PUBLISHED VERSION

PUBLISHED VERSION

http://hdl.handle.net/2440/95479

S.C. Cunningham, R. Mac Nally, P.J. Baker, T.R. Cavagnaro, J. Beringer, J.R. Thomson, R.M.

Thompson

Balancing the environmental benefits of reforestation in agricultural regions Perspectives in Plant Ecology, Evolution and Systematics, 2015; 17(4):301-317

© 2015 The Authors. Published by Elsevier GmbH. on behalf of Geobotanisches Institut ETH, Stiftung Ruebel. This is an open access article under the CC BY-NC-ND license

Originally published at:

http://doi.org/10.1016/j.ppees.2015.06.001

PERMISSIONS

http://creativecommons.org/licenses/by-nc-nd/4.0/

ContentslistsavailableatScienceDirect

Perspectives in Plant Ecology, Evolution and Systematics

jou rn a l h om ep a g e :w w w . e l s e v i e r . c o m / l o c a t e / p p e e s

Review

Balancing the environmental benefits of reforestation in agricultural regions

S.C. Cunningham

^a,b,∗

, R. Mac Nally

^b

, P.J. Baker

^c

, T.R. Cavagnaro

^d

, J. Beringer

^e

, J.R. Thomson

^b

, R.M. Thompson

^b

aSchoolofLifeandEnvironmentalSciences,DeakinUniversity,Burwood3125,Australia

bInstituteforAppliedEcology,TheUniversityofCanberra,Bruce,ACT2617,Australia

cDepartmentofForestandEcosystemScience,UniversityofMelbourne,Burnley3121,Australia

dSchoolofAgriculture,FoodandWine,UniversityofAdelaide,WaiteCampus,PMB1,GlenOsmond,SA5064,Australia

eSchoolofEarthandEnvironment,UniversityofWesternAustralia,Crawley,WA6009,Australia

a r t i c l e i n f o

Articlehistory:

Received6June2014

Receivedinrevisedform22May2015 Accepted2June2015

Availableonline11June2015

Keywords:

Biodiversity Carbonsequestration Habitatresources Nutrientcycling Revegetation Wateryield

a b s t r a c t

Reforestationisanimportanttoolforreducingorreversingbiodiversitylossandmitigatingclimate change.However,therearemanypotentialcompromisesbetweenthestructural(biodiversity)andfunc- tional(carbonsequestrationandwateryield)effectsofreforestation,whichcanbeaffectedbydecisions onspatialdesignandestablishmentofplantings.Wereviewtheenvironmentalresponsestoreforestation andshowthatmanipulatingtheconfigurationofplantings(location,size,speciesmixandtreedensity) increasesarangeofenvironmentalbenefits.Moreextensivetreeplantings(>10ha)providemorehabitat, andgreaterimprovementstocarbonandwatercycling.Plantingamixtureofnativetreesandshrubsis bestforbiodiversity,whiletraditionalplantationspecies,generallynon-nativespecies,sequesterCfaster.

Treedensitycanbemanipulatedatplantingorduringearlydevelopmenttoacceleratestructuralmaturity andtomanagewateryields.Adiversityofhabitatswillbecreatedbyplantinginavarietyoflandscape positionsandbyemulatingthepatchydistributionofforesttypes,whichcharacterizedmanyregions priortoextensivelandscapetransformation.Areaswithshallowaquiferscanbeplantedtoreducewater pollutionoravoidedtomaintainwateryields.Reforestationshouldbeusedtobuildforestnetworksthat aresurroundedbylow-intensitylanduseandthatprovidelinkswithinregionsandbetweenbiomes.

WhilethereareadequatemodelsforCsequestrationandchangesinwateryieldsafterreforestation, thequantitativeunderstandingofchangesinhabitatresourcesandspeciescompositionismorelimited.

Developmentofspatialandtemporalmodellingplatformsbasedonempiricalmodelsofstructuraland functionaloutcomesofreforestationisessentialfordecidinghowtoreconfigureagriculturalregions.To buildsuchplatforms,wemustquantify:(a)theinfluenceofpreviouslanduses,establishmentmethods, speciesmixesandinteractionswithadjacentlandusesonenvironmental(particularlybiodiversity)out- comesofreforestationand(b)thewaysinwhichresponsesmeasuredatthelevelofindividualplantings scaleuptowatershedsandregions.Modelsbasedonthisinformationwillhelpwidespreadreforestation forcarbonsequestrationtoimprovenativebiodiversity,nutrientcyclingandwaterbalanceatregional scales.

©2015TheAuthors.PublishedbyElsevierGmbH.onbehalfofGeobotanischesInstitutETH,Stiftung Ruebel.ThisisanopenaccessarticleundertheCCBY-NC-NDlicense(http://creativecommons.org/

licenses/by-nc-nd/4.0/).

Contents

1. Introduction...302

2. Structuralchangesfollowingreforestation...302

2.1. Developmentofforeststructure ... 302

2.2. Diversityofunderstoreyplants...304

2.3. Diversityofterrestrialanimals...304

∗Correspondingauthorat:DeakinUniversity,SchoolofLifeandEnvironmentalSciences,Burwood3125,Australia.Tel.:+61392445523.

E-mailaddress:shaun.cunningham@deakin.edu.au(S.C.Cunningham).

http://dx.doi.org/10.1016/j.ppees.2015.06.001

1433-8319/©2015TheAuthors.PublishedbyElsevierGmbH.onbehalfofGeobotanischesInstitutETH,StiftungRuebel.ThisisanopenaccessarticleundertheCCBY-NC-ND license(http://creativecommons.org/licenses/by-nc-nd/4.0/).

2.4. Diversityoffreshwateranimals...305

2.5. Physicalandbiologicalstructureofsoils...305

2.6. Increasingregional(␥)-diversityofagriculturalregions...305

3. Functionalchangesafterreforestation...307

3.1. Carboncycling...307

3.2. Nutrientcycling...309

3.3. Stand-scalewatercycling...310

3.4. Wateryieldofcatchments...310

3.5. Waterquality...311

4. Balancingtheenvironmentalbenefitsofreforestation...311

Acknowledgements...314

References...314

1. Introduction

Extensiveareasofnativeforestecosystemshavebeencleared andconvertedtootherlanduses,suchasagriculture,plantation forestryandcities,atrendthatwillcontinuewithincreasinghuman populations.Forestareaisestimatedtohavedecreasedbyathird over the past three centuries in China, the Middle East,North Africa,theeastern UnitedStates ofAmerica andSoutheast Asia (RamankuttyandFoley,1999).Deforestationhassubstantialand widespreadnegativeimpactsonclimate,hydrology,soilsandbio- diversity,with consequentimpacts on societiesand economies (Meyfroidtand Lambin,2011).Aconsiderableproportionofthe remainingnativeforestshasbeenseverelydegradedtoproduce primaryresources.Intheircurrentrestrictedanddegradedstate, theremainingnativeforestsfacethepotentiallyrapidandextreme stressofclimatechangeand increasedclimatevariability (Dale etal.,2001).Inrecentyears,there havebeenseveralimportant agreementsthatsuggesttherewillbeextensivereforestationto addressthisissue(CBD,2010;GPFLR,2013;UN,2014;UNEP,2014).

Itisimperativethattheenvironmentalimpactsofglobaldefor- estationaremitigatedbyacombinationofactive(i.e.treeplanting) andpassive(i.e.regrowthfollowinglandabandonment)reforesta- tion.Here,wefocusonactivereforestation,whichwedefineasthe plantingofforestsonlandsthathistoricallyhadforestsbutthat havesincebeenconvertedtootherlanduses(IPCC,2007).This excludesafforestationofareasthat wereformerlynativegrass- landsorshrublands,whichgenerallyisdetrimentaltobiodiversity (Bremer andFarley,2010; Gerstneretal.,2014).Landhasbeen reforestedactivelyformanyreasonsincludingforplantationsfor timber,riparianplantingstoreducestreampollution,uplandplant- ingstoreducesoilerosionandsalinity,andtoincreasehabitatfor nativespecies(Jacksonetal.,2005).Reforestationmayimprove links among existing remnant forest patches,increasing move- ment,geneflowandeffectivepopulationsizesofnativespecies (Gilbert-Norton etal.,2010).Re-establishingforestscanrestore biogeochemicalcyclingofcarbon,oxygenandnutrientsamongthe atmosphere,biomass,pedosphereandhydrosphere(Arnethetal., 2010).Reforestationofagriculturallandcanimprovebiodiversity, whichcanresultinincreasedprimary production,reducedsus- ceptibilitytoinvasionbyexoticspeciesandincreasedecological resistancetopressuressuchasclimatechange(Hooperetal.,2005).

