Geomorphology and forest management in New Zealand's erodible steeplands

15 Pages·2017·4.57 MB·English

GEOMOR-06094;NoofPages15
Geomorphologyxxx(2017)xxx–xxx
ContentslistsavailableatScienceDirect
Geomorphology
journal homepage: www.elsevier.com/locate/geomorph
Geomorphology and forest management in New Zealand's erodible steeplands:
An overview
ChrisPhillipsa,⁎,MichaelMardenb,LesR.Basherc
aLandcareResearch,P.O.Box69040,Lincoln7640,NewZealand
bLandcareResearch,P.O.Box445,Gisborne4040,NewZealand
cLandcareResearch,Nelson7042,NewZealand
a r t i c l e i n f o a b s t r a c t
Articlehistory: InthispaperweoutlinehowgeomorphologicalunderstandinghasunderpinnedforestmanagementinNew
Received5April2017 Zealand'serodiblesteeplands,whereitcontributestocurrentforestmanagement,andsuggestwhereitwillbe
Receivedinrevisedform31July2017 ofvalueinthefuture.Wefocusonthehighlyerodiblesoft-rockhillcountryoftheEastCoastregionofNorthIs-
Accepted31July2017
land,butcoverotherpartsofNewZealandwhereappropriate.Weconcludethatforestrywillcontinuetomakea
Availableonlinexxxx significantcontributiontoNewZealand'seconomy,butseveralissuesneedtobeaddressed.Themostpressing
concernsaretheincidenceofpost-harvest,storm-initiatedlandslidesanddebrisflowsarisingfromsteeplandfor-
Keywords:
estsfollowingtimberharvesting.Therearethreeareaswheregeomorphologicalinformationandunderstanding
EastCoast
Erosionprocesses arerequiredtosupporttheforestindustry—developmentofanimprovednationalerosionsusceptibilityclassi-
Forestmanagement ficationtosupportanewnationalstandardforplantationforestry;terrainanalysistosupportimprovedhazard
Landslides andriskassessmentatdetailedoperationalscales;andunderstandingofpost-harvestshallowlandslide-debris
Terrainzoning flows,includingtheirpredictionandmanagement.
©2017ElsevierB.V.Allrightsreserved.
1.Introductionandbackground (Pinusradiata).Thesecondplantingboomoccurredinthe1960swith
muchofthisonlandthathadbeenidentifiedaserosion-proneandgen-
ErosionratesinNewZealandareveryhighbyworldstandardsbe- erallyinsteeperhillcountry.Thethirdplantingboom(1992–1997)oc-
causeofnaturalandanthropogenicfactors.Ahigh-energygeomorphic curredmainlyintheEastCoastregion(Fig.1)followingCycloneBola
environmentisaconsequenceoflocationonanactiveplateboundary (MardenandRowan,1993).Nativeforestnowcoversabout25%of
in the mid-latitude zone of strong westerly winds (Basher, 2013) NewZealand,withmostinreservesornationalparks,manyofwhich
resultinginadominanceofsteepslopes,highratesoftectonicactivity, areinthesteepmountainlandsandeffectivelynolongerusedfortimber
generallyhighrainfallandcommonhighintensityrainstorms(Soons production.Currentlyexoticforestcovers1.7Mha(about8%oftheland
andSelby,1992;Hicksetal.,2011).Erosionrateswereintensifieddur- area;(NewZealandForestOwnersAssociation,2016).
ingEuropeancolonisationinthelatenineteenthandearlytwentieth AwidevarietyoferosionprocessesoccurinNewZealand,andthe
centurieswhenlarge-scaledeforestationoccurredovermuchofthe types and activity of erosion show strong regional patterns first
country,followedbytheintroductionoflargenumbersofgrazingani- recognisedbyCumberland(1943,1944)thatrelatetogeologicalset-
malsandintensivelanduseinsomelocations(Mosley,1978;Pageet ting,climate,andlanduse.Massmovementsareregardedasthedomi-
al.,2000;Glade,2003).Inthatperiod,mostofthelowlandnaturalfor- nanterosionprocessinsteeplands(Basher,2013).
estswereclearedandloggingprogressedintothehillier,steeperparts Theuseofforestsforcontrollingerosionorreducingnaturalhazards
ofthecountry.Whilesustainedyieldmanagementofnativeforests iswellestablishedinNewZealandasitisinmanyotherpartsofthe
waspracticed,itbecameapparentthatinordertoprovideacontinuing world(O'Loughlin,1991).NewZealandforestryhasbenefitedsignifi-
supplyoftimbertomeettheneedsofagrowingcountry,forestsneeded cantlyfromexperiencesgainedfromthemountainlandsofEuropeto
tobeplanted.ThisledultimatelytotheestablishmentoftheStateForest thosefromthePacificNorthwestoftheUSAandCanada.However,
Servicein1921(Roche,1990;TeAra,2017).Whatfollowedwerea NewZealandhasalsocontributedtoglobalunderstandingofthisrela-
numberofplantingbooms,initiallyinthecentralNorthIsland,but tionship,owinginnosmallparttoitsrecentcolonialhistoryandthe
laterspreadingelsewhere.Themainspeciesplantedwasradiatapine naturally high rates of geomorphic processes (O'Loughlin, 1995,
2005a,2005b;Basher,2013).
⁎ Correspondingauthor. Globally,considerablescientificendeavourhasfocusedonunder-
E-mailaddress:[email protected](C.Phillips). standingrelationshipsbetweenforestsandgeomorphology,including
http://dx.doi.org/10.1016/j.geomorph.2017.07.031
0169-555X/©2017ElsevierB.V.Allrightsreserved.
Pleasecitethisarticleas:Phillips,C.,etal.,GeomorphologyandforestmanagementinNewZealand'serodiblesteeplands:Anoverview,Geomor-
phology(2017),http://dx.doi.org/10.1016/j.geomorph.2017.07.031
2 C.Phillipsetal./Geomorphologyxxx(2017)xxx–xxx
Fig.1.MapofNewZealandincludinginsetofEastCoastregionofNorthIsland.
landscapesandforestcomposition(e.g.,Turner,1989),steeplandforest hydrological),andforestmanagement(estatemanagement,roading,
management including harvesting (Swanson and Swanston, 1977; planning)withanemphasisonplantedforests.
Grantetal.,1984;Sidleetal.,1985;Swanson,1998;Wempleetal.,
2001; Sidle, 2005); and the value of forests and trees to provide a 2.Historicalcontext
widevarietyofecosystemservices,includingwatershedprotection
anderosioncontrol(O'Loughlin,1994;Roche,1997;Marden,2012; GeomorphologicalunderstandingofNewZealand'ssteeplandshas
Basher,2013;Phillipsetal.,2013). playedasignificantroleinthemanagementofindigenousforestsand
Inthispaperweprovideaselectiveoverviewandhistoricalaccount mountainlandsandinthedevelopmentoftheforestindustrybasedei-
ofhowgeomorphologicalunderstandinghasunderpinnedforestman- theronindigenousforestsorintroducedspecies.Forexample,therela-
agementinNewZealand'serodiblesteeplands,outlinethecurrentfor- tionshipsbetweenforestremoval,introducedanimals,anderosionhave
estry situation in New Zealand, and then suggest where an beenwelldocumented(Cumberland,1943,1944;Wallis,1966;James,
understandingofgeomorphologywillbeofvalueinthefuture.Inpar- 1969;McCaskill,1973;Jane,1979;Holloway,1982;McSaveneyand
ticularwefocusonthehighlyerodiblesoft-rockhillcountryofthe Whitehouse,1989;McKelvey,1992).IntheearlydaysofNewZealand's
EastCoastregionofNorthIsland(Fig.1),thoughincludeothergeologi- colonisationanddevelopment,vasttractsofsteeperodiblehillcountry
calterrainswhereappropriate.Ourapproachisfocusedonthreemain forestswerecutandburnttomakewayforpastoralfarming,inmuch
areas: terrain assessment, process understanding (erosion and thesamewayasoccurredinothercountriesexceptthatNewZealand's
Pleasecitethisarticleas:Phillips,C.,etal.,GeomorphologyandforestmanagementinNewZealand'serodiblesteeplands:Anoverview,Geomor-
phology(2017),http://dx.doi.org/10.1016/j.geomorph.2017.07.031
C.Phillipsetal./Geomorphologyxxx(2017)xxx–xxx 3
colonisationwasrelativelyrecent(withinthelast150years)andthere- processesandreducingsedimentyield—theveryreasonforwhich
sponsetoforestremovalislikelystillimpactingslopeandchannelpro- theywereestablishedalsoneededtobeunderstood.
cesses(Mardenetal.,2014b).Inmanyplaces,thenaturallandscape
respondedbysignificantlyincreasingerosion,sedimentload,andag- 3.1.Terrainassessment
gradationinriverchannelsandresultinginconsequentdownstream
floodingissues. Geologicalandgeomorphologicalinformationhasbeenusedindif-
SomeofthemostdramaticresponsesoccurredintheEastCoastof ferentpartsofNewZealandtoinformforestmanagementandplanning.
