ContentslistsavailableatJournalofAridEnvironmentsjournalhomepagewwwels - PDF document

1 ContentslistsavailableatJournalofAridEnv
ContentslistsavailableatJournalofAridEnvironmentsjournalhomepage:www.elsevier.com/locate/jaridenv Aracetotheunknown:Contemporaryresearchontreeandforestdroughtresistance,anIsraeliperspectiveTamirKleinDepartmentofPlant&EnvironmentalSciences,WeizmannInstituteofScience,76100,Rehovot,IsraelARTICLEINFOTreephysiologyTreegenetics E-mailaddress: milder(IMSreport).IntheMediterraneanregion,includingIsraelatitssoutheastcorner,precipitationcontinuedtodecreaseandevaporationcontinuedtoincrease.MeteorologicalrecordsfortheMiddleEastshowanongoingwarming,predictedtoreachalevelof+4°Cbylate21stcentury(Evans,2009Lelieveldetal.,2012).Dryingisexpectedatthenorthoftheregion(Turkey,Syria,andLebanon),andinspiteofamildincreaseinprecipitationpredictedforsouthoftheregion(e.g.theArabpeninsula),thewetseasonisbecomingshorter(Evans,2009etal.,2012).Thedroughtyearof2000followedthe1999droughtFig.1),andbroughtuponsignsofwaterstressandevenmortalityepisodesinsomeforests.Theabove-averageprecipitationyearsof2004relievedthislong-termstress,butonlyforashorttime:Thenextyearsupuntil2010(2013inthesouthofIsrael)hadallbelow-averageprecipitation.Infact,onlytheperiodof19581963wasdrierinthehistoricalmeteorologicalrecorded.Theseconddecadeofthe21stcenturyhadsofarsomeabove-averageyearsinthesouth(2013butacontinueddroughtinthenorth,ongoingsince2014(Fig.1Droughtismorethanadecreaseinannualprecipitation.Temporaldistributioniskey,andmoreoverinthehighlyseasonalEastMediterranean,wherea46monthslongdryseasonmeanszerorainforthisextendedlengthoftime(Raz-Yaseefetal.,2010b).TheMt.Carmelforestrein2010,whichisthelargestforestlossinthehistoryofthestateofIsrael(~1000ha),servesasanexample.Thereerupted,anddistributedatunprecedentedrate,onthebackgroundofoneofthedriestautumnperiodseverrecorded.Essentially,thismeanta~10monthslongdryperiod.Bythetimethereerupted,theforestwassowater-depleted,thattreebiomassrapidlyturnedintofuelwood.Pestoutbreaks,e.g.thatoftheIsraelipinebastscale(Matsucoccusjosephicanalsoberegardedassecondaryeectsofdrought,whichinturnreducestreepestresistancemechanisms(Mendeletal.,1997).Theepidemicwhichseverelyaected�1400haofMedi-terraneanforest,mostlyPinushalepensis,peakedin19901991,fol-lowingthedroughtyearsof19891991,andthoseof19841987earlierintherecord.Takentogether,climatictrendssince2000virtuallyrealizethedisturbingpredictionsgivenearlierfortheregion(Cohenetal.,2002withlessrainfall,longerdryperiods,andmoreextremeheatwaves.Drought-inducedtreemortalityhasbeenobservedindiusepatternsinforestsacrossthecountry,typicallyaecting~5%ofthetreesineachforest(Dormanetal.,2013a).Insemi-aridforestsinthesouth,rateswereupto20%.Amongspecies,CupressussempervirensPinuspineaP.brutiaweretypicallymoresensitivethanTetraclinisarticulata;AmongEucalyptspecies,Eucalyptusocci-E.camaldulensis,andE.gomphocephalaweremoresensitiveE.spathulataE.torquata,andE.sargentii.Mortalitywasalsoob-servedoutsideplantedforests,e.g.inQuercuscalliprinosinmaquisinthenorth(SeverandNeeman,2008),andinAcaciaraddianaintheAravavalley(Wintersetal.,2015).Muchwassaidandwrittenabouttherateoftreeadaptationontheonehand,andtherateofclimatechangeontheother(Matyas,1996).Concernstemsfromthesimplefactthattrees,beinglong-livedorganisms,adaptovertime-scalesofcenturies,whereascurrentclimaticchangesdeveloponadecadaltime-scale.Thecurrentopenquestioniswhethertreeandforestresearchcanclosethegap,oeringnewwaysforsustainableforestsunderthedryingconditions.TheobjectiveofthefollowingreviewistoprovideanIsraeliper-spectiveonthemajordevelopmentsinresearchonforestdroughtre-sistanceinthe21stcentury,inthecontextofglobaldesertiNaturally,asinglepaper,asextensiveasitmaybe,cannotprovideanexhaustivelistcoveringallresearchactivities.Manyimportantstudies,atlocaltoregionalscalesandformsingletomultiplespecies,arenotdiscussedhereduetolengthlimitations.Instead,thetextfocusesonkeystudiesthatpushedtheeldforward,andinsomecasesconveyedabroadermessageandconsequencesoutsideIsrael.Insomecases,in-ternationalcontributionswhicharerelevanttoforestsinIsraelarediscussed,butthemainfocusofthisreviewisonresearchactivitiesinIsrael.Thesecontributionsarefurtherdividedintovecategoriesofforestdroughtresistanceresearch,describedintherespectivechapters:(1)treephysiology;(2)treegenetics;(3)forestuxes;(4)remotesensingofforest;and(5)forestmanagement.1.1.ForestdroughtresistancethroughtreephysiologyresearchThemolecularbiologyrevolutioninbiologicalresearch,whichstartedinthe1990s,madenumerousbreakthroughsinmanyeldsofbiology,includingtheplantsciences.Yetthiscausedamajorshiftofresourceallocationtogeneticandmolecularapproaches,onexpenseofthemoretraditionalphysiologicalandanatomicalapproaches.Itwasonlyafteradecadeortwothatthelackoftreephysiologyresearchbecameevident,andareactiontrendstarted.Infact,contemporarytreephysiologistsarenowbeingaskedtoclosea~20-yearsgapinresearchoftreestructureandfunction.Thiscallbecomesincreasinglyurgentinfaceoftheexistingchallenges,anddroughtstressinparticular.Amongtreedroughtresistancemechanisms,waterrelationstraitsarenaturallythemajorsubjectofstudy.Improvementstosapmeasurementtechniquesinthe1990s(GranierandLoustau,1994CermakandNadezhdina,1998)enabledtheaccuratemeasurementoftreewater-usepatterns,inbothmesicandxericconditions.Cohenetal. Fig.1.Forestsunderincreasingwaterlimitation.ChangesinannualprecipitationforthepastsevendecadesoverthethreemajorforestareasinIsrael:BeerSheva(south),Jerusalem(Center),andZefat(North).Das

2 hedlinesrepresentthelong-termmeanforeach
hedlinesrepresentthelong-termmeanforeacharea.Asignicantlong-termdryingtrendhasalreadybeenobservedintheNorth(solidline;=0.045).DataarefromtheIsraelMeteorologicalServicereport(2015)anditsdatabase.T.Klein measuredsapowinsixmajorforesttreespeciesinIsraelandprovidedthespecies-specicradialgradientofsapvelocity.Althoughnotdirectlylinkedtotreedroughtresistance,thisimportantstudyprovedvitalininterpretingsapowpatternsinotherstudies.Inaseriesofstudies,Schilleretal.(2002,2003,and2007)reportedthewater-usepatternsofthesclerophyllousbroadleavedspeciesPhillyrealatifoliaQuercuscalliprinos,andQ.ithaburensis,respectively.Withthesepio-neeringsapowapplications,Schilleretal.providedtheannualpat-ternoftree-levelandstand-scalewater-useforeachofthethreespecies,anditsresponsetothelongsummerdrought.InQuercuscalliprinos,themostimportanttreespeciesofthelocalmaquisvegetation,water-usepersistedthroughoutthedryseason,albeitatsixthofthemaximumrate.Theannualtreewater-usewas44and75%oftheannualrainfallamountinwetanddryyears,respectively(Schilleretal.,2003Thinningdecreasedwater-useatthestand-scalebutincreasedthetree-levelwater-use,alongwithincreasedgrowth.Thewater-usestudiesonthesclerophyllousbroadleavedspecieshelpedunderstandingtheirintrinsicdroughttolerance.Thesespeciesaremostlyslow-growing,deep-rooted,andanisohydric-like,i.e.theirxylemcanwithstandverynegativetensions,e.g.awaterpotentialof7.0MPainthecaseofPhillyrealatifolia.ResearchonPinushalepensisthemajoraorestationtreespecies,revealedarathercontrastingpic-Kleinetal.(2005)showedstrongstomatalregulationinthispinespecies,limitingsummerwater-usetonear-zeroamountsinasemi-aridsite(Maseyketal.,2008)andclosingstomataat2.8MPa(Kleinetal.,).Suchaconservativebehaviorinatreeshouldconferhighersafety;butnotinthedrought-vulnerableP.halepensis,with50%lossofxylemhydraulicconductivityalreadyat3.1MPa(i.e.anarrowhy-draulicsafetymargin).SowhatmadeP.halepensisthesuccessstoryofIsraeliaorestationinspiteoftherecurringdrought?Recentresearchhintstoitsuniqueabilitytorecoverembolisminitsxyleminyetun-precedentedratesofafewhours(Kleinetal.,2016).ThephysiologicalerencesbetweenP.halepensisQuercuscalliprinos,thetwomostcommontreespeciesinthecountry,cantranslateintolargeeectsontheforestwaterbalanceanddroughtresilience(Kleinetal.,2013Fig.2).Xylemanatomystudiesshowthatthesearid-landtreeshavenarrowerconduitsthantheirmoremesicrelatives(etal.,2016P.halepensisVillar-Salvadoretal.,1997forthecloselyQ.cocciferawiththepinetracheidsbeingtypicallynarrowerthantheoakvessels.Sofar,theoakmaquisprovedmoredrought-re-sistant(Kleinetal.,2019);butthepineforestusesconsiderablylesswater,athighercarbonuptakerates,i