Restorationof forest ecosystemscould directly mitigatecli- matechangebysequesteringatmosphericcarbon,bothabove-and below-ground.Treessequesterandretainmoreatmosphericcar- bonintheirbiomassthandocropsorpastures(Panetal.,2011).

Undercarbontradingorcarbonemissionreductionschemes(e.g.

UnitedNationsREDD+programme),itispossiblethatwidespread reforestationwillbecome economicallyviable (Bradshawet al., 2013).Increasinguncertaintiesincropyieldswithclimatechange mayencourage landholders todiversifyinto otherinvestments suchas‘carbonfarming’.Reforestationcouldprovideanimportant toolformitigatingclimatechangeintheshort-termwhilefostering

alow-carboneconomyandimprovingenvironmentalconditions jointlyinthelong-term(Mackeyetal.,2013).

Howreforestationisapproachedhaslong-termconsequences withcompromisesbetweenthestructureandfunctionoftheforest.

Thenumberoftreesandthetypesoftreespeciesplanted(exoticvs native,mixedvssinglespecies)andwhethershrubsareincluded arekeydecisions.Plantationsoffast-growingproductionspecies cansequestercarbonfasterthannativemixed-speciesplantings butoftenhavelittlebiodiversityvalue(Lindenmayeretal.,2003).

Reforestationofriparianzonescanleadtolargerincreasesinbio- diversitybutgreaterreductionsinstreamflowthanreforestation inupslopeareas(Scott,1999;PalmerandBennett,2006).Perma- nentrestorationplantingsarelikelytoprovidemoreenvironmental benefitsthanharvestedplantations(Kanowskietal.,2005).

Issuesassociatedwithpassivereforestationorlandabandon- menthavebeencoveredindepthbyotherreviews(e.g.Bowen etal.,2007;Rey-Benayasetal.,2007;MeyfroidtandLambin,2011).

Here,weexploretherangeof potentialresponsesofecosystem structureandfunctiontoactivereforestationofagriculturalland.

Structureincludesthediversityofspeciesinanarea,includingani- mals,plants,fungiand bacteria,andthespatialarrangementof thesecomponentsfromtheplanting(<0.1km²,e.g.canopystrata) totheregionalscale(10⁵–10⁶km²,e.g.forestnetworks).Function includesthebiogeochemicalprocessesresultingfrominteractions betweenspeciesandthephysicalenvironment,suchasproduction, decompositionandnutrientdynamics.Weoutlinehowthebenefits ofreforestationmaybemaximizedbypractitionersinagricultural regionsgiventhepotentialcompromisesbetweenstructureand function,andcurrentspatialandtemporalconstraints.Extensive reforestation ofagricultural landis limited by social, economic andpoliticalobstacles,whicharecoveredelsewherebya grow- ingliterature(e.g.BarrandSayer,2012;Knightetal.,2010).We finishbypresentingamodellingframework,andtheknowledge required,thatwould allowlandmanagerstoquantifythecom- promisesamongstructure(biodiversity)andfunction(carbonand water)underdifferentreforestationscenariosandhencebalance theenvironmentalbenefitsofwidespreadreforestationinagricul- turalregions.

2. Structuralchangesfollowingreforestation 2.1. Developmentofforeststructure

Maturenative forestscontain strata of different-sized trees, shrubsandagroundlayer,whichcreatearangeofmicrohabitats andmicroclimatesbeneaththecanopy(OliverandLarson,1996;

Franklinetal.,2002).Thestructuralcomplexityofaforestincludes the density, spatial arrangement, size and height distribution, speciesrichness(seefollowingsectionsondiversity),canopycover, canopystrataanddebrisoftrees(McElhinnyetal.,2005).Forest structureandtheassociatedhabitatresourcestakedecadestocen- turiestodevelopfollowingreforestation.Theexpectedsequence

Fig.1.Conceptualmodeloftemporaldevelopmentofecosystemstructure(habitatresourcesandtaxonomicgroups)followingreforestationatthepatchscale.Theexpected timeforresourcestodevelop(squareboxes)andtherichnessofnativespeciesfromdifferenttaxonomicgroupstoincrease(wedges)areindicated.Taxonomicgroupsthat maynotrespondtoreforestationaredenotedbydashedborders.Timesforhabitatresourcestodevelopmentwilldifferamongforesttypesandwithmanipulationofstand structure.Thesewerecalculatedfromthefollowingstudiesoftemperateforests:^aMacNallyetal.,2009;^bHardingetal.,1998andSutherlandetal.,2002;^c,dVesketal.,2008;

e,fEyreetal.,2010;^gKanowskietal.,2006;^hWattsandGibbs,2002;ⁱBrunet,2007;^jMacDonaldetal.,2009.

ofstructuraldevelopmentafterreforestationisillustratedinFig.1.

Treeplantingsmaynotdevelopthefullstructuralcomplexityof amaturenativeforest,ortheymaytakealongtimefornatural recruitmentandsenescenceprocessestogeneratethatstructure.

Canopyclosureisanimportantdevelopmentalstepbecauseit reducesirradiance,soiltemperatureandwindspeedbeneaththe canopy (Vetaas,1992).Plantings ofmany species reachcanopy closurewithintwodecades(Fig.1,OliverandLarson,1996).Fast- growing tree species, particular in the tropics, can be planted toaccelerate canopy closure in plantings (Haggaret al., 1997).

Todevelopforest-interiorconditionsbeneaththecanopy,plant- ingsmustbelarge(>100mwideand >10ha)becausetheeffect of agricultural land on microclimate can extend >50m into a forest(Murcia,1995).Reforestationalongstreams mustbelon- gitudinally extensive toreduce water temperatures, which are influencedbytheamountofshadingseveralkilometresupstream (Allan,2004).

Mosthabitatresources areslow todevelop, withtemperate treesoftentaking50yearstoproducesubstantialfloralresources andatleasta centurytobecomelargetrees(Fig.1,Vesketal., 2008).Theinclusionofshrubsintheinitialplantingaccelerates the availability of floral resources, including seeds and nectar.

Plantlitter isanimportantstructural componentofforestsand thicklayerscanaccumulatewithintwodecadeswhentaxasuch asEucalyptusareplanted(Cunninghametal.,2012).Reforestation maytake>30yeartoincreasefallentimberonforestfloors,which

isakeyresourceformanytaxa(Harmonetal.,1986),andcenturies toattainlevelsinstreamssimilartothosewithinmatureforests (Davies-Colleyetal.,2009).

Large trees with large branches and reiterated trunks are significantstructuralfeaturesofmatureforests,whichprovidedis- tincthabitatresources(e.g.hollows,Franklinetal.,2002).Many treespeciestakeoveracenturytodevelopthesecharacteristics (Fig.1,Humphrey,2005),soplantingtreesaroundfocalremnant trees can provide these resources while theplanting develops (LindenmayerandHobbs,2004).Withinplantings,treehollowscan besimulatedbyartificialstructuresandsometreescanbeinten- tionallywoundedtopromotetree-hollowdevelopment(Gibbons andLindenmayer,2002).

Thedevelopmentofstructuralcomplexityafterreforestation dependsonwhichtreespeciesareplantedandtheageoftheplant- ing.Woodplantations(includingagroforestry)havelessstructural complexitythanmatureremnantforestsorthannaturallyregen- eratingforestsofequivalentageduetothemoreuniformspatial and temporal establishment ofplantations (Aubin et al.,2008).

Whenlefttomature(>50years),woodplantationscandevelop similar canopycover, litter mass and strata toremnant forests (Kanowski et al., 2003). Planting a mixture of tree and shrub speciesprovidesahigherstructuralcomplexitythanplantations ofproductiontreespecies(Kanowskietal.,2003)andcanleadto thediverse-sizestructureassociatedwithamatureforestfaster (Munroetal.,2009).

Manipulatingtreedensityisanimportant toolfor accelerat- ingthe developmentof manyhabitat resources.Tree plantings typicallyareplantedathighdensities(ca1000treesha⁻¹,Smith etal.,1996)toensureadequaterecruitmentratesbutdensethickets oftenformthatrestrictstructuraldevelopmentduetostagnating growth.Activethinningofdensestandsduringearlydecadescan acceleratethedevelopmentoftheremainingtrees,makethemless vulnerabletodrought(Horneretal.,2009)andwindthrow(Wilson andOliver,2000),whiletheretentionofcutstemsprovidessome fallentimber,whichisakeyhabitatcomponent.Plantingtreesat lowerdensitiesallowstreestodevelopwidecrownsandbranches faster,andshrubsandunderstoreytaxatoestablishbetweentrees (OliverandLarson,1996).