NorthIsland(e.g.,Allsop,1973).Here,acombinationoffactorsinclud- Inthebroadestsense,landuseorforestestateplanningiswheresetsof
ingtectonic(e.g.,earthquakes,upliftrates,faulting),lithologic(rock constraintsandoptionsareconsideredandthenmappedtoproduce
type),adynamicclimateinfluencedbytropicalcyclones,andtherecent someoptimumpatternoflanduseorspeciesplantingpattern.Thesec-
clearanceofvegetationfromsteepslopespredisposesthisregiontoero- ondaspect,operationalplanning(Pearce,1981),isconcernedwithde-
sion.Dramaticincreasesinsoilerosionundergrasslandultimatelyledto cisionsratherthanoptions—examplesincludefollowingaparticular
the establishment of a major government-led reforestation project roadalignmentorchoosingthetypeofmachinerytologaforestcom-
largelywithexoticconifersintheWaipaoaRivercatchmentinthe partment.Loggingorharvestplanninginwhichharddecisions,short
1950s and 1960s (Phillips and Marden, 2005). Reforestation was timescales,andfinespatialresolutionarerequiredcreatethepullfor
regarded as the most economical way of controlling erosion geomorphologicalinformation;butappropriatedatathatcanbequickly
(CommitteeofEnquiry,1970),andwhiletheseforestswereprincipally interpretedorusedatthescaleneededisnotoftenavailable.Somespa-
for‘protection’itwasacknowledgedthatapartfromcreatingemploy- tialmappingapproachesareusefulforbroad-scaleplanningbutarenot
ment,partsofthoseforestswouldbeabletobeusedfortimberproduc- suitableforoperationalplanning,whichwouldnormallyrequireinfor-
tion.Subsequentpolicyenabledreforestationtobeextendedtoother mationatscalestypicallybetween1:10,000and1:5000.
catchmentsintheregion.Chronologiesoftheattemptstocontrolwide- Earlyterrainzoningapproacheswereusedinplanningfornative
spreadsoilerosionintheEastCoastofNorthIslandhavepreviously forestharvestingaspartoftheWestCoastBeechUtilisationProposals,
beendocumented(Phillipsetal.,2013;PhillipsandMarden,2005).For- whichwerelocatedinareasonSouthIslandwithrecognisedhighero-
estremovalandreforestationisnotuniquetoNewZealand(McKelvey, sionhazardsonerodiblerocktypesandlowerhazardsonsimilarterrain
1992;PhillipsandMarden,2005;Phillipsetal.,2013)buthasalsobeen underlain by other rock types (Fitzsimons and O'Loughlin, 1984;
experienced elsewhere (e.g., European Alps, Japan, United States, Fitzsimons et al., 1985; Eggers, 1987; O'Loughlin and Gage, 1975;
France)andiswelldocumented(Kittredge,1948;Matheretal.,1999; O'Loughlin and Pearce, 1976). A three-class system of zoning was
Piégayetal.,2004;Liebaultetal.,2005). foundusefulasanoperationalplanningtoolforthesebeechforests,
In1987theNewZealandForestService(NZFS)wasdisestablishedas andzoningmapsofthreeclasseswerepresentedon1:10,000scaleto-
partofgovernmentreforms(Roche,1990)withstate-ownedexoticfor- pographicmaps.Thethreeclasseswere:noerosionorwaterandsoil
estsinitiallygoingtoagovernmentbusinessentity(ForestryCorpora- conservationconstraintsonclearfelling,ifthatwasdesirable;wherese-
tion),butwithin3yearsittoowasdisbandedandtheforestcrops lectionloggingwouldbeacceptable,butwhereclearfellingwaslikelyto
(butnottheland)soldtomany,mostlyoverseas-ownedcompanies. causeerosionproblems;andaprotectionzoneclasswherelogging
Withthischange,aphilosophicalshiftoccurredfromforeststhathad shouldnotbepermitted.Theapplicationofthisinformationinanoper-
previously been considered as ‘protection forests' to one of much ationalsensewasnotfullytestedasthebeechschemewasdiscontinued
moreutilitarian‘productionforests'(Poole,1960),inpartbecausethe asaresultofpublicpressuretostopharvestingindigenousforests(Te
forestsplantedonerosion-pronelandhadgrownmuchbetterthanan- Ara,2017).
ticipatedwhenfirstplanted.Today,manyoftheseforestshavebeen OneareainneedofterrainassessmentwasintheEastCoastofNorth
‘returned’totheindigenouspeopleofNewZealand(Māori)aspartof Island,particularlyintheupperreachesoftheregion'smajorriver
theTreatyofWaitangisettlements(e.g.,NgātiPorou,2010)andin catchments(Fig.1).Inordertosupportthenewreforestationpolicies
somecasesonsoldtoprivateinterests.Othersstillremainunderforeign onformerpastoralland,questionsbegantoberaisedaboutthefuture
managementandallarenowmanagedforwoodproduction.Inrecent oftheseprotectionforestsforproducingtimber.Whiletheoriginalob-
yearsastheseforestshavebeenharvestedmanyofthegeomorphicfac- jectiveswereprimarilyremedialandprotective,prospectsthatatleast
torsthatcontributedtotheirestablishmenthavebeguntomanifest partsoftheseforestswouldeventuallybesuitableforproductionforest-
themselvesinwaysforeseenbysomebutlargelyforgottenbyothers rylookedgood.Tosomedegree,thiswouldcompensateforthelossof
duringthetransitiontocommercialplantationforestry(Pearce,1977; agriculturalproductivityfromlandtakenoutofpastoralfarming.How-
Phillipsetal.,2012). ever,thechoiceofwhichareastoleaveasprotectionandwhichareas
weresuitableforproductionrequiredanassessmentofthelandscape
intermsofitssusceptibilitytoerosionfromvariousprocesses(Gage
3.Researchandinformationneeds andBlack,1979;O'Loughlin,2005a).
Inthe1970s,TerrainStabilityZoningwasdevelopedincorporating
SeekingtounderstandNewZealand'sforestedenvironmentsand theunderlyinggeologyandgeomorphologicalfactorstoaccountfordif-
howmanaffectedthesehasbeenanintegralpartofresearchwithin ferencesinslopestabilityanderosionalbehaviourwithinforests(Gage
theNZFSandothergovernmentagenciesfornearlyacentury.Fascina- andBlack,1979).Terrainstabilityzoning,an8-classterrainclassifica-
tionwiththemountainandsteeplandenvironmentsinparticularand tionintermsofslopestabilityanderosional(ordepositional)behav-
theirerosionalhistoryledtoarangeofresearchandtheestablishment iour,wasinitiallydevelopedatMangatuForestandlaterappliedto
ofdedicatedagenciestoconductthatresearch(e.g.,TussockGrasslands other East Coast exotic forests (Pearce, 1977; Phillips and Pearce,
andMountainLandsInstitute;ForestandRangeExperimentalStation; 1984a,1984b).WhileextensiveareasofStateForestintheEastCoastre-
NationalWaterandSoilConservationOrganisation).Allthesededicated gionwereassessedandreportsandmapsproduced,inclusionofthis
researchagencieshavesubsequentlybeendisestablishedasaresultof knowledgeintoforestmanagementwasmostlyignoredbecausethe
governmentpolicyandfundingchanges. timingofpublicationcoincidedwithaperiodofmajorchangeinthefor-
Theearlyerosioncontrolreforestationprogrammesestablishedby estindustry:thedisestablishmentoftheNZFSandtheselling-offofthe
theNZFSinplacesliketheEastCoastalsotriggeredspecificresearch forestestate(outlinedabove).However,inrecenttimes,thisinforma-
needs.Apartfromobviousquestionsaboutspeciesselection,establish- tionhasbeenusedbysomeforestmanagers,particularlyinrelationto
mentandmanagement,andtheirsubsequentperformanceandwood harvestplanning,whereitprovidedanindicationofareasthatmightre-
quality, how these new forests performed in modifying erosion quireadditionalanalysisofslopestability(Phillipsetal.,2012;Marden
Pleasecitethisarticleas:Phillips,C.,etal.,GeomorphologyandforestmanagementinNewZealand'serodiblesteeplands:Anoverview,Geomor-
phology(2017),http://dx.doi.org/10.1016/j.geomorph.2017.07.031
4 C.Phillipsetal./Geomorphologyxxx(2017)xxx–xxx
etal.,2015).Callstomakesuchapproachesmandatoryastheyarein
Canada and the USA (Fannin et al., 2007; Schwab and Geertsma,
2010)havealsobeenmade(Mardenetal.,2015).