.e.ithashigherwater-useeciency.Consideringtheongoingdryingandwarming(Lelieveldetal.,),thedecreasedprecipitationwillnotsucetosustaintheoakmaquis(Fig.2),whileextremedroughtsmightkillmorepinetreesKleinetal.,2019Thescienceofwoodanatomyoriginallystemmedfromthemoreeldofdendrochronology.Tree-rings,beinganaturalar-chiveoftree-climateinteractions,arestillapowerfultoolinthequestforimprovedtreedroughtresistance.Essentially,thestrongforcingexertedbythelimitedprecipitationfacilitatesinterpretationoftree-ringdatafromthearidzone(Kleinetal.,2005Touchanetal.,2008Dormanetal.,2013a).Plottingtree-ringwidthsagainstprecipitationdynamicsforJuniperusphoeniciaPinushalepensis,andQuercusaegilopsfromJordanyieldedusefulcorrelations(TouchanandHughes,1999Inturn,thesecorrelationspermittedclimatereconstructionsforthelastfourcenturies,highlightingclimatevariabilityinperiodsthatprecededmeteorologicalmeasurements.Asimilarapproachappliedonden-drochronologiesfromTunisia,identiedthepastconditionsaridforestsendured,includingdroughtyearsinthe19thcentury(Touchanetal.,).Lookingclosertoidentifyintra-ringgrowthdynamicscanpro-videmoreinformationonspecieseco-physiology.InIsrael,theuseoftree-ringcarbonisotopecompositionasanindexforwater-useeciencywasvalidatedthisway(Kleinetal.,2005);andcambialactivitywasdecipheredforthedeserttreeinanaridPatagoniawoodland(Giantomasietal.,2012).Recently,dendrochronologiesarebeingusedtoidentifyandpredicttreemortalityrisksonalocalscaleinIsrael(Dormanetal.,2013a)andontheglobalscale(Cailleretetal.,).Thelattermeta-analysisfoundthatagradualgrowthdeclineandanincreaseininter-annualgrowthvariabilitycanbeusedasearly-warningsignalsofdrought-inducedmortalityingymnospermspecies.Treedroughtresistancemechanismsgofarbeyondwaterrelations.Forexample,Maseyketal.(2008)notedadecouplingbetweenstemandneedlegrowthphenologiesinP.halepensis:stemgrowthfollowedthewetseasonandadjustedtothelocalwinterconditions,counter-intuitivelyformingthecoolestgrowthseasonacrossEurope(andYakir,2010),whereasneedlegrowthwasstilloccurringthroughoutthelongdryseason.Thismeantthatneedlelengthservesasagoodproxyfordroughtstressinthisspecies(Kleinetal.,2014a),butalsoraisedafundamentalquestionaboutgrowthatverylowassimilationrates.Atree-scalecarbonbalancerevealedthatcarbonstorageinlatewinterwascrucialforsubsidizingneedlegrowthinsummer(KleinandHoch,2015).SuchdetailedinformationiscrucialtounderstandhowinIsraelwillrespondtofutureconditions.Forexample,highertemperaturesinthedryseasoncouldexacerbatedroughtstress,butinthewetseasontheymightenhancegrowth;andashorterwetseasoncanbemoredetrimentaltotreesthanabelow-averagerainfallyearRaz-Yaseefetal.,2010a Fig.2.Oak/pinedierencesinwater-usetrans-latetohigherscales.Expectedchangesinthepar-titioningbetweenhydrologicaluxcomponentsin

3 responsetostanddensitychangesforaPinusha
responsetostanddensitychangesforaPinushale-forestandaQuercuscalliprinosmaquis.Theincreasinglyhigherstand-leveltranspirationamountinanoakvs.pinestandcomesonexpenseofthewateryield.Attypicalstanddensityof300treesandannualprecipitationof500mm(verticalandhorizontaldottedlines,respectively),thewateruseofP.halepensisandaQ.calliprinosstandsareexpectedtobe357and459mm(longdashlines),leaving143and41mmaswateryield,respectively(arrows).Suchanalysisalsoindicatesthecarryingcapacityforthedierentstands(e.g.,~350and~550treeshaforoakandpineat500mm).AdoptedfromKleinetal.2013.T.Klein 1.2.TreegeneticsforforestdroughtresistanceThefactthatcertaintreegenotypescanperformbetterunderdroughtthanothers,makesthegeneticapproachveryappealingtoforestmanagementprograms.Therststepsinthisdirectionwereattheecotype,ratherthanthegenotypelevel.Thecaseofthepan-P.halepensisprovenancetrialsshowsnicelyhowthiselddevelopedalongthepasttwodecades.Originally,IsraeljoinedtheUNFAOprogramonP.halepensisseedcollectionprovenancetrialsfromacrosstheMediterranean,contributingtwoprovenances(ElkoshandShachariya)andtheeldtrialsinBeitDagan,Yatirforest,andafewothersites.Schilleretal.