Reforestationshouldmimictheheterogeneityofstandstruc- turesfoundwithinnativeforestsbyplantingamosaicofdifferent mixturesoftreespeciesandtreedensitiesoveranarea.Plantings shouldaimtohaveasimilarrichnessoftreestosurroundingrem- nantforestsbutwerecognizethatthiswouldnotbepracticalfor highlydiverseforestsofthesubtropicsandtropics.Moreimpor- tantly,plantings shouldcontainrepresentativespeciesfromthe differentfunctionaltypesfoundinremnantforests.Alternatively, the‘frameworkspecies’approachdevelopedintropicalAustralia (GoosemandTucker,1995)useslocalknowledgetochoosespecies thathavehighsurvivalandgrowthratesonclearedland,rapidly producedensecanopytocontrolcompetingvegetationandpro- duce fruitsand flowersthat attract seed-dispersing animals to promoteestablishmentofnewspecies(Elliottetal.,2003).

Appropriatespeciesforthecurrentandfutureenvironmental conditionsshouldbeplanted.Thereneedstobeawarenessthat speciesthatoccurredpriortolandclearancemaynotsuccessfully developintomatureforestsdue toclimatechange.Treespecies withinaplantingshouldincludefast-growingspeciestorapidly amelioratethemicroclimateandlong-livedspeciestoprovidethe structureofmatureforests(Haggaretal.,1997).Manipulatingtree densityatplantingorduringearlydevelopmentcreatesarangeof standstructuresfromthicketsforsheltertowidespacingforunder- storeyestablishment(OliverandLarson,1996),andacceleratesthe developmentofsomehabitatstructures(e.g.Horneretal.,2010).

2.2. Diversityofunderstoreyplants

The development of a nativeplant understorey within tree plantingsis dependentondispersal fromlocal sourcesbecause nativeseedbanksusuallyareimpoverishedonagriculturalland (Cramer etal., 2008).Understorey speciesrichness of plantings decreaseswithdistancefromthenearestremnantforest(Brunet, 2007).Theabundanceofseedinplantingsdecreasesfromwind- dispersed,bird-dispersedtoheavyseeds(Battaglia etal.,2008).

Manyunderstoreyspeciesneedtobeplantedbecausethenearest sourceforestoftenistoodistantortoodegradedtoprovideseeds.

Establishmentofnativeplantspeciescommonlyislimitedby theexistingcoverofexoticspeciesorthenutrientenrichmentof soilsassociatedwithlong-termagriculturalmanagement(Prober etal.,2005).Agriculturalweedsmaybecontrolledusingherbicides andtargetedgrazingbuttheseactionscanbedetrimentaltonative species.Canopyclosureincreasescompetitionforsoilmoistureand light,generallyfavouringnativespeciesover agriculturalweeds adecadeafterreforestation(Brunet,2007,Fig.1).Canopyopen- ingsareneededtorecruitnativeshrubs(OliverandLarson,1996) andtomaintaincoverofaquaticmacrophytesinriparianplantings (BroadmeadowandNisbet,2004).

Reforestationcommonlydevelopsaplantcompositionthatis intermediatebetweentheadjacentfieldandremnantforest,but plantingsmaynotbecomenativeforest.Althoughspeciesrichness ofolder(>80years)plantedforestsmaybesimilartothatofrem- nantforests(Brunet,2007),manyforestandwoodlandspeciesare

missingfromtheunderstoreyoftreeplantingsacenturyafterrefor- estation(Harmeretal.,2001).Consequently,theremaybelower diversityofplantsamongolder(ca70years)plantedwoodlands thanamongnearbyremnantwoodlands(DzwonkoandGawronski, 1994).

Thenativediversity oftheunderstoreyis determinedpartly bywhichtreespeciesareplanted.Plantationsofsingleproduc- tionspeciesusuallycontainasimilarnumberofnativeunderstorey speciestoadjacentfields,andmoreexoticandfewernativespecies thannativeforests(Feltonetal.,2010).Withinaregion,plantations ofsomeforesttypes(e.g.broadleaf)increase␣-diversity(under- storeyrichnesswithinplantings)morethanothers(e.g.conifer).

However,eachforesttypepromotesadistinctunderstoreyflora (Pensaetal.,2008),helpingtoincreaseboth␤-diversity(among plantings)and␥-diversity(regional).

Increasingthestructuralcomplexityofreforestation,byplant- ingseveralspecieswithinaplanting,andarangeoftreespecies andtreedensitiesacrossaregion,islikelytoincreasethediversity ofunderstoreyplantspeciesthatbecomeestablished(Brockerhoff etal.,2013).Treeplantings mustprovideconditionsfavourable for theestablishmentof nativeplants,includinggrowingspace andappropriatenutrientlevels,todevelopadiversenativeunder- storey.Manynativeplantspeciesneedtobeactivelyplanted,dueto alackoflocalseedsources(FlinnandVellend,2005).Plantingthe fulldiversityofforestunderstoriesisnotpractical,especiallyfor highlydiversetropicalforests,soplantedspeciesshouldbechosen toprovidespecificstrataandresources(e.g.perchtrees).

2.3. Diversityofterrestrialanimals

The response of terrestrial animals to reforestation differs amongtaxonomicgroups(Fig.1).Speciesrichnessusuallyishigher in tree plantings than in surroundingfields for mammals(e.g.

Christianetal.,1998),birds(e.g.Loynetal.,2007)andreptiles(e.g.

Kanowskietal.,2006)butlowerthaninremnantforests.Thecom- position(butnotrichness)ofbeetleschangeswithreforestation, withplantingshavingcompositionsmoresimilartoremnantsthan tofields(WattsandGibbs,2002;Gollanetal.,2011).Amphibians andbatshavesimilarrichness,compositionandactivityindevel- opingplantings(<30years)tosurroundingagriculture(Kavanagh etal.,2005;LawandChidel,2006).Treeplantingstypicallyaredom- inatedbyanimalspeciesthatarealreadyabundantonagricultural land(Christianetal.,1998).

Tree plantings mustprovide appropriate physical and biotic requirements(habitat)for foraging,shelterand nestingforani- malstocolonize.Animalsoftenrequiredifferenthabitatorhabitat resourcesfortheseactivitiesandtheneedsofanimalscandiffer seasonallyand throughthelife cycle(Law andDickman,1998).

Manyanimals areassociatedwithparticularplantspecies (Koh etal.,2004)orstandstructures(Sullivanetal.,2001),soplanting arepresentativerangeofnativeplantspeciesanddensitiesshould increasetherichnessofnativeanimalscolonizingtreeplantings.

Habitatvalueofreforestationisdeterminedpartiallybylandscape position.For example, birdsare more abundantin gullies than ridgesduetotallertrees withhollowsandpossiblyhighernec- taravailability(MacNallyetal.,2000),whilereptilescanbemore abundantonrockyridges(Michaeletal.,2011).Plantings,while lackingsomehabitatresources,providemorehabitataroundrem- nants,which isparticularlybeneficial duringsomeseasons(e.g.

overwinteringormigration)andfollowingdisturbanceeventsin remnants(e.g.fire).

Colonizationoftreeplantings bynativeanimalsisrestricted bythetemporaldevelopmentoftheirrequiredhabitatresources (Fig.1).Afteradecade,atreeplantingprovidesadequatecanopy cover,particularlyifshrubsareplanted,forsomenativebirdsand ground-dwelling mammals (Eyre etal., 2010).Ground-foraging

insects increase 20 years after reforestationafter which a sub- stantial litter layer has accumulated (Watts and Gibbs, 2002).

Hollow-dependentanimals,suchasbirds,arborealmammalsand bats,maycolonizewhentreesarematureenoughtodevelophol- lows(>75years,Vesketal.,2008).Thepresenceofreptilesdepends onthe availability ofspecific microhabitats, with somespecies usingbarkfeaturesthat developafterafewdecadeswhileoth- ersrequirelargerocks,whichwillnotbeprovidedbyreforestation (Kanowskietal.,2006).Amphibiansmayrespondmoretothechar- acteristicsofwater-bodiesthantothepresenceoftreesperse(Mac Nallyetal.,2009).

Nativeanimalsmaycolonizeplantingsbutthis maynotlead totheestablishmentof successfulbreedingpopulations.Breed- ingsuccessrarelyismeasuredintreeplantingsandinformation largelyhasbeenlimitedtobirds(e.g.SergioandBogliani,2000).

In a long chronosequence (9–111 years) of tree plantings in south-easternAustralia,breedingactivitywasdominatedbyopen- countryspecies,withbreedingbydecliningwoodlandspeciesonly evidentin plantings>100 years(Selwoodetal., 2009).Thereis aneedforcomparisonsofarangeoftaxaamongtreeplantings, agriculturallandandremnantforeststodeterminethedegreeto whichreforestationinducesbreedingbecausewecurrentlyhave littleideaofwhetherplantingsare‘sinks’ratherthan‘sources’for decliningspecies.