3.2.Erosionprocessunderstanding
Alongsidethedevelopmentoftoolstoassessterrain,questionswere
alsobeingaskedabouthowindigenousdeforestationandexoticrefor-
estationaffectratesoferosionandhydrologicalprocessesandabout
theoverallvalueofsuchforestsforprovidingarangeofservicesto
thewidercommunity(NWASCO,1970;SwaffieldandFairweather,
2000).ThissparkedseveraldecadesofexperimentalworkacrossNew
Zealand,muchofwhichwasundertakenintheWaipaoacatchment
andmorespecificallyinplaceslikeMangatuForest(Fig.1).Here,re-
searchersbegantoexaminetheon-siteeffectivenessofreforestation
inmitigatingerosion(earthflow,slump,gully)processes(DeRoseet
al.,1998;Zhangetal.,1991a,1991b,1993;Mardenetal.,2005,2008a, Fig.2.CycloneBola(1988)damagetoyoungplantationslessthanabout6yearsold,inland
2008b,2011,2012,2014b;Herzigetal.,2011),itsinfluenceonhydro- TolagaBay,EastCoast,NorthIsland,NewZealand.Photo:NoelTrustrum.
logical processes studies (Pearce et al., 1987), and the role of tree
rootsinsoilreinforcementofsoilsandtheircontributiontoslopestabil- theCoromandelregionofNewZealandthatwasaffectedbyasevere
ity(Ekanayakeetal.,1997;EkanayakeandPhillips,1999,2002;Phillips storminMarch1995withfewslopefailuresoccurringwithinclosed-
etal.,2011).Promptedbyconcernsoftheoff-siteeffectsofsediment canopyexoticforest(MardenandRowan,2015).Theseresultssuggest
anditstransportationandsupplytothePovertyBayfloodplain(an thattheleveloflandslidedamagelikelytooccurwithinaforeststand
areausedintensivelyforhorticulture)andtotheocean(Hicksetal., isdependantatleastinpartontheage,density,andmaturityofthe
2000,2004;HicksandShankar,2003),furtherstudieswereundertaken treesatthetimeofamajorstorm.Duringsuchanevent,themagnitude
bynationalandinternationalresearcherstocomparemodern-daypro- ofinterceptionlossacrossdifferentclosed-canopyvegetationcommu-
cessratesinthecontextofthelonger-term(last~15,000years)geo- nitiesasapercentageofrainfallissmall(Roweetal.,1999).Several
morphologicalhistoryofthisarea(Berrymanetal.,2010;Marsagliaet otherstudiesconfirmedthatarelationshipexistsbetweenlandslide
al.,2010;Mardenetal.,2011,2014a).Understandingmechanismsof densityandvegetationtype,whereshallowsoilsexistonsteepslopes
sedimentgeneration,storage,andfluxtotheoceanwasakeygoalof subjected to intense rainstorms (Selby, 1976; Salter et al., 1983;
the MARGINS Source-to-Sink Programme, which focused on the Harmsworthetal.,1987;HancoxandWright,2005).
Waipaoacatchment(Marutanietal.,2001).
Thegeneralunderstandingofhowreforestationhascontributedto
effectiveerosioncontrolintheEastCoastregionisnowrelativelywell
advanced.Whatfollowsisabriefsummaryofthatunderstanding:
3.2.1.Shallowlandslides
Vegetation,andtreesinparticular,improveslopestabilityandre-
duce erosion (e.g., Greenway, 1987; Marden and Rowan, 1993;
PhillipsandMarden,2005).Ingeneralterms,thebelow-groundcompo-
nents(roots)providemechanicalreinforcement(Watsonetal.,1999)
andarethemeansbywhichtreesextractsoilmoisturefromthesoil
toreduceporewaterpressures(Ekanayakeetal.,1997),whilethe
above-groundcomponentsofvegetation(canopy)reducetheability
ofrainfalltocauseshallowlandslidingthroughtheprocessesofinter-
ception(Kelliheretal.,1992)andtranspiration(Pearceetal.,1987).An-
nualevaporationfromaclosed-canopyforestinthisregionaccountsfor
85%ofrainfallwithinterceptionat35%(Pearceetal.,1987)andtree-
transpirationat50%(WhiteheadandKelliher,1991),significantlyre-
ducingtheriskoflandslideinitiation.Combined,theseprocessesbe-
come most effective when full root occupancy (lateral roots of
adjacent treesoverlap) andcanopy closure(canopies of individual
treestouch)firstoccurs,andtheyimprovewithincreasingtreeage
and/orplantingdensity(Kelliheretal.,1992).Thiswasaptlydemon-
stratedbyMardenandRowan(1993)whonotedthatwherehillslopes
plantedinpinesN8yearsoldandhadattainedcanopyclosurebefore
CycloneBola,wereconsiderablylesspronetorainfall-inducedland-
slides than were either pastured hillsides or young stands of pine
wherecanopyclosurehadyettooccur(Figs.2and3).Onfurtherinves-
tigation,theyfoundthatyoungforeststandsb6yearsoldandpastured
hillslopesatthetimeofCycloneBolasustainedsimilarlevelsofland-
slidedamageandupto16-timesgreaterthanforeitherexoticplantings
orindigenousforestwherethecanopywasfullyclosed.Thisrelation-
shipwasparticularlystrongforhillcountryunderlainbyTertiary-age
sedimentarybedrockwherelandslidesaretypicallyshallowandtrans-
Fig.3.Storm-initiatedlandslidesonpasturecontrastingwithclosedcanopyforests,
lational(Mardenetal.,1991).Asimilarresultwasfoundinanareaof northernHawke'sBay.Photo:PFOlsens.
Pleasecitethisarticleas:Phillips,C.,etal.,GeomorphologyandforestmanagementinNewZealand'serodiblesteeplands:Anoverview,Geomor-
phology(2017),http://dx.doi.org/10.1016/j.geomorph.2017.07.031
C.Phillipsetal./Geomorphologyxxx(2017)xxx–xxx 5
3.2.2.Earthflowmovementstudies Landslideswererelativelysmallscale,generating~≤5%oftheannual
Researchonearthflowsshowedthatforestedearthflowshadsurface sedimentbudget(Tayloretal.,2012,thisissue).Phasesofgullyexpan-
movementratesthatwere2–3ordersofmagnitudelessthanthoseon sionareinvariablyfollowedbyperiodsofinactivity,andfurthermore,
grassedearthflows,onsimilarterrain(Zhangetal.,1993,Fig.4).Subsur- theresponsetimerequiredtostabiliseagullyfollowingreforestation
facedeformationofforestedearthflowsaccountedforb25%oftotalsur- ishighlyvariable.Thesignificanceofgulliesinsedimentproductionin
facemovementandresultslargelyfromcompressionflowcomparedto these environments arises because sediment supplied from gullies
extension flow on grassed earthflows (Zhang et al., 1991a, 1991b, tendstobepersistentandactivatedbysmall,frequentrainfallevents
1993).Araftingmechanisminwhichblocksofrootsfromindividual (Trustrumetal.,1999),andgullyerosionasawholeisthedominant
treesinteractwiththoseofneighbouringtreestoretardsurfacemove- processofsedimentdeliverytotheserivers(Trustrumetal.,1999;
ment(i.e.,creatinganetworkofraftblocksconnectedbylateralroots) Gomezetal.,2003;Mardenetal.,2005;Pageetal.,2007).Fortherefor-
wassuggestedtoexplainobserveddeformationprofilesofforested estationperiodspanning1957to1997gullysourcesgeneratedthe
earthflows.Once‘stabilised’,forestedearthflowscouldbereactivated equivalentof43%,49%,and54%oftheaverageannualsuspendedsedi-
byanynumberoffactors(Mardenetal.,2008b)—thoughclimate, mentyieldoftheWaipaoa,Waiapu,andUawacatchmentsrespectively
treeremoval,andotherforestmanagementfactorssuchasroadcon- (Mardenetal.,2008c).