(1986)werethersttostudyallelefrequenciesincertainloci,showingrelativelylittlevariation,andawestern/easternpopulationsubdivision.AnalysisofgenomicDNAandproteinpolymorphisminlocalpopulationsshowedanincreaseinthepercentofpolymorphicloci,from12%to48%,withsitedryness(etal.,2002).ThisstudyfurthershowedhowtheperipheralpopulationinYatirforestisunderselectionpressurethateliminateshomozygousandfavorsheterozygousgenome,inturnhighlightingtheroleofplas-ticityintreedroughtresistance.Phenotypicdierentiationbecameevidentasprovenancetrialsmatured,showingthehigherperformanceofcertainGreekprovenancesandthehigherdroughtresistanceoflocalvs.foreignpopulations(Atzmonetal.,2004SchillerandAtzmon,).Usingthesametrials,Kleinetal.(2012)providedthephysio-logicalcontexttotheseobservations,showinghigherxylemvulner-abilitytoembolisminprovenancesfromItalyandAlgeria,comparedtothosefromGreeceandIsrael,inturnwithhighergrowthrateorhigherwater-useeciency,respectively.TheinferiorityoftheprovenancesfromItalyandAlgeriacanrelatetothewetter,or,cooler,conditionsattheirsitesoforigin,respectively(Voltasetal.,2018),aswellastotheirlowerheterozygosity(Koroletal.,2002P.halepensisprovenanceresearchdidnotstopthere.Israeliforestscientists,incollaborationwithinternationalexperts,continuedusingthistemplatetofacilitategenotypeselectionforfutureaestationfromtheonehand,andtoimproveourunderstandingofun-derlyingmechanismsontheotherhand.Upuntil2016itwasclearthatxylemresistancetoembolismisthekeytotreedroughttolerance,butitwasstillunknownwhatthexylemstructuresunderlyingthistolerancewereexactly.Inconiferousxylem,waterowsthroughnarrowandshorttracheidsthroughmembrane-equippedpits,calledtorusandmargo,respectively.Studyingthexylemanatomyoftheexactsametreesofthesameprovenances,itwasfoundthatthosevulnerableprovenancesfromItalyandAlgeriahadsignicantlyloweroverlapbetweenthetorusandpitaperture,comparedtothosefromGreeceandIsrael(David-Schwartzetal.,2016).Essentially,thismeanthighertorusmargoclosingpressureinthelatterprovenances.Asinglethreadcon-nectstheanatomicalobservations(David-Schwartzetal.,2016)tothephysiologicalmeasurements(Kleinetal.,2012),andtheeldob-servationsshowingdierentialsurvivalratesoftheexactsamefourprovenances(SchillerandAtzmon,2009Fig.3).Takentogether,theseresultshighlightthestrengthofcontinuous,interdisciplinaryresearchontreedroughtresistanceinIsrael.manyways,theaforementionedmolecularbiologyrevolutionskippedtheeldofforesttreeresearch.Indeed,foresttreespresentmultipleobjectivechallengestomoleculartechniques:duetotheirre-lativelyslowgrowthrateandmaturation,transformationandpropa-gationtechniquesaremorecomplexinforesttrees;andthefactthatconifershavethelargestknowngenomesamongalllivingorganismsisanothercaveat.Yetlocal,contemporaryresearchseemstonallytakeonsomeofthesechallenges.Recently,theP.halepensisdroughtneedletranscriptomehasbeenconstructed(Foxetal.,2017).Thiselaborateprojectopensthegatetodetailedresearchofnumerousgenecandidatesinmultiplebiochemicalpathways.Agoodplacetostartwouldbethekeystructuralprocessesidentiedinpastphysiologicalresearchoftreedroughtresponse,suchascarbohydratemetabolism,aquaporinchannelactivity,cellwallsuberization,endodermisandexodermisstructure,xylogenesis,dierentialabove-andbelowgroundgrowth,andothersSchwinningandEhleringer,2001MasedaandFernández,2006).Inaddition,themolecularapproachholdsthepotentialtoexposenovel,yetunknownpathways.Byidentifyingthemajorcellularplayersindrought-exposedtrees,wearegraduallyimprovingourunderstandingoftheunderlyingmechanismsintreedroughtresistance.1.3.ForestuxesunderdroughtObviously,aforestismorethanthesumofitstrees.OneofthemajoradvancesinrecentresearchonforestdroughtresistanceinIsraelisinthequanticationofmajorforestuxes.Theseuxesareinvolvedinbiogeochemicalcyclingofelements(e.g.carbon,nitrogen)orcom-pounds(water),andtheirmonitoringislargelyfacilitatedbyrecentimprovementsinmicro-meteorologicaltechniques.Theimplementa-tionofeddy-covariance,infra-redgasanalysis,andisotopicanalysisinforestsinIsraelmarkedthetransitionfromclassicalforestecologytouxbiogeochemistry,andmuchofithasbeendirectedtowardtheresearchofforestdroughtresistance.Thecalltousethesouthern-mostpineaorestationatYatir(Koroletal.,2002)asalaboratoryforforestfunctionunderdroughtwaswellreceived,anda