Reforestationmustcreateadiversityofhabitatsandresources withinthemtomaximizecolonizationbynativeanimals.Arange offoresttypesandlandscapepositionsneedstoberestoredina region,ideallywithamixtureofnativetreesandshrubswithina planting.Theareaofplantingsshouldbelargeenoughtoprovide sufficienthabitatfor‘interior species’(>100mwide and>10ha, Murcia,1995).Manyhabitatresourcesareslowtodevelop,partic- ularlyintemperateareas(Vesketal.,2008,Fig.1),althoughsome canbeacceleratedbyreducingtreedensity(e.g.Horneretal.,2010), andothersareabsent(e.g.fallentimber),needingtobeaddedto encouragecolonizationbyanimalspecies.

2.4. Diversityoffreshwateranimals

Giventheexpectedchanges inmicroclimate(reducedirradi- anceandwatertemperature)andincreasedresourceinputs(e.g.

litter,Allan,2004),reforestationalong stream-banksislikelyto changeassemblagesoffreshwateranimalssubstantially.Recently reforested (<25 years) streams can have a similar richness of macroinvertebratestoagriculturalstreams(Parkynetal.,2003), andcanhavedistinctmacroinvertebratecompositionsandlower

␥-diversitycomparedwithforestedstreams(BeckerandRobson, 2009).Streamsinmatureforestshavehigherrichnessandmore pollution-sensitive species than agricultural streams (Harding etal., 1998), suggestinga long-termpotential forreforestation.

Trendsforfishareequivocal,withforestedreacheshavingeithera higher(Sutherlandetal.,2002)orlower(Hardingetal.,1998)abun- danceandrichnessthanagriculturalreaches,whichmaydepend onthelocationofpotentialsourcepopulations.

Colonization by native freshwater animals following refor- estation would be improved by concurrently restoring habitat resourceswithinstreamchannels(Lakeetal.,2007).Fallentim- ber,whichprovideshabitatandfoodformanyfreshwateranimals, maytakeacenturytoaccumulateinstreamsfollowingreforesta- tion(Davies-Colley etal.,2009)and needstobesupplemented.

Clearanceofforestsforagriculturegenerallyincreasessedimen- tation,cloggingstreamchannelsandcrevices,whichareusedby many freshwateranimals (Burcher et al.,2008).Riparianrefor- estationpromotesanherbaceouslayerthatincreasesinterception ofsediment(BroadmeadowandNisbet,2004),whileaccumulated sedimentloadsinthestreamchannelneedtoberemovedactively orbyhigh-flowevents.

The longitudinally connected nature of stream networks restrictsthepotentialforsmall-scale(<0.1km²)reforestationto improvenativespeciesdiversityofindividualreaches(Lakeetal., 2007).Compositionofstreamassemblagesoftenismorestrongly relatedtotheamountofupstreamforestcoverthantotheextent oflocalriparianvegetation(Thomsonetal.,2012).Pastlanduse canbeastrongerpredictoroffishandmacroinvertebrateassem- blagesthancurrentlanduse,suggestinglong-term‘legacyeffects’

of agriculture onstreams (Harding et al.,1998).Restoration of nativestreamassemblageswillbemostsuccessfulwhenseveral kilometresofastreamarereplantedandsurroundingagricultural practicesarechangedtominimizenegativeimpacts.

2.5. Physicalandbiologicalstructureofsoils

Soil structure is a primary determinant of water and air movement,stabilityoforganicmatter,rootgrowth,seedlingestab- lishmentanderosion(Lal,1991).Agriculturalpracticesaltersoil structureduetoregulardisturbancebreakingsoilaggregatesand fillingporespaces,compactionfrommachineryandlivestock,and irrigationdissolvingaggregates(YoungandYoung,2001).Refor- estationislikelytoimprovesoilstructurebecauseforestsoilshave higheraggregatestabilityduetolargerlitterinputsandreduced soildisturbance,lowerbulkdensity(lesscompacted)andhigher porositythanagriculturalsoils(Lichtfouseetal.,2011).However, reforestationmaynotleadtoasubstantialdecreaseinbulkdensity withinthefirstthreedecades(Limaetal.,2006).

Forestsoilstendtohavemorediversesoilassemblagesthan agriculturalsoils(Bardgettetal.,2005).Soilsareverybiodiverse predominantly duetosoilmicrobes(fungi,bacteria,archeaand viruses),buttherearealsoprotozoa,nematodes,microarthropods, enchytraeids,earthworms,termitesandmillipedes(Andreetal., 2002).Soilanimalsandmicrobesplaya keyroleinecosystems, affectingplantsdirectlythroughparasitism,pathogens,herbivory andsymbiosesandindirectlybycyclingnutrientsfromsoilorganic matter(Wardle,2002).Reforestationofagriculturalfieldsislikely to increase the heterogeneity of soil resources, which usually increasesthespeciesrichnessofsoilassemblages(Bardgettetal., 2005),becausetreesredistributeresourcesthroughuptakefrom theirextensiverootsystemsanddepositionofleaf,stemandroot material.

Soilfaunaandmicrobesshowrapidchangesinabundanceand biomasswithinafewyearsaftercroppinghasceased(Kardoletal., 2005).Compositionalchangesinsoilassemblagesafterreforesta- tion maybeslower,withchanges in fungiseen aftera decade (MacDonaldet al., 2009, Fig.1)and in earthwormsafter three decades(Pizl,2001).Ashifttowardsafungal-dominatedassem- blagefollowingreforestationofpasture(MacDonaldetal.,2009)is associatedwithchangesinplantlitterquality(e.g.C:Nratio),cessa- tionofcultivation,fertilizeradditionandgrazing(Sixetal.,2006).

Establishmentofa diverse,nativesoilassemblage maybeslow following reforestation dueto limitations ofdispersal, resource availabilityorabioticfactors(e.g.pH,Lauberetal.,2008).

Distinctsoilmicrobialcommunitiesareassociatedwithforest types(Hackletal.,2005)andtreesspecies(Ushioetal.,2008).Con- sequently,plantingarangeofmixed-speciesplantingsislikelyto increasetheheterogeneityofphysico-chemicalpropertiesofthe soilandhencethediversityofsoilassemblages.Inoculationoftree specieswithsymbioticbacteriacanovercomedispersallimitations anddoubleestablishmentsuccess(Thralletal.,2005).

2.6. Increasingregional()-diversityofagriculturalregions

Reforestationisconstrainedbypresentlanduse, makingthe area,configurationandhabitatqualityofforestpatchescriticalto speciesuseof,andmovementthrough,regions(Fig.2,Bennettetal.,

Fig.2. Plantingconfigurationstomaximizespeciesdispersalacrossareasshowing(a)distributionofremnantforests,(b)forestnetwork,(c)ecologicalzonesand(d) continentalbiolinks.‘Forestnetworks’areareasinwhichreforestationisusedtoincreasetheforestextentandtodecreasetheisolationamongforestpatches.‘Ecological zones’areareastheextentofnativevegetationisincreasedandtheintensityoflanduseisreduced.‘Biolinks’arefocalzonesforreforestationthatextendacrossregions andcontinents.Theareaincludesremnantforests(black),rivers(bluelines),reforestation(darkgreen)andecologicalzones(lightgreen).Atthecontinentalscale,remnant forests(grey)andpotentialbiolinks(dashed)areshownforeasternAustralia.(Forinterpretationofthereferencestocolorinthislegend,thereaderisreferredtotheweb versionofthearticle.)

2006).␥-Diversityofforest-dependentanimalsdecreasesrapidly belowaforestcoverof10–30%withinanarea(Radfordetal.,2005;

SwiftandHannon,2010).Apragmaticapproachistoconcentrate reforestationinareaswithinexistingmoderateforestcover(>30%) andpotentiallyhigherremnantdiversity.However,clearancehas focusedonspecificareasandforesttypes,typicallythoseonthe morefertileandwell-wateredlowlands,sothatfocusedreforesta- tionof theseareas maymaintainor providehigher ␥-diversity (VeskandMacNally,2006).

Reforestation(includingrestorationplantingsandwoodplan- tations) can be used to improve connectivity among remnant

forestsbycreatinga‘forestnetwork’(Fig.2b),whichalthoughnot formingacontinuouscorridorincreasestheextentofforestand reducestheisolationofpatchesinanarea.Theeffectivenessofa forestnetworkofremnantsandreforestationdependsonthequal- ityofhabitatprovided fortaxa. Thespreadof individualnative plantspeciesthroughsuchanetwork mayberestrictedbysoil type,moistureandnutrientavailability,andthepresenceofother species(DzwonkoandGawronski,1994).Manyremnant forests aredegraded,withdepauperateunderstories,impoverishedseed banksandfewanimals(Proberetal.,2005),andneedtoberestored toensuretheviabilityofnativepopulations.Thestructureoftree

plantings,includingtreedensityandsizedistribution,determines whetheranimalsmovethroughaforestnetwork.Currently,most restorationplantingsaretooyoung(<30years)toprovidemany habitatresources(Fig.1,Vesketal.,2008)andsomeresourcesmay needtobesupplied(e.g.nestboxes).Agroforestrypresentsaneco- nomicsolutiontoincreasingforestcoverinagriculturalregionsbut theirtypicallysmallareasandharvestduringearlydevelopment provideslimitedhabitatresources(Bhagwatetal.,2008).