structionweresuggestedasbeingthemostimportant.Inonestudy, Theeffectivenessofreforestationinstabilisinggulliesisbasedon
norelationshipwasfoundbetweentheinitiationofearthflowactivity modelsdevelopedtoestablishthetime(yearsafterplanting)required
andtheharvestingofforestssurroundingtheearthflow(Mardenet before a wide range (0.07–60.5 ha) of gully sizes ‘closed-down’
al.,2008b),anditwasconsideredunlikelythepracticeofharvestingin (Mardenetal.,2005,2008c,2011).Theconceptofgullystabilisationis
itselfwouldinitiateneworrenewedactivityonstableflows.However, basedonthemeasuredchangein‘active’gullyareabetweenmeasure-
duringthepost-harvestperiodanduntilevapotranspirationrecovers mentperiodsasafunctionofincreasingtreecanopysize(resultingin
sufficientlytoinfluencesoilmoisturelevels(about2years),elevated areductioningullysize),orconversely,gullyreactivation(resultingin
soilmoisturewilllikelyresultinincreasedmovementratesofalready anincreaseingullysize)overthelengthofarotationofexoticpines.
mobilisedearthflowsandwillprolongtheiractivityintoperiodstradi- Thesemodelsshowedthatthetimerequiredto‘stabilise’gullieswas
tionallyassociatedwithsoilmoisturedeficit. stronglyassociatedwithgullysizeandtheduration(years)sinceplant-
ing,andforgulliesofequivalentsize,thedurationsinceplantingissim-
3.2.3.Gullies ilarinbothgeologicalterrains.Also,lineargulliesarelikelytostabilise
BettsandDeRose(1999)definedgulliesasareasofactivelyeroding earlierthantheiramphitheatre-shapedcounterparts.Usingtheresults
baregroundthatarecontiguouswiththechannelsthatdrainthem. oftheseformerstudies,Herzigetal.(2011)developedamodeltoretro-
Gullies form through the complex interaction of fluvial and mass spectivelyassesstheeffectivenessofpreviousgovernment-fundedre-
movementprocesses(Hicksetal.,2000;Poesenetal.,2003).Thus,an forestationschemes(1957–1997)inreducinggully-derivedsediment
increaseingullydepth(orgullyarea)isrelatedtogreaterrunoffrates massandappliedittoevaluatetheeffectivenessofdifferentreforesta-
following deforestation, and the converse is true when land tionscenariosinreducingthesedimentmassgeneratedfromremaining
surrounding a gully is reforested (Parkner et al., 2007). Based on untreatedgullieslocatedwithingully-pronehillcountryprioritisedfor
shape,twotypeswereidentified:linear(lengthexceedswidth)and futurereforestation.Theoverallsuccessofreforestationinameliorating
amphitheatre-shaped (width exceeds length). Linear gullies often gullyerosion(Fig.5)canalsobeattributedtotheselectionoffast-grow-
beginasaseriesofcoalescinglandslidesinareasofconvergentdrainage ingtreespecies(whichareharvestableca.24–27yearsafterplanting),
whereverticalincisionislargelyconfinedwithinexistingdrainagelines. idealgrowingconditions,andtheplantingstrategyadopted.Thatis,
Lineargulliesaremainly,butnotexclusively,locatedwithintheTertiary gullystabilisationisachievedfirstbyplantingasmuchofthegullywa-
terrainwherestructuralandbedrockcontrolsdeterminetherateand tershedareaasphysicallypossibleandsecondbydelayingwithin-gully
modeofgullyexpansion,whichispredominantlybyfluvialprocesses. plantingsuntilanoticeablereductioninrunoffandsedimentsupplyto
Giventherightcombinationofpredisposingsiteconditions,juxtaposed thechannelisobserved.Whileregionwidereforestationeffortshavere-
lineargulliesoftenenlargeintooneamphitheatre-shapedgully.Once sultedina~42%netreductionincompositegullyarea,theinitiationof
initiated,thesegulliesrapidlybecomeentrenchedintotheweakbed- newgulliesandexpansionofnumerousgulliesthatremaineduntreated
rock resulting in repeated rotational collapse of gully sideslopes throughout the measurement period (1957–1997), collectively
destroyingallformsofvegetationcover(Bettsetal.,2003)andwhere accountedforanoverall~27%netincreaseingullyarea.Ifallremaining
rillingandgullyingtendtobethemostpersistentandwidespreadof gulliesinthesecatchmentsweretobereforestedby2020,thevolumeof
theerosionprocessesgenerating~≥80%oftheannualsedimentbudget. gully-derivedsedimentwouldhalveby2030,butifleftuntreatedthe
Debris flows generated ~≤15% of the annual sediment budget. yieldwoulddouble(Herzigetal.,2011).
Fig.4.WetherRunearthflow,MangatuForestbeforeandafterreforestation(afterMarden,2004).
Pleasecitethisarticleas:Phillips,C.,etal.,GeomorphologyandforestmanagementinNewZealand'serodiblesteeplands:Anoverview,Geomor-
phology(2017),http://dx.doi.org/10.1016/j.geomorph.2017.07.031
6 C.Phillipsetal./Geomorphologyxxx(2017)xxx–xxx
Fig.5.Pre-(1961)andpost-reforestation(1972,2004)photographyofamedium-sizedgullyinTeWeraroaStream,MangatuForest.Pinusnigrawasplanted(1962)onthelesser-eroded
interfluvessurroundingthisgully;thenin1966Pinusradiatawasplantedonthesteeperandmoreseverelyerodingslopesimmediatelyflankingthegullyandwithinthegullyitself.By
1972theeffectivenessoftheplantingsinstabilisingthegullywereapparentanditschannelincisedbelowthelevelofthefanatthemouthofthegully.Furtherwithin-gullyplantings
(1974)ofP.radiatawereundertakenontheremainderofbareslopesandofthefan.Beforeplanting,thisgullywas7.6hainsize,andby1988reforestationhadreducedtheareaof
activeerosionto0.8ha.Thelatestphotographshowsthatinspiteofamajorcycloniceventin1988(CycloneBola),thisandsimilarlyreforestedgulliesofthissizehaveremained
stable.Theelevationdifferencebetweenstreamlevelandtheridgeattheheadofthisgullyis280m(1961and1972photographscourtesyofJ.Johnsandreproducedbypermission
ofNewZealandForestResearchInstituteLimited;2004photographwastakenbyR.HamblingandreproducedbypermissionofR.Hambling,MinistryofAgricultureandForestry)
(afterMardenetal.,2005).
