Eveniftreescannotwalktheyarestillonthemove.
InpartsoftheArcticentireforestsaremovingnorthward.AcrosstheArctictemperaturesarerisingfasterthananywhereelseintheworld.Asthathappensthetreelinethatmarkswhereforestsstopandthetreelesstundra(冻土地带)startshasbeenshiftingnorthward.Treesgrowingalongthetreelinemustprotectthemselvesfromthecoldwind.Todothisplantstendtogrowhorizontal(水平的)brancheslowtotheground.Theenergyittakesfortreestogrowthiswaymeanstheydon'thaveenoughenergytomakeseeds.
ButasEarth'sclimatehasbeenwarmingtreesnolongerhavetojustgrowhorizontally.Manycaninsteadgrowuptowardthesky.Thistakeslessenergy.Andwithalltheleftoverenergy.thesetreeshavestartedproducingmoreseeds.Thishappensespeciallyinplaceswherethewhitespruce(白云杉)grows.
WhitesprucewhichisaNorthAmericantreeisquiteabletoproducealotofseedswhichcanmovelongdistancesinthewind.Whenwind-blownseedsenduponthetundrabeyondthetreelinetheyeventuallycansprout(发芽)newtrees.Thisexplainshowaforestcanmove.Ofcoursetheprocesswouldworkonlyifthetundrawerewarmenough.Butinrecentyearsthewholeplanethasbeenwarming.
Newtreeswillprovidesheltersforsomesnowkeepingthesun’sraysfrommakingthewhitesurfacedisappear.Insteadthetreesabsorbthesun'sheat.Thiswarmsthesurroundingair.Theextrawarmthencouragesevenmoretreestoproduceseeds.Thatfurtherboostsaforest'sabilitytoexpand.Inadditionmoretreeswilltrapmoresnowpreventingmuchofitfrombeingblownaway.Snowcantrapheatinthesoilbelowwhichencouragestreestogrow.Therecentriseoftemperaturehashelpedmoretreesgrow. Therecentriseoftemperatureshashelpedmoretreesgrowpastthetreeline.Peopleworryaboutimpactsontheanimalsthatdependonfrozenconditionsforfoodandshelter.
25.WhatisthemaincauseofthetreesintheArcticmovingnorthward?
A.Therisingtemperature. B.Theshrinkingofnortherntundra.
C.Thefierceandcoldwind. D.Theabundantresourcesinthenorth.
26.Whatdoestheunderlinedword“this”inParagraph2mean?
A.Toproducemoreseeds.
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