Thelocation of tree plantings relative tothenearest source population of native taxa is an important factor in producing advantageousreforestation outcomes(Lindenmayer and Hobbs, 2004; Brunet, 2007). Mobile taxa, such as birds, can disperse through networks of forest fragments. However, the dispersal ability of most other taxa is poor, withwoodland herbs mov- ing<2myear⁻¹ (Singletonetal.,2001).Althoughadultformsof freshwatermacroinvertebratescandispersemorethanakilome- trealongastream,manymacroinvertebratesusuallydonotmove betweenstreams(ParkynandSmith,2011).Treeplantingsadjoin- ing remnant forestshave higher species richness and augment habitatarea,potentiallyimprovingthechancesofcolonizationby nativespeciesandreducingtheadverseeffectsofsurroundingland usesonremnants(LindenmayerandHobbs,2004).Riparianplant- ingsaremostlikelytobeeffectivewhenlocateddownstreamof existingremnantsbecausefreshwateranimalsaremuchinfluenced byupstreamprocessesanddegradation(Lakeetal.,2007).

Reforestationcanovercomedispersal limitationsbycreating

‘forestcorridors’orlinkagesthatconnectpatchesofremnantforest (Fig.2b).Dispersalofplants,invertebratesand non-avianverte- brateswashigher,whilethatofbirdswasunaffected,withforest corridorsinameta-analysisof78 pairedcomparisonswithand withoutcorridors(Gilbert-Nortonetal.,2010).Widerforestcorri- dorsweremoreeffective,withrichnessoftropicalrainforestbirds andmammalsincreasingrapidlyuptocorridorwidthsof500m (LeesandPeres,2008).Althoughwoodplantationsprovidefewer habitatresources than restoration plantings, theymay provide functionalforestcorridors(LindenmayerandHobbs,2004).Poor dispersers canbeintroducedtoreforestedpatchesonce appro- priatehabitatresourcesandenvironmentalconditionshavebeen createdordeveloped.

␤and␥diversitiesareproductsoftheheterogeneityofhabi- tats at all scales, from microhabitatswithin patches (<1ha)to physiographicregions(10⁵–10⁶km²),whichareassociatedwith variationinnutrients(e.g.soiltype),wateravailabilityanddisturb- ancehistory(Tewsetal.,2004).Forestnetworksshouldemulate thisspatialheterogeneitybyestablishingavarietyofnativemixed- speciesplantingsindifferentlandscapepositionstoincreasethe

␥-diversityofagriculturalregions.Targetedreforestationofmore productive locations, suchas floodplains (Vesk and Mac Nally, 2006),hasthepotentialforhighergainsinspeciesrichness,but otherlocationssupportdistinctassemblagesthattogetherwould increase␥-diversityofaregion.Reforestationneedstoaccountfor thedifferenthabitatrequirementsofanimals(i.e.foresttypes),and thetime takenfor matureforeststodevelop.Optimizationpro- cedurescanbeusedtoguideschedulingofreforestationinspace andtimetoavoidfuturepopulationbottleneckscausedbydelayed provisionofcriticalresources(Thomsonetal.,2009).

Forestnetworksbasedonwidebelts(>500m)ofnativetrees through areas would be ideal because such belts substantially increase thedispersal of native species(Lees and Peres,2008).

Severalextensivereforestationprogrammes havebeeninitiated recently,withChina’sGrainforGreenProgrammeestablishing24 millionhaofplantationsinlessthanadecade(Chenetal.,2009).

Moretypically,reforestationissmallscale(<5ha,e.g.Bhagwatetal., 2008;Cunninghamet al.,2014), discontinuous,and alongroad- sidesorriparianzones(Fig.2b).Thisforms‘stringy’networksof smallpatchesthatdonotsupportmostnativespecies,especially

interiorspeciesandlargeanimals(Turner,1996),andmayfacilitate theexclusionofsmallanimalsbyaggressivenativeanimals(Maron etal.,2013).Thedesignofforestnetworksmaybebestinformed bytheneedsof‘umbrellaspecies’(e.g.apexpredators)thathave thelargestarearequirementforhabitat(Lambeck,1997).

Semi-natural vegetation, such as clumps of remnant trees, scruborabandonedfields,canprovideusefulresourcesforsome forest species(Humphrey,2005).Similarly, plantingsmall ‘tree islands’(<0.01ha)infieldscanimprovetherecruitmentofnative treesbyalteringtheimmediatemicroclimateandattractingseed- dispersingbirds(ZahawiandAugspurger,2006).Oneapproachto increasingtheeffectivesizeofforestnetworksistocreate‘eco- logicalzones’inwhichtheextentofnativevegetationisincreased andtheintensityoflanduseisreducedconcurrently(Fig.2c,sensu Bennett,1999).Thiscouldbeachievedthroughthemaintenance andrestorationofhabitatresourceswithinremnants,creatingnew habitatswithreforestation(activeandpassive)andreducingthe intensityoflanduse(harvesting,cultivatingandchemicaladdi- tions)inthesurroundingagriculturalland(Fig.2c).Thiswouldnot necessarilyexcludereforestationofareasofhigh-intensitylanduse ordegradedconditionoutsidetheseecologicalzones.

For reforestation to increase the dispersal of native species and toimprove␥-diversity ofa region,reforestationpreferably shouldincludeplantingsthatprovidesufficienthabitatforinterior species (>100mwide and >10ha, Murcia, 1995), aspireto cre- ateextensivehabitatfragmentsforumbrellaspecies(>50ha,Ray etal.,2005),coveralargeextent(>30%ofhistoricalextent,Swift and Hannon,2010),arefunctionallyconnectedand areallowed to mature (notharvested) to offer an adequate range of habi- tatsandresources.Afunctionallyconnectedforestnetworkmay bedevelopedbyincreasingtheextentofforestandconcurrently reducingtheimpactofsurroundingagricultureonnativespecies within‘ecologicalzones’.Suchecologicalzonescouldformpartof alargernetworkthatprovides‘biolinks’amongbiomesofaconti- nent(Fig.2d,sensuBreretonetal.,1995),whichmayallownative speciestomigrateasclimateschange.

3. Functionalchangesafterreforestation 3.1. Carboncycling

Net ecosystemproduction offorestsis estimatedglobally to be1.0tCha⁻¹year⁻¹ (Panetal.,2011)whileimprovedpastures andcroplandssequesteronaverage0.5tCha⁻¹year⁻¹(Lal,2004).

Therefore,reforestationofagriculturallandprovidesanopportu- nitytosequestersubstantialamountsofatmosphericcarbon(C) andpotentiallytomitigateclimatechange(Mackeyetal.,2013).

Carbonsequestrationfollowingreforestationisdependentonthe balancebetweentheaccumulationofbiomassandlitterandlosses fromrespirationanddecompositionoflitterandsoilC.Thisbal- ancecanbealtered bydifferencesinestablishmentofplantings (e.g.single-vsmixed-species,Piotto,2008;PaquetteandMessier, 2010)andinpreviouslanduses(e.g.pasturevscropping,Pauletal., 2002;Laganièreetal.,2010).

Treeplantingsaccumulatelargeamountsofbiomass,withPinus species storing ca 250tCha⁻¹ after a century (Fig. 3a, Hooker and Compton,2003; ThuilleandSchulze,2006).Productivityof forests, and consequentlycarbon sequestrationpotential,varies widelyamongclimatezonesandforesttypes(1–30tCha⁻¹year⁻¹, ChurkinaandRunning,1998).Monoculturesofproductiontrees generallyaccumulatebiomassfasterthannativetreespeciesdue to treebreeding and silviculture (Paquette and Messier,2010).