4 17muxtowerwasconstructedthereintheyear20
17muxtowerwasconstructedthereintheyear2000(Fig.4).Yatirforestisamongthedriestcanopy-formingforestsworldwide,withleafareindexof~1.5atannualprecipitationof285mm(Sprintsinetal.,).TheroleoftheuxtoweratYatirgoesfarbeyondlocalorre-gionalinterest,asitrepresentsavastglobalareaofsemi-aridforests Fig.3.ThepowerofinterdisciplinaryapproachintreedroughtresistanceThehigherdrought-inducedmortalityratiooftheItalianandAlgerianAleppopineprovenances,vs.theIsraeliandtheGreek,observedbySchillerandAtzmon(2009;top)hadaphysiologicalbasisofhighersensitivitytoembolism(Kleinetal.,2013;middle).Embolismlevelwasprovenance-spe-cacrosssemi-arid(SA)andThermo-Mediterranean(TM)sites.Inturn,thehighersensitivitywasexplainedbylowertorustopitapertureoverlapinthexylem(David-Schwartzetal.,2016T.Klein RotenbergandYakir,2010).Overtheyears,majoruxeshavebeenmeasuredonsite,exposingmultipleecosystem-levelprocessesofdroughtresistance.Constructingthehydrologicalbalanceofaforestisprerequisitefortheevaluationofitsdroughtresistance.ResearchinYatirforestalongthepasttwodecadesdemonstratesthefastevolutionofresearchcap-abilities,whichfollowedthetechnologicaladvances,fromsap-SchillerandCohen,1998)tosoilevaporationandecosystemevapo-transpiration(Raz-Yaseefetal.,2010a,b)andsoilhydrology(etal.,2014b).Theremarkableprecipitation-useeciencyofthesemi-aridforest,with�90%oftherainfallcyclingtoevapotranspiration,and~60%oftherainfalltranspiredbytrees,wastherstdiscoverySchillerandCohen,1998Raz-Yaseefetal.,2010a,bUngaretal.,Kleinetal.,2014b).Focusingontheinterplaybetweencanopycover(afunctionofstanddensity),soilevaporation,andtreetran-spiration,hasalreadyidentiedanupperlimittostanddensityunderthelowwaterbudget(Raz-Yaseefetal.,2010a).Furtherworkhigh-lightedtheroleofrainwaterinputrate,showingthatrainstormsweremoreeectiveforforestwateravailabilitythanahighernumberofsmallerrainevents(Raz-Yaseefetal.,2010b).Inthatsense,climateextremessuchasrainstormsmightcompensateforanoveralldryingconditionsforecastedfortheregion.Deeperanalysisofthesoilstruc-tureinYatirexposedtheroleofaclay-richlayer,at0.5mbelowground,inholdingthewateraboveit(Kleinetal.,2014b).Accuratelyquanti-fyingthesoilwaterdynamicsallowedforpredictionoftreedroughtstress,includingtheobservedmortalityin2009(Fig.3).Inasense,thisapproachextendsontheearlierstudyofbedrockasamajorsitequalityparameter,showinghigherpineperformanceonmarlandchalk�dolomite�limestone(Schiller,1982Forestsareamajorcomponentinthecarboncycle,andtheirdroughtresistanceinteractswiththeirabilitytostorecarbon,whilecontinuouslyreceivingmoreanthropogenicCOfromtheatmosphere.Usingtheeddy-covariancemethodologyontopoftheYatiruxtower,Grünzweigetal.(2003)haveshownanunexpectedlyhighcarbonse-questrationratesof~0.2kgCm,inlinewiththeratesmea-suredovertemperateforests.Thissingleobservationwasarevolutionbyitself,replacingourpreviousviewoflocalforestsbeingapoorfractionofthefromhigherlatitudes,andemphasizingthevastpotentialofsemi-aridforestswithintheglobalcarboncycleandclimatesystem(andYakir,2010).Inaddition,welearntthatforestsinIsraelarealreadyadaptingtothelong-termdroughtconditionsinawaythatdisqualimeasurementsoftemperateforestresponsetodroughtepisodesasamodel(Grünzweigetal.,2009Kleinetal.,2014c).Whichfactorscontributedtotheobservedenhancedcarbonstoragefollowingaestation?Fieldmeasurements,complementedbyisotopicanddecom-positionexperiments,indicatedhighsoilorganiccarbonaccumulationGrünzweigetal.,2007)andhighbelowgroundcarbonallocation,ac-countingfor~40%ofcanopyCOuptake(Grünzweigetal.,2009Unexpectedly,thisenhancedcarbonstoragewasatotherwiseun-changedsoilnitrogenconcentration(Grünzweigetal.,2007).Thishighnitrogenuseeciencymightberelatedtohighsynchronizationbe-tweennitrateavailabilityandtreeactivityinthewetseason,inturntingfromattenuatednitrateproductionfromammoniaoxidationGelfandandYakir,2008).Clearly,abioticandbioticrhizosphereprocessesholdgreatpotentialforfurtherimprovingforestdroughtre-sistanceinIsraelandbeyond.Despiteofitsmanydiscoveries,theuxapproachtoforestdroughtresistanceisfarfromrealizingitsfullpotential.Asmeasurementsintheatmosphereandsoilintensify,morefocusedresearchbecomesavail-able.Asoneexample,focusingonshortheatwaveperiods,etal.