncausingtheoveralldeclineinyellowcedarpopulationhasnotyetbeendetermined.Theproposedreasonsinthereading,thoughresponsibleforthep
13、oorhealthofsomeindividualplants,maynotaccountforthedeclineofthewholespeciesthroughouttheNorthAmericanContinent.此段功能：（Listeningpassage后文提到用L代替）（Readingpassage后文用R代替）以L为根本，指出L反对R的结论，说造成黄杉树群总数下降的原因还未确定。随后复述R的观点，指出这些原因只能造成不健康的树木的死亡而不能造成北美总数量的下降。此段结构：首段第一句话以L出发，复述了L的结论，确认逻辑为相反。第二句复述R的结论，使读者明白了L和R要争论的点。随即
14、准备讨论。Paragraph2First,healthyyellowcedartreescansecreteachemicalthatispoisonoustoinsectsfeedingonitsbark.Hencezitisunlikelythatthecedarbarkbeetlecaneverattackoneofthesetreesbeforeitfallsillordies.Thereadingthusfindsamisleadingcausalrelationshipbetweentheinsectandthetree.此段功能：在第一个分论点中,L列举证据显示健康的黄杉能分泌出
15、毒液防止虫害。所以其实这些虫子基本不可能导致健康树的死亡。随后指出并不像R说的那样，其实树的死亡和虫害没什么关系。此段结构：此段为分总结构。为了证明R的逻辑的不严密性，先列举了L中的真实证据，得出L的结论，最后说明R的结论是误导。Paragraph3Secondzbearscannotbeblamedforthelarge-scaleddeathofyellowcedarsacrossNorthAmerica,althoughtheymightberesponsiblefortheaccidentaldeathofsomeindividualtrees.Anotherconditionthat
16、canabsolvebearsofthisaccusationisthatyellowcedarsgrowingonbear-freeislandsarealsodyinginlargenumbers.Herebythereadingmaterialfailstoidentifytheprimaryreasonagain.此段功能：在第二个分论点中，作者还是做总结，列举了L的证据和观点，熊是不能造成大规模黄杉死亡的，最多也就只能造成个别死亡，因为没有熊存在的岛屿，黄杉也在大量死亡。这样看来，R的结论自然不对。此段结构：此段为总分总结构，先总结L的结论的正确性并列举证据，最后一句又总结了R是不正确的。Paragraph4Third,coldclimatesalsocannotbeblamedforthegeneraldeclineincedarpopulation.Theproofcanbefoundin