In low-rainfallareas (<800mmyear⁻¹),nativespecies arelikely tobeequallyproductive,andlessvulnerabletodroughtandcli- matechange,thanproductiontreesduetothetrade-offbetween growthrateanddroughttolerance(e.g.Stercketal.,2011).Limited

Fig.3. Conceptualmodelsof(a)carboncyclingand(b)watercyclinginfieldsandafterreforestation.Differencesinarrowwidthbetweenanagriculturalfield(white)and areforestedfield(black)indicatetherelativeincreaseordecreaseinastockfollowingreforestation.NPP=netprimaryproduction.Estimatesforstocksandflowsofcarbon underbothlandusesaregiveninbracketswhereasonlychangesinwatercyclingfollowingreforestationareindicated.Thesewerecalculatedfromthefollowingsources:

aLal(2004);^bPanetal.(2011);^cBondeauetal.(2007);^dPregitzerandEuskirchen(2004);^eHagenetal.(2010);^fBenfield(1997);^gZhangetal.(2001);^hJacksonetal.(2005);

iIlstedtetal.(2007);and^jGeorgeetal.(1999).

meta-analyses (19 studies) suggested that the same species producemorebiomassinmixed-speciesplantingsthaninmono- culturesoftheindividualspeciesbecauseinterspecificcompetition generally is less intense than intraspecific competition, which probably is due to niche differentiation among species (Piotto, 2008; Hulveyet al., 2013). Higherdiversity does not generally meanincreased productivitybecause outcomes dependonsite

productivityand ecological niche differentiationamong species (Pretzsch,2005).Specieschoicedeterminesgrowthrates,withthe inclusionofnitrogen-fixingtrees(e.g.Acacia),asintheabovemeta- analyses,oftenincreasingproductivityduringearlydevelopment (Forresteretal.,2006).

AsubstantialamountofCaccumulateswithplantingageasleaf andstem litter ontheforest flooruntila steadystate between

deposition and decomposition is reached (Fig. 3a). Afforested streamstendtohavehigherinputsoflitter(Allan,2004)andlarger accumulations oforganicmatterin thesediment thanadjacent agriculturalstreams(Gilingetal.,2013).Litteraccumulationdif- ferswidelyamongforesttypesandspecies,with17tha⁻¹undera mixedEucalyptusplantingafter30years(Cunninghametal.,2012) and40tha⁻¹underaPiceaabiesplantingafter90years(Thuille andSchulze,2006),butcenturies-old,mixed-deciduousoakforests hadonly10tha⁻¹(FacelliandCarson,1991).Suchdifferencesin accumulationpredominantlyreflectdisparitiesinlitterincorpora- tion,withslowerdecompositionratesinevergreenspeciesthan fordeciduousspecies(Cornwelletal.,2008),andatlowertemper- aturesandlowerrainfall.

SoilgenerallyprovidesamorestableCstorethanplantbiomass, whichissusceptibletocatastrophicdisturbances,andsoilcontin- uestoaccumulateC afterforestmaturity(Schulzeetal.,2000).

SubstantialdecreasesinsoilC oftenfollowconversionofforest toagriculture(30%,Murtyetal.,2002),soreforestationhasmuch potentialforsequestration(Fig.3a).Estimatesofthechangeinsoil Cafterreforestation rangewidely(−3%to+26%),reflecting dif- ferencesinsequestrationamongclimateregions,soiltypes,forest typesandpreviouslanduse(Pauletal.,2002;Laganièreetal.,2010).

SoilCstocksgenerallyshowlittlechangeuntil30yearsafterrefor- estation(Hoogmoedetal.,2012)butcanincreaseby20%within50 years(Laganièreetal.,2010).

SpecieschoiceintreeplantingsisimportanttoCsequestration insoil,withthelargestincreasesunderbroadleafspecies(27%), intermediatevaluesundereucalypts(12%)andlittlechangeunder conifers(2%,Laganièreetal.,2010).Plantingsthatincludenitrogen- fixingtreespeciescanhavehigherproductivity(Forresteretal., 2006)andhigher retentionoforiginalsoilC stocks(Reshetal., 2002),suggestingthattheirinclusionmayacceleratetheaccumu- lationofsoilC.Regularadditionoffertilizerandwaterincreases productivityofagriculturallandoverthatofthepre-clearanceveg- etation,suchthatreforestationmaynotachievethesamesurface soilCcontentaspasturesinsomeareas(e.g.Trumboreetal.,1995).

ReforestationmayincreasethestabilityoftheexistingsoilC stockbychangingitsphysicalandmolecularformwithinthesoil.

Physical(e.g.soilaggregates)andbiologicalprotection(e.g.fun- galbiomass)ofsoilCmaybemoreimportantthanthechemical structure,withligninandsugarshavingthesamemeanresidence timeof10–50years(Schmidtetal.,2011).Reforestationmaynot changeaggregatestabilityinthefirst20years(Kaseletal.,2011) butthesubstantiallyhigheraggregatestabilityofforestsoilscom- paredwithagriculturalsoils(Lichtfouseetal.,2011)suggeststhat reforestationmayincreasetheprotectionofsoilCinthelonger term.IncreasesinsoilCfollowingreforestationareoftensubstan- tialin the‘light’fractionof partly decayedmaterial (Berthrong etal.,2012).However,increasesinthemorestable‘heavy’frac- tionofhumicmaterialmayoccur20yearsafterreforestationwith deciduoustrees(DelGaldoetal.,2003)buthavenotbeenfound withevergreentrees(Kaseletal.,2011).TheC:Nratioofsoilsoften increasesafterreforestation(e.g.Cunninghametal.,2012),which suggestsdecreaseddecompositionandincreasedstabilityofsoilC.

Fungal:bacterialratiosinsoilcanbe50%higherfollowingrefor- estationofpastures(MacDonaldetal.,2009)implyinghigher C sequestration(Jastrowetal.,2007).

Establishment choices (location, species) determine the C sequestrationpotential of reforestation.Planting in higherpro- ductivity regionsleads tofaster accumulationof biomass (Paul etal.,2002)whilereforestationondegradedsoilscandoublesoilC content(Jiaoetal.,2012).Evergreenanddeciduoustreesspecies have a similarrange ofbiomass accumulationrates (Reich and Bolstad,2001)whiledecompositionproductsofleavesofdecidu- ousspeciesareincorporatedfasterintostablesoilC(DelGaldoetal., 2003).ProductionspeciesgenerallyaccumulatebiomassCfaster

thannativespecies(PaquetteandMessier,2010),makingthem desirableifCsequestrationwerethegoal.However,production speciesareunlikelytoprovidecomparablebiodiversitybenefitsto nativespecies(Lindenmayeretal.,2003),andarelikelytobemore vulnerabletodroughtandclimatechange(e.g.Stercketal.,2011).

Aneffectivestrategywouldbetoplantamosaicofproduction-tree monoculturesforbiomassaccumulation,diversenativeplantings oftreesandshrubsforbiodiversity,andmixedplantingsofproduc- tivenativetreespeciesforbothcarbonandbiodiversity.

3.2. Nutrientcycling

Reforestationchangesnutrientcyclingonagriculturallanddue tochangesinquantityandqualityofinputs,andratesofuptake.

Plantlitterincreasesontheforestfloorandadjacentstreambeds afterreforestationoffields(Allan,2004;ThuilleandSchulze,2006).

Decompositionoflitterand,therefore,thecyclingofnutrientsare slowerafterreforestationbecauseofthelignifiednatureoflitter fromtreesandtheshifttowardsafungal-dominatedassemblagein thesoil(MacDonaldetal.,2009).Thetreespeciesusedinaplant- ingaffectsthebreakdownoflitterduetodifferencesinquality(e.g.

deciduousvsevergreenspecies,Cornwelletal.,2008).However, environmentaldifferencesamongplantings(e.g.soilmoistureand streamtemperatures)mayhavealargereffectonlitterdecomposi- tionthanthespeciesmixoflitter(Lacanetal.,2010).Theeventual (>30years)increaseinsoilCafterreforestationwillimprovethe physicalpropertiesofsoil,theamountof nutrients,cationsand traceelements,andconcentrationoforganicacidsthatmakemin- eralsavailableandpreventleachingfromthesystem(Leeperand Uren,1993).

Losses in soil nitrogen (N) are expected after reforestation becauseconcentrationsusuallyarehigheronagriculturallandthan inforests(GartenandAshwood,2002)duetotheadditionofagri- culturalfertilizers.SoilNdecreasedby20%underPinusplantations butnotunderEucalyptus,otherangiospermsorotherconifersbased onmeta-analysisof153treeplantings(Berthrongetal.,2009).A century-longchronosequenceofreforestationwithPinusstrobus showedthatecosystemNstockswereconservedbytranslocation ofNfromthesoiltoabovegroundbiomass(HookerandCompton, 2003).ChangesinsoilNafterreforestationmaydependonsitepro- ductivity,withlossesinhighrainfallareas(>1200mmyear⁻¹)but gainsinlow-rainfallareas(<800mmyear⁻¹,Berthrongetal.,2012) andondegradedsoils(Jiaoetal.,2012).