(2016)managedtodecoupletheeectsofsoilandatmosphericdroughtonthesemi-aridpineforest.Forestcarbonandwatervaporexchangesshowedastrongresponsetoairdryness,thatweakenedassoilmoisturedecreased.Thelarge(~60%)decreasesincarbonex-changeandcanopyconductivitytowatervaporduringaheatwave,typicallyrecoveredtopre-stresslevelswithinasingleday,demon-stratingthehighecosystemplasticityandresiliencetodroughtTatarinovetal.,2016).Therichdatasetscomingfromuxmeasure-mentsarealsoagoldmineforeco-physiologicalandeco-hydrologicalmeasurementandmodeling(Fig.5).Incorporationofsuchmodelsinpredictingforestresponsetodroughthasalreadyhelpedupscalingspatialmeasurements(Kleinetal.,2014b),andshouldintensifyinthenearfuture.TwoofthebestcandidatesitesforintegratingamodelingapproacharethestanddensityexperimentsinKdoshimandYatirfor-ests.Thesethinningandgrazingexperimentsarenotonlylarge-scalebutalsolong-termandconsistentinmonitoringprocedures,underthelong-termecologicalresearch(LTER)framework.Theseeldmanip-ulationsholdgreatpromiseforsustainableforestryunderclimate2.Forestdroughtresistance:resultsfromremotesensingWhenstudyingvegetationpatternsoverlargeareas,suchasinforests,remotesensingtoolsbecomeincreasinglyimportant.Th

5 eon-goingincreaseinthediversityandresolu
eon-goingincreaseinthediversityandresolutionofsensorsontheonehand,andintheabilitytoanalyzeandsynthesizetheiroutput,alsousingmodels,ontheotherhand,meansamoreintensiveutilityinfu-tureresearch.AerialphotographyofforesthasbeenusedinthepastinIsrael,andshouldproduceimportantdataonforestdroughtresistanceincomingyears.Itissatelliteimaging,however,thathasrecentlymadeascienticbreakthroughinlocalforestresearch.UsingLandsatsatelliteimagesfor1994Dormanetal.(2013a,b)estimatedforestper-formanceunderdroughtacrossIsraelbythenormalizeddi Fig.4.Studyingforestdroughtresistanceatthefrontierofthedrytim-.Yatirforestin2010:Theuxtowerovertheforestcanopy(top);useAleppopinemortality(middle);Andanear-emptywaterreservoir,theresultofprolongeddrought,highlightingtheforestroleinthelocalwaterbalance.Photos'credits:TamirKlein,EmanuelaNegreanu.T.Klein vegetationindex(NDVI).Forestsinthesouth,atthedrytimberline,weremoreaectedthanthoseincentralIsrael(mostlywestofJer-usalem),whichneverthelesshadhighersensitivitytoprecipitationthanforestsbothsouthandnorthofthem(Dormanetal.,2013a).Locally,higherperformanceinnorthernvs.southernaspectswasobservedinwetyearsbutnotindroughtyears(Dormanetal.,2013b).Thein-tegrationofNDVIdatawithtree-ringchronologies(Dormanetal.,)provedpowerfulinafollow-upstudyfocusedondrought-in-ducedtreemortality(Dormanetal.,2015a,b).Thediusetreemor-talityinLahavandDvira,twodrytimberlineforests,washigherinolder-agedsparsestands,onsouthernaspects,andondeepersoils.Thesepatternsexposedthehighcomplexityindecipheringmortalityrisks,aswellastakingmeasurestoreducesuchrisks,e.g.thebenetsofforestthinning.ThepotentialofremotesensinggoesbeyondtheNDVI.Recently,additionalsatellitedatahavebeenutilizedtogetherwithmeteor-ologicaldataandadvancedmodelingapproaches.Usingtheenhancedvegetationindex(EVI)combinedwithNDVIderivedfromthemod-erate-resolutionimagingspectrometer(MODIS)onboardtheTerrasa-tellite,itwasshownthatwateryieldrangedfrom48%to8%ofpre-cipitationinlocalhumidanddryforests,respectively(Helmanetal.,).Inaddition,thedrierforestsshowedhigherhydrologicalresi-liencetodroughtthanthemorehumidforests.Oneofthestrengthsofthatnational-scaleanalysiswasthatmanyofits36predenedforestareasweredominatedbyPinuspineaPinusbrutiaratherthanthemorecommonP.halepensis,thoughnosignicantspecieseectwasobserved.Next,asimilarinterdisciplinarymethodologywasappliedinacomparativestudyoftheP.halepensisforestandtheoak()maquisonMt.Carmel(Helmanetal.,2017b).Theremotesensingapproachconrmedtheupscalingofeco-physiologicalmea-surementsshowinghigherwater-useeciencyandlowerwater-useofpinevs.oak(Kleinetal.,2013Fig.2).Last,thisanalysispredictedreductionsof16%intheproductivityofbothforests,forprojectedincreaseinmeantemperatureof1°C,asresultofglobalwarming.3.ForestmanagementforimprovedforestdroughtresistanceTherecentdroughteectsonforests,andthe2010Carmelaftermath,echoedintheIsraeliforestmanagementcommunity,bringingtogetherforestscientists(Osemetal.,2011)andforestman-agers(Poratetal.,2017).Oneoftheresultsofthiscollaborativeworkwasthecreationofaforestmanagementcode(Osemetal.,2013)anditsadoptionbythenationalforestserviceKKL,withguidelinesforapplication(Poratetal.,2017).TherisksinvolvedinanAleppopinemonocultureapproachwerealreadyknown,includinghighability(Carmeletal.,2009)andpestvulnerability(etal.,).Forthisreason,Osemetal.