3.2.4.Splashandsurfaceerosion someindigenoustreespecies.Investigationsexaminedrootarchi-
Surfaceerosionunderforestshasreceivedmuchlessattentionthan tecture(Fig.6;WatsonandO'Loughlin,1990;PhillipsandWatson,
massmovementerosion.Otherthansurfaceerosionrelatingtoforest 1994), root tensilestrength (Watson andMarden, 2004), loss of
managementpracticessuchasfromroadsorfromharvestingopera- root tensile strength following tree removal (O'Loughlin, 1974;
tions(discussedlater)theonlystudyofsurfaceerosionunderforests O'LoughlinandWatson,1981;Watsonetal.,1999),sheartestingof
was by Mosley (1982a,1982b)who assessed the kineticenergy of rootedsoil(Ekanayakeetal.,1997),rootsiteoccupancy(Phillipset
waterdropsandtheimplicationforsurfaceerosion.Itwasconcluded al.,2011),rootgrowth(Phillipsetal.,2014;Mardenetal.,2016),
thatrates of soil detachment by splash under forest would exceed androotbiomass(WatsonandO'Loughlin,1985).Morerecentstud-
thoseintheopen,especiallywherethelitterandhumuswereremoved iesutiliseddatafromsomeoftheearlierrootstudiestoconstructand
ordisturbedandthemineralsoilexposedatthesurface. validatemodelsofrootreinforcement(EkanayakeandPhillips,1999,
2002;Schwarzetal.,2016)ortodeterminetheroleoftreerootsin
contributing to slope stability (O'Loughlin and Ziemer, 1982;
3.2.5.Treerootstudies O'LoughlinandZhang,1986;Phillipsetal.,2015).Soilswithtree
Aspartoftheneedtounderstandhowforestsmodifiederosional rootswillundergolargersheardisplacementsbeforefailingthan
andhydrologicalprocesses,exploratorystudieswereinitiatedinthe soil without roots (Ekanayake et al., 1997) confirming why tree
1980stodeterminehowtreerootsreinforcedsoilsandwhatrole rootsareconsideredtobeamajorcontributortosoilstrengthand
rootarchitectureplayedinslopestability.Initialeffortsfocusedon slope stability in the zone where roots are present (O'Loughlin,
the predominant exotic species Pinus radiata but also included 1985;PhillipsandWatson,1994).
Attentionwasalsofocusedonwhathappenedoncetreeswere
harvested. Studies concluded that plantation forests located on
steeplands were more prone to shallow landsliding for several
yearsfollowingharvestingthanatanyothertimeinthegrowing
cycle(e.g.Phillipsetal.,2012).Treeremovalallowedsoilmoisture
conditionstobecomewetterforlonger(Pearceetal.,1987;because
ofthelossofinterceptioncapacityofthecanopyandreductionsin
evapotranspiration).Thiscoupledwithrootdecay—inthecaseof
softwoods like Pinus radiata this is fairly rapid (O'Loughlin and
Watson,1979;Watsonetal.,1995)—increasedthepotentialfor
shallowslopefailuretooccur.Thispost-harvestperiodbeforethe
replantedcropbecomesfullyfunctionalandwhererootreinforce-
mentislow,hasbeenreferredtoasthe‘windowofvulnerability’
(O'Loughlin,1985;Sidle,1991;Fig.7).However,therelationshipbe-
tweenforestremovalanddeep-seatedlandslidingislessclearthan
withshallowlandslides.Onceshallowlandslidesareinitiated,they
have the potential to transform into debris flows, which in turn
erode ephemeral gullies and riparian margins, delivering large
amountsofsedimentandwoodydebrisfartherdownthecatchment.
Suchoccurrencesareoftenwidespreadduringlargeregionalstorms
(e.g.,CycloneBola)orasa result oflocalisedintenserainstorms
(MardenandRowan,2015).Theseverityoflandslidesanddebris
flowsduringthewindowofvulnerabilitybringsintoquestionthe
roleofforestryasasustainablelanduseandraisesquestionsabout
whatpracticescanbeusedtomanagetheseeffects(Collins,1988;
Fig.6.Excavated25-year-oldPinusradiatatreefromMangatuForest,EastCoast(after
Phillipsetal.,2015). Phillipsetal.,1989,2012).
Pleasecitethisarticleas:Phillips,C.,etal.,GeomorphologyandforestmanagementinNewZealand'serodiblesteeplands:Anoverview,Geomor-
phology(2017),http://dx.doi.org/10.1016/j.geomorph.2017.07.031
C.Phillipsetal./Geomorphologyxxx(2017)xxx–xxx 7
reductioninthelatterperiodofreforestationwhensmall-tomedium-
sizedgulliesbecamelessactive.However,largeamphitheatre-shaped
gulliesfailedtostabiliseandcontinuedtodominatethesedimentsup-
ply, indicating a strong coupling with channels. Contribution of
earthflowstothestudybasinsedimentloadwasseveralordersofmag-
nitudelessthanforgulliesandwasfurtherdecreasedbyanorderof
magnitudefollowingcanopyclosure.
Althoughreforestationprovedeffectiveinstabilisingshallowland-
slides,smaller(1–5ha)gullies,andearthflows,theresultantreduction
intheircontributiontobasinsedimentyieldloadandtooverallcatch-
mentyieldwasminimal(Mardenetal.,2014b).Thegreatestreduction
insedimentloadoccurredduringthelaterphasesofreforestationasthe
numeroussmall-tomedium-sizedgulliesstabilised,coincidentwith
canopyclosureabout 8–10yearsafterplanting.However,thevery
large gully-mass movementcomplexes failed to stabilisefollowing
planting, and they remained the dominant source of sediment
(Mardenetal.,2014b).Theconclusiondrawnfromthesestudieswas
thatspecifictargetingofgully-massmovementcomplexesinhighsed-
iment-yieldingcatchmentscouldleadtosignificantreductionsinsedi-
mentyieldwithinthreedecadesofforestestablishment.
ReforestationoftheupperWaipaoaRiverbasinindicateda62%re-
Fig. 7. An example of typical changes in forest vegetation root strength or root ductionintheareaeroding,a51%reductioninerosionrate,andanes-
reinforcementaftertimberharvesting(afterPhillipsetal.,2015).Initialcurvesfrom timated12%reductioninsedimentyieldoftheWaipaoaRiverfrom6%
O'Loughlin(1985)andSidle(1992,2005)andmodifiedbyWatsonandothers.Net ofitscatchmentarea(Mardenetal.,2014b).
strengthorreinforcementisthesumofthedecayandrecoverycurves.Thewindowof
vulnerabilityforNewZealandplantationsisestimatedtobeintheperiod1–6years
afterharvesting,butthisisspeciesanddensitydependent. 3.3.Foresthydrologyandwaterquality
Asmentionedabove,contemporaneouswithresearchonerosion
3.2.6.Reforestationeffectsoncatchmentsedimentyield
processesandtreeroots,studieswerealsoundertakentodetermine
Oneoftheanticipatedresponsesofreforestationisareductioninthe
the role of forests in the hydrological cycle. Many of these studies
sedimentsupplyfromslopestoriverchannelsandincatchmentsedi-
wereinitiallyinstigatedinresponsetoaplantoharvestlargetractsof
mentyield.ThemostcomprehensiveNewZealandstudythatassessed
beechforestintheSouthIsland(mentionedearlier),aschemethat
thecontributionsfromdifferenterosionprocesseswasthatofMarden
didnotproceed.Smallpairedcatchmentstudieswereestablishedat
etal.(2014b)thatfocusedonanalysingtheeffectofreforestationofa
MaimaiinnorthWestlandandatBigBushnearNelsontodetermine
small area of extensively eroding land in theupperWaipaoa River
theeffectofdifferentharvestingoptionsfornativebeechforeston
catchmentonsedimentyieldintheentirecatchment.Inthatstudy,an
erosionrate(ty−1)andadenudationrate(mmy−1)wascalculated waterquantityandwaterquality.Thisresearchlaidthefoundationfor
thecurrentunderstandingofhowindigenousforests(andforestsin
foreachoffourerosionprocesses(gullies,earthflows,slumps,shallow
general)inNewZealandaffectbothofthesehydrologicalattributes
landslides)andcomparedtothosederivedfromthewiderWaipaoa
(Faheyetal.,2004;QuinnandPhillips,2016).Studiesincludedstorm
Rivercatchmentandlongertermratesforthepost-glacialperiod.Sedi-
runoffandwaterquantity(PearceandMcKerchar,1978;Fahey,1994;
mentloadbyprocessandtimeperiodwasthenexpressedasapercent-
FaheyandJackson,1997),waterquality(FaheyandJackson,1997;
ageofthesedimentyieldfortheWaipaoaRivercatchment(Fig.8).
Parkyn et al., 2006; Baillie and Neary, 2015), management effects
Withcompletionofreforestation,asignificant(about43%)decrease
(Nearyetal.,1978),hydrologicalpathwaystudies(McDonnelletal.,
insedimentsupplyfromgullieswasobservedalongsideagreaterrateof
1998),rainsplash(Mosley,1982b),andinterceptionandevapotranspi-
ration(Pearceetal.,1980;Rowe,1983;Faheyetal.,2001).Someofthe
conclusionsfromtheseearlierstudiessuggestedthattheinfluenceof
forestcoveronhydrologyisasecondaryone,whichisoverriddenbyre-
gionaldifferencesinrainfallanditsvariability,topography,soilparam-
eters,andunderlyinggeology(PearceandO'Loughlin,1978).
