Farmers,especiallyindevelopingcountries,areoftencriticizedforcuttingdownforests.Butanewstudysuggeststhatmanyfarmersrecognizethevalueofkeepingtrees.

Researchersusingsatelliteimagesfoundatleasttenpercenttreecoveronmorethanonebillionhectaresoffarmland.Thatisalmosthalfthefarmlandintheworld.Earlierestimatesweremuchlowerbutincomplete.Theauthorsofthenewstudysayitmaystillunderestimatethetrueextentworldwide.

ThestudyfoundthemosttreecoverinSouthAmerica.NextcomesAfricasouthoftheSahara,followedbySoutheastAsia.NorthAfricaandWestAsiahavetheleast.

Thestudyfoundthatclimateconditionsalonecouldnotexplaintheamountoftreecoverindifferentareas.Norcouldthesizeofnearbypopulations,meaningpeopleandtreescanlivetogether.Thereareareaswithfewtreesbutalsofewpeople,andareaswithmanytreesandmanypeople.Thefindingssuggestthatthingslikelandrights,marketsorgovernmentpoliciescaninfluencetreeplantingandprotection.

DennisGarrity,whoheadstheWorldAgroforestryCenter,saysfarmersareactingontheirowntoprotectandplanttrees.Theproblem,hesays,isthatpolicymakersandplannershavebeenslowtorecognizethisandtosupportsuchefforts.

Thesatelliteimagesmaynotshowwhatthefarmersareusingthetreesfor,buttreesprovidenuts,fruit,woodandotherproducts.Theyalsohelppreventsoillossandprotectwatersupplies.Evenunderdrought(干旱)conditions,treescanoftenprovidefoodandawaytoearnmoneyuntilthenextgrowingseason.

Sometreesactasnaturalfertilizers.Theytakenitrogen(氮气)outoftheairandputitinthesoil.ScientistsattheCentersaytheuseoffertilizertreescanre-ducetheneedforchemicalnitrogenbyuptothree-fourths.Treesalsocapturecarbondioxide,agaslinkedtoclimatechange.

小题1:Throughthestudy,theresearchersfoundthat .