(2009)testedthepotentialoftransformingexistingplantationsintomixedMediterraneanforeststhroughnaturalregeneration,indicatinglargedierencesbasedonthelocalwateravailability(seealsoer,2012).Thelargedrought-in-ducedforestlossintheCarmelrewasamajorcatalystpushingforachangeinexistingpractice.Prescribedthinningandgrazing,togetherwithconservationofthemaquisvegetation,anddelineationofsafetyrebelts,wereamongthechiefaftermathrecommendationsPerevolotskietal.,2011).Traditionally,grazinginsemi-aridwood-landshasbeenassociatedwithlanddegradationandsoilerosion(asintheGreekislandofCrete;Hilletal.,1998).Theremovalofforestun-derstoryplantsandtreelitterbyheavygrazingaectsecosystempro-cessesfromcarbonsequestration(Nosettoetal.,2006)toinsectcom-munitycomposition(Molinaetal.,1999).Yettheseeectsappearmorecomplexthanpreviouslythought,withsoilorganiccarbonandrootcarbonvirtuallyunaectedbygrazing(Nosettoetal.,2006).Instead,controlledgrazingbecomesanimportantmanagementtoolinsemi-aridThenationalmasterplan22forforestandaorestation,whichwassignedin2006,laidthefoundationforaforestmanagementcodeOsemetal.,2013).Manyoftheprinciplesofthiscode,includingthefacilitationofsustainabilityandadaptivemanagement(Osemetal.,),weredesignedtoincreasethedroughtresistanceoftheforest.Howdidtheseprinciplestranslateintomanagementpractices?Poratetal.(2017)describetheplanningofthinningandgrazingap-plicationsinKdoshimforestasanexample.Wherepossible,treepruningisyetanothermanagementoption.InadrylandwoodlandinSudan,pruningofProsopisjulitreesincreasedtheirleafproductionby60%andimprovedtheirhydraulicstatus(Elfadl&Luukkanen).Drought-inducedtreemortalitycanmarktheendofaforest,butcarefulplanningcanhelpcreatingamoresustainableforestatthesamesite.State-and-transitionmodelsareoneofthetoolstotestmanage-mentalternativespriortotheirapplicationintheeld(Cobbetal.,).Last,thevalueofcontinuousmonitoring,whichwasacknowl-edgedinthecode,facilitatedtheestablishmentofanationalnetworkof80monitoringplots(Sprintsinetal.,2014).Theseplots,whichareindependentfromtheperiodicalforestinventorysurveys,willprovidemoreinformationo

6 ndroughtstressanditsforestresponse.Monit
ndroughtstressanditsforestresponse.MonitoringoftreehealthshouldbefurtherimprovedwiththeadoptionofthestandardsoftheEuropeanforestmonitoringnetwork(ICPForests;4.SynthesisandfutureperspectivesTreephysiologists,forestecologists,andforestmanagersacrossaridenvironments,andinIsraelinparticular,arerunningaracetotheunknown.Toensurelocalforestsustainabilityforfuturegenerations,thegapbetweentheincreaseddroughtriskontheonehand,andtheratherstationaryforestdroughtresistanceontheotherhand,mustbeclosed.Thisreviewpaperopenedwithtwo,rathercontrasting,nationalaridlandaprograms:China'sThreeNorthsShelterbeltWangetal.,2010),andIsrael'sgreeningprogram(Osemetal.,2011Formanygoodreasons,bothprojectsshouldbehighlightedalsoout-sideoftheirlocations.Nevertheless,muchofthescienticknowledgeavailabletodaywasmissingatthetimetheseprojectsstarted.WhatmadeChina'saridaorestationlesssuccessfulthananticipated?Thisinvestigationisongoing.WhatmadeIsrael'saorestationsensitivetoprolongeddroughtandre?Recentresearchfromthelocal,regional,andglobalscienticcommunityprovidessomeclues.Amongthemessagesconveyedinthisreview,itisclearthat:(1)seedandplantmaterialweresub-optimalinmanycases.Today,provenanceandspe-ciesselectionaresupportedbycurrentresearch;(2)forestmanagementpracticessuchasthinningandgrazinghavenotbeenfully