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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
17、确的。Paragraph4Third,coldclimatesalsocannotbeblamedforthegeneraldeclineincedarpopulation.Theproofcanbefoundinthetreeswiderrecessioninareasoflowerelevation,wherethetemperatureiswarmerthanathigherelevations.Thoughcoldweathermayhavemadecedarsmoresensitive,itisnottheprimarykillerasthereadingindicates.此段功能
18、：讨论最后一个原因，总结冷天气是不能造成大面积黄杉下降的。原因是,在低纬度温暖地区，黄杉在大量死亡，在高纬度寒冷地区并没有出现比温暖区死亡更多的情况。虽然冷天气造成了黄杉的更敏感,但不是元凶。此段结构：此段为总分结构,第一句复述L的结论。并很好了做了类比,证明L的正确性。三、范文全篇总评1 .逻辑结构全文遵从总分结构，第一段立刻指出L觉得造成黄杉数量下降的原因notyetbeendetermined,所以对R的态度主要是，R的解释都不充分（notadequate）.在接下来3段的分论点中，L分别对R的结论中的证据进行了更新，说明R之前做出的粗略的结论是不充分的。真实的原因还有待考察。2 .语言
19、表达本文语言表达，没有用复杂的句式，也没有大量采用难词生词，论证却十分准确。分段开头用1st,2nd,3rd虽然老套,但是叙述十分清楚。其中一些列举证据，论点可以借鉴的句式：1 （论点A）Theproofcanbefound.（证据A）.Thedefendantisnotguilty.Theproofcanbefoundwhennewevidenceisprocessed.2 .absolve（某物，某人，某事件）ofthisaccusation免除（某）的责任。Thenewlyfoundevidencecanabsolvethedefendantofhisaccusationofmurder.
