Unit6單元高考閱讀閱讀提示：本文為說(shuō)明文，根據(jù)一項(xiàng)研究，自20世紀(jì)30年代以來(lái)，加利福尼亞已經(jīng)失去了一半的大樹(shù)，氣候變化似乎是其主要因素。Californiahaslosthalfitsbigtreessincethe1930s，accordingtoastudytobepublishedTuesdayandclimatechangeseemstobeamajorfactor(因素）.Thenumberoftreeslargerthantwofeetacrosshasdeclinedby50percentonmorethan46，000squaremilesofCaliforniaforests，thenewstudyfinds.Noareawassparedorunaffected，fromthefoggynortherncoasttotheSierraNevadaMountainstotheSanGabrielsaboveLosAngeles.IntheSierrahighcountry,thenumberofbigtreeshasfallenbymorethan55percent；inpartsofsouthernCaliforniathedeclinewasnearly75percent.Manyfactorscontributedtothedecline,saidPatrickMcIntyre,anecologistwhowastheleadauthorofthestudy.Woodcutterstargetedbigtrees.Housingdevelopmentpushedintothewoods.AggressivewildfirecontrolhasleftCaliforniaforestscrowdedwithsmalltreesthatcompetewithbigtreesforresources(資源）.①ButincomparingastudyofCaliforniaforestsdoneinthe1920sand1930swithanotheronebetween2001and2010,McIntyreandhiscolleaguesdocumentedawidespreaddeathofbigtreesthatwasevidenteveninwildlandsprotectedfromwoodcuttingordevelopment.Thelossofbigtreeswasgreatestinareaswheretreeshadsufferedthegreatestwatershortage.Theresearchersfiguredoutwaterstresswithacomputermodelthatcalculatedhowmuchwatertreesweregettingincomparisonwithhowmuchtheyneeded,takingintoaccountsuchthingsasrainfall,airtemperature,dampnessofsoilandthetimingofsnowmelt(融雪).②Sincethe1930s,McIntyresaid,thebiggestfactorsdrivingupwaterstressinthestatehavebeenrisingtemperatures,whichcausetreestolosemorewatertotheair,a