3.4.Forestmanagementeffects
AlongsideearlyEastCoastresearch,studieswerealsotakingplacein
otherregionsofNewZealandthatfocusedondifferentaspectsofforest
management. These included assessments of sediment yield from
paired catchment studies with and without different management
treatments(O'Loughlinetal.,1980);sedimentproductionfromforest
roadsincludingsurfaceerosion(FaheyandCoker,1989,1992;Coker
et al., 1993; Fransen et al., 2001) and mass movement erosion
(Mosley,1980);storminfluencesfollowingharvestingonlandingfail-
ures(Cokeretal.,1990)andmassmovements(Phillipsetal.,1996;
Mardenetal.,2015);assessmentoftheeffectsofloggingonstream
ecology(Graynoth,1979;Roweetal.,2002);surfaceerosion(Fransen,
Fig.8.Trendsinrelativespecificerosionratesfordifferentprocessesbeforeandafter 1998;Mardenetal.,2006,2007);andsedimentproductionacrossa
reforestation(afterMardenetal.,2014b). number of different rock types, land uses, and regions (Visser and
Pleasecitethisarticleas:Phillips,C.,etal.,GeomorphologyandforestmanagementinNewZealand'serodiblesteeplands:Anoverview,Geomor-
phology(2017),http://dx.doi.org/10.1016/j.geomorph.2017.07.031
8 C.Phillipsetal./Geomorphologyxxx(2017)xxx–xxx
McConchie,1993;FransenandBrownlie,1995;MardenandRowan, • post-harveststorm-initiatedlandslidesanddebrisflowsposea
1997). threattothesociallicensetooperatefortheforestindustrylocat-
Monitoringofsedimentyieldfollowingforestoperationsinbeech edonsteep,erosion-pronelands(seenextsections);and
forestshowedthattrackconstructionrequiredforskidderextraction • forestsareimportantforerosioncontrolinNewZealandandcan
resultedinsubstantialincreasesinsuspendedsedimentoutputeven deliverarangeofotherecosystemservices.
withprovisionofa20-mstreamsideprotectionzonefromwhichno
woodwasremoved.Observationsalsoindicatedthatwithinmany
forestedareasatthattime,massmovementswerethepredominant 4.Currentsituation
erosionprocessunderundisturbedconditions,withsmalldebris
slide/debrisavalanchesbeingtriggeredbystormswithreturnpe- 4.1.Overview
riods of several years to several tens of years (O'Loughlin et al.,
1978).Similarly,inclearfelledcatchments,numeroussmallslides Since 1987, when New Zealand's exotic plantation forest estate
weretriggeredbyeventsthatcausednomass-movementactivity changedfromlargelybeingunderonecentralagency,theapproachto
inuncutcatchments. forestmanagementbegantovarywithincreasingnumbersofnewcom-
Results of these earlier studies confirmed other international paniesemerginginthesector.Manyinitiallyemployedex-NZFSstaff,
findings that elimination of forest cover frequently produces in- andsomeinstitutionalknowledgeremained.Overtime,ownership
creasesinerosionratesthatrangefromonetothreeordersofmag- changes,differingdemandsofshareholders,anddifferentpersonnel
nitude, and that smaller changes in erosion rate might well be sawtheearlier‘consistency’inforestmanagementapproachesacross
regardedasnaturalfluctuationsaboutanaveragerate(Pearceand NewZealandchange,andwiththose,theuseandplaceofgeomorpho-
O'Loughlin,1978).Whilenotclarifyingtheroleofsedimentstorage, logical information in all aspects of forestry. In 2004, 66% of New
suchstudiessuggestedthattherelativesizeofthestoragecompo- Zealand's exotic forestry estate was owned by private companies,
nentsofthesedimentsupplysystemlikelyvariesfromregiontore- manyofwhichweresmall,largelygeneratedbyasmall-growerplant-
gion but is a necessary feature that smooths out the larger ingboombetween1990and2005.In2016,18companieshad42%of
fluctuationsofsedimentinputsothatoutputchangesaresmoothed thetotalestate(NewZealandForestOwnersAssociation,2016)andof
andlaggedintimefrominputchanges. the1.7Mha,155,079ha(9%)wasintheEastCoastregion.
Insummary,theseearlystudieslaidthefoundationforthenext InplacesliketheEastCoastofNorthIsland,manyforestsplantedas
30–40yearsofresearchandconcludedthatinacountrysuchas partofearlyreforestationeffortshavenowbeenthroughonerotation.
NewZealandwithlargevariationingeology,topography,soiltype EarliestplantingsatMangatuForestarenowapproachingtheharvest
andthickness,andclimate,theeffectsofforestcoveronthehydro- phaseofthesecondrotation.Apartfromthedeep-seatedamphitheatre
logicregimeanderosionratesandprocesseswillgenerallybesec- gulliesthathavenotbeenfullycontrolled,sedimentcontributionsfrom
ondarytothosefourfundamentalfactors.Theseconclusionswere mostotherslopeprocesseshavereducedsignificantly.However,large
notoutofstepwithinternationalthinkingatthetime,andthecon- quantitiesofsedimentarestoredinchannelsthatwillcontinuetosup-
tributionsNewZealandresearchmadetothewidebodyofunder- plysedimenttoriversliketheWaipaoa,Uawa,andWaiapufordecades
standing cannot be underestimated (Swanson and Fredricksen, orcenturiestocome(Mardenetal.,2014a).Awayfromthoseearlyre-
1982;Sidleetal.,1985). forestationeffortsintheheadwaters,onthesteeper,shorter-sloped,
Inrecentyears(post-2000),attentionhascentredondetermining Tertiaryterraintotheeast,andinotherpartsofNewZealand(including
sedimentyieldduringorfollowingforestharvesting(Basheretal., BayofPlenty,Northland,Nelson,Marlborough),newissuesareemerg-
2011; Fahey and Marden, 2006; Phillips et al., 2005; Quinn and ingespeciallyasaresultofharvesting.
Phillips,2016)andoncomparingsedimentyieldfrompastureandhar- Oneofthoseispost-harvestshallowlandslidesanddebrisflowstrig-
vestedcatchments(Faheyetal.,2003;FaheyandMarden,2006);deter- geredduringrainstorms(inthewindowofvulnerability)thatcandeliv-
miningslopewash(raindropimpact,slopewash,andrilling)duringthe erlargequantitiesofloggingslash(frombranchestowholetrees)and
post-harvestperiod(MardenandRowan,1997;Mardenetal.,2006, sedimenttoriverchannelsanddownstream-receivingenvironments
2007);post-harvestlandscaperesponse,particularlyinrelationtoland- suchasbeachesorriverfloodplains.Thegeomorphologicalprocesses
slidesanddebrisflows(Phillipsetal.,2012;Basheretal.,2015c);policy atplayarenotnewandarethesameasthoseinnaturalforeststhat
relatingtoharvesting,reforestation,andtheroleofforestryforerosion alsocontinuetoshapethelandscapesofNewZealand(Mardenetal.,
controlandcatchmentprotection(Marden,2012;Phillipsetal.,2013);
andtheprovisionofecosystemservices(Smailletal.,2014).Thesestud-
iessuggest
• storm-initiatedlandslidesarethemostimportanthillslopeprocesses
generatinganddeliveringsedimenttostreamsandthatmostgenerat-
edsedimentisretainedontheslope;
• the degree of soil disturbance from harvesting operations and
theamountofsurfaceerosioninthefirstyeararestronglycor-
related;
• theamountofsedimentproducedon-sitebysurfaceerosionand
massmovementcanbemuchgreaterthanthatleavingthecatch-
mentassedimentyield;
• forestsedimentyieldsatharvesttimecanincrease5timesover
pre-harvestrates,decliningtopre-harvestlevelswithinabout
2–3yearsafterreplanting;
• inexoticforests,post-harvestsedimentyieldsvaryfrom10sto
100softkm−2y−1acrossarangeofgeologies;
• atsmall-catchmentandstorm-eventscales,forestedcatchments
yieldsignificantlylesssedimentthanpasturecatchments(50–
80%less); Fig.9.Photooflogjamfollowingastormin2010.