Fig.5.Themultiplescalesofforestdroughtvulnerability.frameworkforadvancedassessmentofforestdroughtvulnerability.Suchas-sessmentintegratesspecies-specichydraulicanddrought-acclimationdatawithresponsesandwaystobuildresilienceattheforestscale.T.Klein implemented.Fieldexperimentsnowconrmtheirroleinensuringforestsustainability;and(3)forestrydatamanagementreliedonthetraditionalinventorysurveys.WiththeremotesensingtoolsandGeo-graphicalInformationSystemcurrentlyavailable,forestmanagementismuchimproved.Still,aridlandforestsinIsrael,andotherplaces,havebeenhighlysuccessfulinsequesteringcarbon,channelingittolong-termstorageinthesoil.Inmanycases,thiswasachievedwithlittleectonthewateryield,whichissoimportantinaridenvironments.Thecontemporaryresearchreviewedherehighlightsthefollowing1.Newtoolssuchastranscriptomicsandforesttreetransformationopenthegatetothediscoveryofnewdrought-responsegenes.Amolecularapproachshouldbepromoted,butunlikemanyexampleswithannualplants,itmustbeaccompaniedwithaneco-physiolo-gicalapproach.Therolesoftreeaquaporinsandcarbohydrateme-tabolismenzymesinregulatinghydraulicconductivityandcarbonstorage,respectively,arealreadyunderinvestigation.2.Therhizosphereofaridlandforestsisstilllargelyunexplored,andholdsgreatpotentialtothefutureofforestsunderdrought.Rootprocessessuchassoilwaterexploration,carbonstorage,andexu-dation,canbemoreimportantfortreedroughtresistancethanabovegroundprocesses.Also,moreresearchshouldbededicatedtoidentifyinglocalmycorrhizaspeciesandquantifyingtheirroleinincreasingwatersupplytotrees.3.Forestthinning,pruningandgrazingpracticesarealreadybeingappliedinmanysites.Yet,moreresearchisneededtotesttheirimpactonforestdroughtresistance.Thisconsiderstheobservationofdrought-inducedtreemortalityacrossmultiplestanddensities.4.Newremotesensingtools,fromdronestosatellites,willbecomeincreasinglycriticalinforestdroughtresistanceresearch.Asnewsensorsprovidenovelspectralproductsatever-increasingspatialandtemporalresolutions,preciousdataaccumulateandawaitanalysis.Indeed,remotesensingtoolsareincompletewithoutana-lyticalcapabilitiestomatch.Theseshouldbefurthercomplementedbygroundobservations.Forexample,theKKLGISmapswillin-troduceanewlayer,entirelydedicatedtotreemortalityreporting.5.TheMediterraneanmaquisecosystem,whichhasshownhigherdroughtresistancethanthecoexistingconiferforest,attractedfarlessresearchsofar.Nevertheless,underthenewforestmanagementcode,thisvegetationtype,alongwithamixedconifer-broadleavedforest,willbecomeincreasinglyimportant.Itshigherdroughtre-sistanceisencouraging,yetatipping-pointscenarioshouldnotberuledout(SeverandNeeman,2008CopyrightstatementAllthematerialsincludedinthisreviewarewiththeauthor.DeclarationofcompetinginterestTheauthorsdeclarenocompetingnancialinterests.TheauthorwishtothanktheMerleS.CahnFoundationandtheMonroeandMarjorieBurkFundforAlternativeEnergyStudies;Mr.andMrs.NormanReiser,togetherwiththeWeizmannCenterforNewScientists;TheKKL-JNFIsraelForestService;TheIsrael-USBSFgrantUsingwaterpotentialofsoilsandplantsasaunifyingtermtoassessforestvulnerabilitytodrought;TheWeizmannSustainabilityandAlternativeEnergyResearchInitiative;TheYeda-SelaCenterforBasicResearch;TheAngelFaivovichFoundationforEcologicalResearch;andtheEdith&NathanGoldbergCareerDevelopmentChair.Thepaperttedfromusefulcommentsonanearlierversion,madebyananonymousreviewer.Atzmon,N.,Moshe,Y.,Schiller,G.,2004.EcophysiologicalresponsetoseveredroughtinPinushalepensisMill.treesoftwoprovenances.PlantEcol.171(1),15Cailleret,M.,Dakos,V.,Jansen,S.,Robert,E.M.,Aakala,T.,Amoroso,M.M.,etal.,2018.Early-warningsignalsofindividualtreemortalitybasedonannualradialgrowth.Front.PlantSci.9,1964Carmel,Y.,Paz,S.,Jahashan,F.,Shoshany,M.,2009.AssessingreriskusingMonteCarlosimulationsofrespread.For.Ecol.Manag.257(1),370Cermak,J.,Nadezhdina,N.,1998.Sapwoodasthescalingparameter-deningaccordingtoxylemwatercontentorradialpatternofsapow?In:AnnalesdesSciencesfor-estieres,vol55.EDPSciences,pp.509521No.5Cobb,R.C.,Ruthrof,K.X.,Breshears,D.D.,Lloret,F.,Aakala,T.,Adams,H.D.,Hartmann,H.,2017.Ecosystemdynamicsandmanagementafterforestdie-o:aglobalsynthesiswi

7 thconceptualstate-and-transitionmodels.E
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