Reforestationwithconifers increasesthe mineralizationand availabilityofphosphoruscomparedwithagriculturalsoils(Chen etal.,2008).Someforesttypesmayaccumulatesoilphosphorus forcenturies, withplantings ofQuercusrobur showingsubstan- tialincreasesbetween100and200years (Wilsonet al.,1997).

Other macronutrients, suchas calcium, potassium and magne- sium,decrease(>20%)inthesoilafterreforestationbecausethese are translocated tobiomass (Berthrong et al.,2009), except on degradedsoils,wheremacronutrientscanincreasebyasmuchas 70%(Jiaoetal.,2012).

Theredistributionof basecations(Ca²⁺,K⁺,Mg²⁺,Na⁺)from thesoiltobiomassafterreforestationwithspeciesofEucalyptus, Pinusandotherconifersacidifiesthesurfacesoil(−0.3pHunits, Berthrongetal.,2009).ReforestationwithPiceasitchensisinScot- land ledtosubstantial decreasesinstream pH(−0.7 pHunits) onceforestcoverexceeded60%ofthecatchment,withanasso- ciated80%decreaseinabundanceofnativebrowntrout(Reesand Ribbens,1995).Similarly,extendeduseofsalinegroundwaterby treesaccumulatessaltintheirbiomass,soilorthegroundwater (JobbagyandJackson,2004).Reforestationcanleadtoanaverage increaseof71%insoilNa,withplantingsofEucalyptusinsomeareas increasingsoilNabyanaverageof250%(Berthrongetal.,2009).

Eucalyptsareplantedtolowersalinewatertablesbecauseoftheir

rapidgrowth,buttheassociatedincreasingsalinizationofsoiland groundwatermayreducethelong-termviabilityofsuchplantings.

Adverse effects of reforestation on nutrient cycling (acidifica- tionandsalinization)maybeavoidedbyplantingnativespecies foundundersimilarenvironmentalconditionsandbyreducingtree density.

3.3. Stand-scalewatercycling

Waterfallingonthelandsurfacecan:(1)returnrapidlytothe atmospherethroughinterceptionbyplantcanopiesandbyevap- orationfromsurfacesoil;(2)infiltratetodeepersoillayers;or(3) runoffthesurfacetoprovidestreamflow(Fig.3b).Ofthewaterthat penetratestodeepersoillayersandthewatertable,muchistaken upbyrootsandtranspiredfromleaveswhiletherestcontributesto baseflowsintostreams(WaringandRunning,1998).Evapotranspi- ration(ET),whichincludestranspirationandinterceptionbyplants andevaporationfromthesoil,canbeasubstantialproportionof annualrainfall(30–80%,Zhangetal.,2001),givingplantsaprimary roleincyclingwaterbetweenthelandandatmosphere.Treeshave agreaterpotentialtoinfluencewatercyclesthanagriculturalcrops andpasturesduetotheirlargerleafareaandextensiverootsystems (Farleyetal.,2005).

Evapotranspirationcanbe50%higherfromforeststhanfrom agriculture(Ellisonetal.,2012,Fig.3b),suggestingreforestation couldshiftthewaterbudgetfromasurplus(ET<rainfall),result- ingindecreasedgroundwaterrechargeandrunoff(Jobbagyand Jackson,2004).However,thedifferenceinETbetweenforestand grasslanddependsonannualrainfall,withforestshaving50%more ETat2000mmyear⁻¹ andlittledifferencebelow500mmyear⁻¹ (Zhangetal.,2001).ManyestimatesofETarefromshort-rotation (<30years)woodplantationsandprobablyoverestimatethelong- termwateruseoftreeplantings.ETdecreasesinolderstands,with ratesdecreasingbyathirdbetween10and54yearsinPinuspinaster forests(DelzonandLoustau,2005).

Waterusedifferswidely(10–200kgday⁻¹tree⁻¹)amongforest typesandamongtreetaxa(Wullschlegeretal.,1998).Intercep- tionofrainfallbycanopiesofevergreenconiferousforestsistwice thatof broadleafdeciduousforests(Komatsuet al.,2011).Tree speciesappropriateforthelocalcurrentandfuturewatercondi- tionsshouldbeplantedinpreferencetoproductionspecies,which typicallycombinehighratesofbiomassaccumulationwithhighET (Jacksonetal.,2005).

Waterinfiltrationisusuallyhigherbeneathtreesthaninagricul- turallandduetoalargenetworkofmacroporesinthesoil(Fig.3b, Vetaas,1992).Clearanceofforestsforagricultureleadstolong- termreductionsininfiltrationowingtosoilcompaction(Bruijnzeel, 2004).Intropicalregions(850–2500mmrainfallyear⁻¹),infiltra- tionofsoilscanincreaseby60mmh⁻¹duringearlydevelopment ofreforestation(Ilstedtetal.,2007).However,mostundisturbed forestsoils have aninfiltration rateof 20mmh⁻¹ (Waring and Running,1998).Evenwithincreasedinfiltrationafterreforestation, theyear-roundwateruseoftreesleadstolowersoilmoistureand reduceddeepdrainagetothewatertablerelative toagriculture (Fig.3b,JobbagyandJackson,2004).

Theincrease inET andinfiltration followingreforestation of agriculturallandsuggestdecreasesinlocalrunoff(Fig.3b).Fast- growingtreespeciessuchasproductionspeciesarelikelytoreduce runoffmorethanslow-growingspecies(Carnusetal.,2006),and maybemoresusceptibletodroughtandclimatechange(e.g.Sterck etal.,2011).Riparianforestcanproducetwicethereduction in runofftoanequivalentareaofuplandforest(Scott,1999)demon- stratingthat landscapepositionisan importantdeterminantof runoff after reforestation. Reductions in local runoff would be achievedbyplantingtreesatlowerdensitiesandwithspeciesthat uselesswater.

3.4. Wateryieldofcatchments

Thereisconcernthatwidespreadreforestationofagricultural land may lead to substantial reductions in water yields from catchments.Aglobalmeta-analysisof504catchmentobservations indicatedthat streamflowgenerally wasreducedby50% at20 yearsafterreforestation(Jacksonetal.,2005).Reforestationcauses largerproportionalreductionsinwateryieldsinmoderaterain- fallareas(−60%,1000–1250mmyear⁻¹)thaninhighrainfallareas (−30%,1250–2000mmyear⁻¹,Farleyetal.,2005).Inlowerrain- fallareas(600–1000mmyear⁻¹)wherewaterismorecritical,little differenceinstreamflowhasbeenpredictedbetweenforestedand agriculturalland(vanDijketal.,2007).Giventhatstreamflowfrom catchmentsusuallyismonitoredforafewyearsandrarelyinplant- ingsolderthan30years,thelong-termeffectsofreforestationon wateryieldareunclear.

Thespeciesusedinreforestationcanhaveasubstantialeffect onthewateryieldofcatchments.Inareviewof94catchmentstud- ies,a10%increaseinextentofEucalyptusandPinusforestcaused a40mmdecreaseinannualstreaminputsrelativetograsslands, while the decrease in deciduoushardwood forestswas 25mm (BoschandHewlett,1982).Inthefirstdecade,Eucalyptusplanta- tionsinducedalargerproportionaldecreaseinstreamflow,and morecease-to-floweventsthandidPinusplantations(Farleyetal., 2005).However,theseproductioneucalyptswerepredominantly plantedoutsideAustraliaandmaycausesmallerreductionsintheir nativeranges.

Focusingonlyonannualwateryieldsfollowingreforestation overlookstheseasonalityofflows,whichismoreimportantforthe lifehistoriesofplantsandanimals.Thisisespeciallytrueinarid areaswherebaseflowsdeterminesurvivalthroughthedrysea- son.Absolutereductionsinyieldfollowingreforestationusually arelargerduringthewetseasonwhileproportionalreductionsare largerduringthecriticaldryseason(Brownetal.,2005).Howmuch reforestationreducesbaseflowsduringthedryseasondepends ontheaccessoftreestogroundwatersystems(Bruijnzeel,2004).

Forestcoverseemstohavelittleimpactonbaseflowsintropical catchments,whichpotentiallyisexplainedbythebalancingeffect ofincreasedevapotranspirationandincreasedinfiltrationunder forests(LocatelliandVignola,2009).Theeffectofforestcoveron stormflowsisunclear(Bradshawetal.,2007;LocatelliandVignola, 2009).

The spatial arrangement of reforestation in a catchment dependsonwhetherthegoalistominimizereductionsinwater yieldortomitigaterisingwatertables.Thehighestwateruseof plantationsoccursatthebottomofslopesornearstreamswhere soilsaredeepandwateraccumulates(Benyonetal.,2007),and theseareasshouldbeavoidedtomaintainwateryields.Recharge zones(e.g.slopesandhilltops) shouldbeplantedpreferentially over discharge zones (e.g. floodplains) to maximize the effec- tivenessof reforestationfor loweringthewater table(vanDijk etal.,2007).Theimpactoftreeplantingsonwatertablesislocal (<30maway),sothatextensivereforestation(ca25%forestcover) is requiredtolower thewater tableby1macrossa landscape (10–100km²,Georgeetal.,1999).