Pleasecitethisarticleas:Phillips,C.,etal.,GeomorphologyandforestmanagementinNewZealand'serodiblesteeplands:Anoverview,Geomor-
phology(2017),http://dx.doi.org/10.1016/j.geomorph.2017.07.031
C.Phillipsetal./Geomorphologyxxx(2017)xxx–xxx 9
2015).However,these‘new’storm-inducedmassmovementsareoc- WhiletheNES-PFhasnotyetbeengazetted,severalforestrycompa-
curringonamanagedlanduse(plantationforestry),anderosionisper- niesinstigatedtheirownapproachestodeterminingerosionsuscepti-
ceivedasnot‘natural’byregulatoryagenciesorbythepublic. bility/environmentalimpactassessmentaspartoftheirnormalforest
Manyoftheseeventsimpactonproductivelowland,affectinfra- operations. These include terrain stability mapping and empirical
structuresuchasroadsandbridges,andcanupsetlocalcommunities modellingapproaches(forexamplesseeAmishevetal.,2014).Geomor-
whenlargeamountsofwoodydebrisenduponlocalbeachesfollowing phologyandearthscienceinformationisthusbeingusedintermsof
rainevents(Figs.9and10;Basheretal.,2015c).Insomecases,thesame broaderestatemanagementandinoperationalplanning.Thediffer-
communitiesareaffectedmultipletimes,particularlythoselocatedim- encesbetweenthevariousapproachesoftenreflectcompanyowner-
mediatelydownstreamofforests.Whileenvironmentalandeconomic ship and philosophy with greater or lesser acknowledgement of
impactsofsucheventsareimmediate,increasingpublicattentionisbe- environmentalriskandhowitshouldbemanaged.However,while
comingfocusedonforestry'ssociallicensetooperate(Edwardsetal., themajorityofforestrycompanieswithlargerestateshaveresponded
2016;andthis,inturn,isbeingreflectedinincreasingscrutinyinre- toaneedforbetterenvironmentalmanagement,asignificantpropor-
sourcemanagementplanningprocesses).Asaconsequence,attention tion(30%)oftheNewZealandplantationestateisinsmallholdings
hasbeenrefocusedonwhatgeomorphologicalunderstandingisneeded b1000ha(MPI,2016).Inthesesituationsalowerlevelofdetailedplan-
tobettermanagetheriskofharvestingforestsonerosion-proneterrain ning,roadandearthworkconstruction,andthusenvironmentalperfor-
(Phillipsetal.,2012;Basheretal.,2015c;Mardenetal.,2015;Paynetal., manceoftenoccurs.TheNES–PFwillattempttoprovidemoreuniform
2015). coverageacrossthecountryaswellasprovidingmorecertaintyforfor-
estryinvestment,whichiscurrentlyatitslowestwiththeplantation
areahavingdeclined5%inthelast10years(MPI,2016)andiscontinu-
4.2.ANationalEnvironmentalStandardforPlantationForestry(NES-PF) ingtodecline.
IncontrasttotheNewZealandsituation,regulatorycontrolsonfor-
TheMinistryforPrimaryIndustries(MPI)inNewZealandiscurrent- estryactivitiesinothercountriesaregenerallymuchmorestringentand
lyleadingaprocesstodelivergreaternationalconsistencyintheman- havebeenineffectformuchlonger.InCanadaandtheUSA,forestprac-
agementofplantationforestryundertheResourceManagementAct ticesarehighlyregulated,withmostattentionfocusedontimberhar-
1991byimplementingaNationalEnvironmentalStandardforPlanta- vest,roadconstruction,andchemicaluse.InNewZealand,theseare
tionForestry(NES-PF).Atpresentforestryismanagedbylocalregional ‘managed’througharangeofgovernmentalactsandindustryguide-
councilswiththeresultthatdifferentrulesapplyindifferentregions lines(e.g.,NewZealandEnvironmentalCodeofPracticeforPlantation
andthatasingleforestmaybesubjecttodifferentrulesifitcrossesre- Forestry,2007).Alsoasageneralrule,countriessuchastheUSA,Cana-
gionalboundaries. da,andpartsofEuroperequireahigherlevelofgeomorphologicalinput
AtthecoreoftheNES-PFisanerosionsusceptibilityclassification intoforestplanningandmanagementthanNewZealand(Fanninetal.,
(ESC)thatdeterminesthesetofrulesforplantationforestryactivities 2007;SchwabandGeertsma,2010).Thisusuallytakestheformofde-
coveringthewholeplantationforestlifecycle.Itcategorisesthesuscep- tailedterrainassessment,follow-upfieldinspection,significantengi-
tibilitytoerosioninfourclassesfromlowtoveryhighrisk.Thehigher neering design input to roads and construction activities and
therisk,thetighterthecontrolofforestryactivities.Thebasisofthe compliancemonitoringofactivities.NewZealandiscatchingupthough,
ESCanditsderivationfromaconceptof‘potentialerosion’andland witheffortssuchastheNES-PF,widespreadadoptionofengineeringde-
usecapability(LUC)datafromtheNewZealandLandResourceInvento- signinput(NewZealandForestOwnersAssociation,2011),anduseof
ry(NZLRI)iscomprehensivelydescribedinBloombergetal.(2011)and erosionandsedimentcontrolplansaspartofharvestplanningallcon-
Basheretal.(2014,2015a,2016).Whileelementsofgeomorphological tributingto‘liftingthebar’intermsofforestmanagementandenviron-
understandingunderpinpotentialerosionandLUC(slope,rocktype, mentalperformance.
erosiontypeandseverity)andhencetheNES-PF,somefundamental
difficultiesremain.Theconceptofpotentialerosion(includingpoordef- 5.Thefuture
inition of the concept and inadequate description of classification
criteria)andthescaleofmappingthatunderpinsit(1:50,000),limit 5.1.Overview
itsusefulnessforeffectiveforestmanagement(Bloombergetal.,2011;
Mardenetal.,2015;Basheretal.,2015b,2015c). ForestrymakesasignificantcontributiontoNewZealand'seconomy
withthesectorbeingthethirdlargestexportearnerwithatotalexport
offorestryproductsin2015valuedat$4.8billionandcontributing1.6%
toGDP(NewZealandForestOwner'sAssociation,2016).Eventhough
forestinvestmenthasdeclinedinrecentyears,thefutureforforestry
isstilllookingpromising(PureAdvantage,2016).Plantedforestscan
anddoplayasignificantroleinenvironmentalimprovement,notonly
intermsofreducederosionbutthroughimprovementsinwaterquality
duringtherotation(O'Loughlin,1995;Parkynetal.,2006;Quinnand
Phillips,2016),carbonsequestration(McLaren,1996),anderosioncon-
trol(Marden,2012).Currentlyradiatapine(PinusradiataD.Don)isthe
dominantspeciesmakingup90%oftheplantedproductionforestarea.
However,infuture,thismaychangeassocietyviewsmonoculturesas
something inherently bad in terms of the environment (Rosoman,
1995)andfinanciallyrisky(Wiltshire,2000),eventhoughmanyexam-
plesofnaturallyoccurringmonoculturesexist–NewZealandindigenous
beechforests,forexample,tendtobenaturallyoccurringmonocultures
(e.g.,McLaren,1996).
Aboutone-thirdoftheNewZealandplantationforestestateislocat-
edonerodiblesteeplands.Manyoftheseforestswereoriginallyplanted
Fig.10.PhotoofdebrisaccumulatingonMidwayBeachinGisborneCityafteralandslide-
toprovideasoilandwaterprotectionfunction.Formostoftherotation,
triggeringrainstorm.PortofGisborneinbackground,withstacksoflogswaitingforexport
torightandleftofwhitebuildings.Photo:BillWheeler. ahighlevelofslopestability,reducederosionandsedimentsupply,and
Pleasecitethisarticleas:Phillips,C.,etal.,GeomorphologyandforestmanagementinNewZealand'serodiblesteeplands:Anoverview,Geomor-
phology(2017),http://dx.doi.org/10.1016/j.geomorph.2017.07.031
10 C.Phillipsetal./Geomorphologyxxx(2017)xxx–xxx
otherbenefitsareconferredbytheseforests(e.g.,Marden,2012).How- debrisflows,tomaintainlocallicensetooperateandmeetinternational
ever,whenforestsareharvested,landslideriskinparticularincreases certificationrequirements(e.g.,ForestStewardshipCouncil,2017),and
(andthepotentialforrainstormstotriggershallowlandslidesandde- tomanagethelikelyimpactsofclimatechangethatispredictedtoin-
brisflowsthatentrainloggingresidues(slash)andcauseimpactsbe- creasetheincidenceoflandslide-triggeringstormsinmanypartsof
yondtheforestboundaryrises;Phillipsetal.,2012;Fulleretal.,2016). NewZealandwhereplantationforestsarelocated(Basheretal.,2012).
Infuture,wesuggestthatarequirementtoimprovetheassessmentof
thatrisktoaidforestmanagementandtofindpracticesthataimtore-
ducetheincidenceandconsequencesoftheseeventswillbenecessary 5.3.Datacollectionandterrainanalysistosupportimprovedhazardand
(Phillipsetal.,2012).Assuch,weseegeomorphologyandearthscience riskassessmentatdetailedoperationalscales
information'sroleinNewZealandforestrylikelytogrowassocietyde-
mandsbetterenvironmentalinformationandperformancefromnatu- Inadditiontosupportingthedevelopmentofanerosionsusceptibil-
ralresourcesectorindustries. ityclassificationabove,detailedterrainanalysisisalsoneededtosup-
Weseethreeareaswhereinformationandunderstandingarestill port improved hazard and risk assessment at detailed operational
requiredtosupporttheforestindustry:developmentofanimproved scales(O'Loughlin,2005a;Mardenetal.,2015).Inthelast50years,ex-
nationalerosionsusceptibilityclassificationspecificallytosupportthe periencesfromCanadaandtheUSAhaveshownamixofregulationto
NES-PF;terrainanalysistosupportimprovedhazardandriskassess- voluntary‘results-basedmanagement’(Wiseetal.,2004)intermsof
mentatdetailedoperationalscales;andunderstandingofpost-harvest howlandslideriskmanagementinlandslide-proneterrainrelatingto
shallow landslide-debris flows including their prediction and forestpracticesiscarriedout.However,coretothishasbeenlandslide
management. hazardidentification,terrainstabilityhazardmapping,andterrainsta-
bilityfieldassessmentstoevaluatepotentialorexistingeffectsofforest
5.2.DevelopmentofanimprovednationalESC development on terrain stability. The question for New Zealand is
whetherterrainassessmentshouldbemandatoryorvoluntary.Howev-
TheESCwasfoundedontheconceptofpotentialerosion.Thiscon- er,eitherway,thegeneralmethodologicalapproachesarelikelytobe
ceptwasdevelopedinNewZealandduringanationwidesurveyof similar.
landresourcesinthe1970sand1980s.Itwasdefinedas‘thepotential Abasicruleintheapplicationofterrainstabilitymappingprinciples
erosionunderanactualorassumedgrasslandcoverwithnosoilconser- isthatanyforestmanagementactivity—notjustharvesting—thatin-
vationmeasuresapplied’(NWASCO,1979).Clearlybecausethedefini- creasesthepotentialforgroundwaterorsurfacewatertoenterland-
tion refers to grassland, its application to forestry is problematic. slide-prone terrain or unstable slopes and on material created by
Whilenoformaldefinitionwaseverpublished,itincorporatedconsid- constructionactivities,increasestheriskofslopefailureorreactivation.
erationsofobservedpresentandpasterosion,underlyingsusceptibility Identificationofpartsofthelandscapethatarelandslide-proneorcould
toerosion,magnitudeandfrequencyoferosion-causingevents,anddif- becomeunstablefollowingsitedisturbanceisthusakeyrequirement.
ficultyofrepairoflandfollowingerosion(Basheretal.,2014).TheESCis Anotherconsiderationofthisapproachwasthattheinformationshould
recognisedashavinglimitationsrelatedto:theunderlyingdataitwas bepresentedinaformthatcouldbeeasilyinterpretedforforestanden-
derivedfrom;thescaleofmapping;andprobablemisclassificationof vironmentalmanagementplanning.Mappingcanbeattwolevels:re-
someland.Becauseofthis,anapproachhasbeendevelopedtomanage connaissanceanddetailed.Theformerisappropriatewhereonlylocal
classification changes and incorporate detailed site-level mapping occurrencesofpotentiallyunstableterrainexistwithinextensivestable
(Basheretal.,2015b). areas.Thelatterisrecommendedforareasthathavealargeproportion
Potentialerosionisnotatermthatisincommonscientificusagenor ofsteeperosion-susceptibleterrainandwheresignificantresources
doesithaveacleardefinition,andthereforeitislargelybasedonsubjec- mightbeaffectedbyslopefailure(Mardenetal.,2015).
tive judgementandopen tointerpretation errors.Bloomberg etal. Someforestrycompanieshavestartedtodevelopoperationallevel
(2011)andBasheretal.(2015c)havearguedthatNewZealandshould hazardidentificationandriskmanagementapproachestotryandbetter
developafit-for-purposeriskassessmentforforestmanagementthat managetheriskoflandsliding,woodyresiduemobilisation,anddebris
independentlyassessesthreethings: flows.Oftentheseapproachesarepartofenvironmentalmanagement
systems(e.g.,RayonierMatarikiForests,2015).Furtherworkisrequired
• susceptibility(theunderlyingsusceptibilitytodifferenttypesofero- todevelopimprovedquantitativehazardidentificationandriskman-
sionlargelydeterminedbytopography,rocktype,andsoils);
agementmethodsthatcanbewidelyappliedandareacceptabletofor-
• hazard(thelikelihoodorfrequencyoflandslidesanddebrisflowsthat
est companies and to environmental regulators (central and local
islargelydeterminedbyclimaticfactors);and
governmentagencies).Oneofthemostpressingneedsisforbetter
• risk(theconsequencesofthehazardbeingrealised,incorporatingon- data on landslides and debris flows and on the conditions under
siteandoff-siteimpacts).
whichtheyaretriggeredbecausethemanagementofothererosionpro-
cesses(earthflowsandgullying)isbetterunderstood.
Underpinningthedevelopmentofimprovedriskmanagementis
Basheretal.(2015c)suggestedthatwhatisneededisascreening aneedforbetterdatacollectionontheincidenceandextentofero-
tool(basedongeomorphicandclimaticanalysistodeterminewhere sioninforests,particularlypost-harvest.Whilesomelocalexamples
detailedsite-levelriskassessmentisrequired)anddevelopmentofan ofdatacollectionontheoccurrenceofpost-harveststorm-induced
accepted methodology for site-level risk assessment. They suggest landslidesanddebrisflowsandontherainfallandforestmanage-
threepossibleapproaches:quantitativelandslide/debrisflowsuscepti- mentconditions(i.e.,roadingorloglandingrelated)thattriggered
bilityzoning,mechanisticslopestabilitymodelling,orterrainstability themexist,nationallevelinformationisstillapriority(Phillipset
analysisbysuitablyqualifiedpersonnel.Suchanapproach,incorporat- al.,2012).Thisshouldincludewhatwasaffectedbysuchevents
ingareconnaissancescreeningtoolanddetailedsite-levelassessment (e.g.,roads,bridges,communities)andwhatthecostsweretoreme-
andbasedonterrainstabilityanalysis,wouldbesimilartotheapproach diate any damage caused. A consistent national monitoring pro-
requiredinNorthAmerica(Fanninetal.,2007;SchwabandGeertsma, gramme with clearly set methodology and criteria and a simple
2010)andwouldbeunderpinnedbydetailedgeomorphicunderstand- processtorecordthisdataandreportatregionalandnationallevels
ingoftheoccurrenceandsignificanceoflandslidesanddebrisflows.De- would meet this need. It potentially could be incorporated into
velopmentofmethodologytoachievethisisurgenttoallayincreasing GeoNet,theNewZealandnationalgeologicalhazardinformationda-
communityconcernsabouttheimpactofpost-harvestlandslidesand tabase(Dellow,2001).
Pleasecitethisarticleas:Phillips,C.,etal.,GeomorphologyandforestmanagementinNewZealand'serodiblesteeplands:Anoverview,Geomor-
phology(2017),http://dx.doi.org/10.1016/j.geomorph.2017.07.031