Productive species provide fast C sequestration but their widespreadplantingcouldleadtosubstantialreductionsinwater yields(Jacksonetal.,2005).Speciesthatoccurinplaceswithsimilar moistureconditions,preferablynativespecies,needtobeplanted toavoidunintendednegativewater-yieldeffectsofreforestation.

Reducingtreedensityeitheratplantingorduringearlydevelop- ment canreduce thenegativeeffects of reforestation onwater yields,particularlyinhigherprecipitationandelevationareas(Zou etal.,2010).

Although reforestation can reduce runoff at the local scale (<2km²),extensiveforestcoveratregionaltocontinentalscales

(10⁵–10⁷km²)mayincreaserainfallandwateryields(Ellisonetal., 2012).Evenpessimisticforecastsofwateryieldsunderextensive reforestationforCsequestrationinNorthAmericawerebasedon veryuncertainpredictionsformeanannualrainfall(Jacksonetal., 2005).Thehighevapotranspirationofforestscanstimulatecloud formationbyincreasingwatervapourandaerosolsintheatmo- sphere,and cancreate areas of low pressure,which attract air currents(Makarievaetal.,2006).Continentalevapotranspiration isamajorsourceofannualglobalrainfall(40%,vanderEntetal., 2010)andofsummerrainfallinlargeriverbasins(48%,Ellisonetal., 2012).Extensivereforestationmaysubstantiallyincreaseregional andcontinentalrainfallandpotentiallyoutweighsmallerdecreases inlocalrunoff.

3.5. Waterquality

Agricultural land use increasesnutrient inputs into streams relative to nativevegetation, with consequent negative effects onmacroinvertebrateandfishassemblages(MaloneyandWeller, 2011),potentiallyresultingintoxicalgalblooms(Cloern,2001).

Reforestationhasthepotentialtoreducetheamountofagricul- turalpollutants(fertilizerandpesticides)appliedtotheland,and tofilteroutsedimentandexcessnutrientsbeforeenteringstreams (Lowranceetal.,1997;Newboldetal.,2010).Increasingforestcover inaregionmayreducesalinedischargeintostreamsbylowering risingwatertables(Tutejaetal.,2003).

Restoringthegroundlayerofvegetationreducessoilerosion (FrancisandThornes,1990)andoccursafterreforestationprinci- pallybytheendingofcultivationandbytheremovaloflivestock.

Treesshouldbeplantedatdensitiesthatprovidesufficientlight forthedevelopmentofagroundlayer(BroadmeadowandNisbet, 2004).Riparianplantingscanreducesediment enteringstreams because vegetationreduces runoff and channelled flow, which increasesinfiltration and depositionof material(Broadmeadow andNisbet,2004).Ariparianvegetationbufferof30mcancap- turenearlyhalfthesuspendedsediment(Newboldetal.,2010).

Theeventualaccumulationoffallentimberinstreamsdecreases transportofsedimentsalongstreams(Montgomery,1997).

Forested reachesreduce N in subsurface water to a greater degree (40–100%) than do agriculturalreaches (10–60%). Wide plantings(30m)overshallowaquifershavethehighestpoten- tialforreducingnutrientloadsinstreams,withretentionratesof almost100%possibleforN(OsborneandKovacic,1993).However, theretentionofNbyplantingsisreduceddramaticallyaftercanopy closureandcontinuestodecreaseasthegrowthrateofthetrees declineswithmaturity(Gundersenetal.,2006).TheremovalofP islesseffectivebecausethereisnomicrobialprocessequivalent todenitrification(Lowranceetal.,1997).Atthecatchmentscale (10⁴–10⁵km²),nutrientloadsaredeterminedmainlybylanduse, sothatextensivereforestationisrequiredtoreducetheimpactof agricultureandimprovewaterquality(Allan,2004).

Reforestationcanlowerlocalwatertables(Georgeetal.,1999) andhasthepotentialtoimprovewaterqualityofstreamsinsalin- izedareas.However,extensivereforestation(ca25%forestcover) isneededformajorreductions(ca25%)instreamsalinity(Tuteja etal.,2003).Preferentialplantingonhillslopesmaybemuchmore effective(+50–600%)inreducingstreamsalinitythanhaphazard plantingacrossacatchment(vanDijketal.,2007).

Reforestationofindividualreacheshasanegligibleeffect on waterquality becausepollutants aretransported longdistances alongstreams(Parkynetal.,2003).Theeffectivenessofriparian plantings in reducing nutrient loads in streams decreases with maturity (Gundersen etal., 2006).Issuesof water qualitymust be addressed at the whole-catchment scale, of which riparian zones are a minor component in terms of total area, with an

understanding of the flow pathsof surface and ground water.

Sourcesofpollution(e.g.sedimentandsalinity)maybemoreeffi- cientlyreducedbyreforestingalongdrainagelinesofuplandareas insteadofrelyingmainlyonriparianzones.Giventhatreforestation (includingrestorationandwoodplantings)mayremainaminor component of mostagricultural regions, improvingagricultural practicesislikelytobeamoreeffectivewayofimprovingwater quality.

4. Balancingtheenvironmentalbenefitsofreforestation

Reforestation presents many potential compromises in the concurrent management of biodiversity, C sequestration, water yields and water quality (Fig. 4). Plantations of production speciessequesterCrapidlybuthavelimitedbiodiversitybenefits (Lindenmayeretal.,2003)andmaycausesubstantialreductions in water yields (Jackson et al., 2005). No single type of refor- estationcansimultaneouslymaximizeallenvironmentalbenefits (Fig.4)butanyformofreforestationshouldprovideenvironmen- talimprovementsoveragriculturalland,withtherelativebenefits changingasplantingsmature.

Ourreviewsuggeststhatpractitionerscanusedifferentplant- ingconfigurations(location,size,speciesmixandtreedensity)to achievedifferentenvironmentaloutcomes(Fig.4).Tomaximize biodiversityoutcomes,individualplantingsshouldbeaslargeas possible,giventhelong-establishedrelationshipbetweenareaand speciesdiversity(Gaston,1996),andremainunharvestedtoallow arangeofhabitatsandresourcestodevelopandmature(Kanowski etal.,2005).Thebiodiversityof aregioncouldbeincreasedby plantinga rangeofnativetrees andshrubs atvariousdensities acrossalllandscapepositions(Fig.4).Maximizingcarbonseques- trationcouldbeachievedbyestablishingplantationsofproduction speciesonthemostproductiveland(highrainfallareasandflood- plains)atmediumdensities(OliverandLarson,1996).Reductions towateryieldscouldbeminimizedbyplantingtreeshavinglow wateruseatlowdensitiesandbyavoidinglandscapepositionswith accesstogroundwater;improvingwaterqualitycouldbeachieved bytheoppositestrategies(Carnusetal.,2006;Benyonetal.,2007;

Zouetal.,2010).

Reforestation in agricultural regions involves compromises among componentsof structureand functionatthelocalscale, buttheregionalbenefitswillfaroutweighlocalnegativeeffects.

Thisinvolvesa mosaicof reforestationapproaches for different outcomes acrossa region(Fig.4)and intermediate approaches forcombinedoutcomes(e.g.plantationsofproductivenativetree speciesforbothCsequestrationandbiodiversity,Lambetal.,2005).

Plantingnativetreespeciesthatgrowundersimilarenvironmental conditionsshouldavoidadversefunctionaloutcomes.Treedensity isakeyattributeoftreeplantingsthatpractitionerscanmanipu- latetoprovidedifferentstandstructures,habitatresources,water yieldsandwaterquality(OliverandLarson,1996;Zouetal.,2010).

Itisdifficultforlandmanagersandpolicymakerstomeasurethe effectsofalternativereforestationscenariosonthestructureand functionofagriculturalregions.Forwidespreadreforestationtobe acceptedandimplementedstrategically,landmanagersandpol- icymakersmustbeabletobeabletopredictthebalanceofrisks andbenefits.Studyofreforestationhasonlybecomewidespread in recent decades, particularly for mixed-species plantings, so thatmostunderstandingislimitedtotheearlydevelopmentof suchforests(Jacksonet al.,2005; Laganière etal., 2010).Com- parisons of remnant forests and clearedland are used toinfer long-termchangesafterreforestation (e.g.Allan,2004)but itis unknownwhethertreeplantingswilldevelopeventuallyintosim- ilarmatureforestecosystems,particularlygivenchangingclimates andthetimeframesinvolved.Previousworkonreforestationhas