Network analysis of wildfire transmission and implications for risk governance

28 Pages·2017·3.34 MB·English

RESEARCHARTICLE
Network analysis of wildfire transmission and
implications for risk governance
AlanA.Ager1*,CodyR.Evers2,MichelleA.Day3,HaiganoushK.Preisler4,AnaM.
G.Barros5,MaxNielsen-Pincus2
1 MissoulaFireSciencesLaboratory,RockyMountainResearchStation,ForestService,UnitedStates
DepartmentofAgriculture,Missoula,Montana,UnitedStatesofAmerica,2 DepartmentofEnvironmental
SciencesandManagement,PortlandStateUniversity,Portland,Oregon,UnitedStatesofAmerica,
3 DepartmentofForestEcosystems&Society,CollegeofForestry,OregonStateUniversity,Corvallis,
Oregon,UnitedStatesofAmerica,4 PacificSouthwestResearchStation,ForestService,UnitedStates
a1111111111 DepartmentofAgriculture,Albany,California,UnitedStatesofAmerica,5 DepartmentofForestEngineering,
a1111111111 Resources&Management,CollegeofForestry,OregonStateUniversity,Corvallis,Oregon,UnitedStatesof
a1111111111 America
a1111111111
*[email protected]
a1111111111
Abstract
OPENACCESS Wecharacterizedwildfiretransmissionandexposurewithinamatrixoflargelandtenures
(federal,state,andprivate)surrounding56communitieswithina3.3millionhafireprone
Citation:AgerAA,EversCR,DayMA,PreislerHK,
BarrosAMG,Nielsen-PincusM(2017)Network regionofcentralOregonUS.Wildfiresimulationandnetworkanalysiswereusedtoquantify
analysisofwildfiretransmissionandimplications theexchangeoffireamonglandtenuresandcommunitiesandanalyzetherelativecontribu-
forriskgovernance.PLoSONE12(3):e0172867.
tionsofhumanversusnaturalignitionstowildfireexposure.Amongthelandtenuresexam-
doi:10.1371/journal.pone.0172867
ined,theareaburnedbyincomingfiresaveraged57%ofthetotalburnedarea.Community
Editor:AsimZia,UniversityofVermont,UNITED
exposurefromincomingfiresignitedonsurroundinglandtenuresaccountedfor67%ofthe
STATES
totalareaburned.Thenumberoflandtenurescontributingwildfiretoindividualcommunities
Received:September9,2016
andsurroundingwildlandurbaninterface(WUI)variedfrom3to20.Communityfiresheds,
Accepted:February11,2017 i.e.theareawhereignitionscanspawnfiresthatcanburnintotheWUI,covered40%ofthe
Published:March3,2017 landscape,andwere5.5timeslargerthanthecombinedareaofthecommunitycoreand
WUI.Forthemajorlandtenureswithinthestudyarea,theamountofincomingversusoutgo-
Copyright:Thisisanopenaccessarticle,freeofall
copyright,andmaybefreelyreproduced, ingfirewasrelativelyconstant,withsomeexceptions.Thestudyprovidesamulti-scale
distributed,transmitted,modified,builtupon,or characterizationofwildfirenetworkswithinalarge,mixedtenureandfirepronelandscape,
otherwiseusedbyanyoneforanylawfulpurpose.
andillustratestheconnectivityofriskbetweencommunitiesandthesurroundingwildlands.
TheworkismadeavailableundertheCreative
Weusethefindingstodiscusshowscalemismatchesinlocalwildfiregovernanceresult
CommonsCC0publicdomaindedication.
fromdisconnectedplanningsystemsanddisparatefiremanagementobjectivesamongthe
DataAvailabilityStatement:Allwildfiresimulation
largelandowners(federal,state,private)andlocalcommunities.Localandregionalrisk
andtransmissiondataaresharedat
ScholarsArchive@OSU,OregonStateUniversity’s planningprocessescanadoptourconceptsandmethodstobetterdefineandmapthescale
repositoryforscholarlymaterials.Datamaybe ofwildfireriskfromlargefireeventsandincorporatewildfirenetworkandconnectivitycon-
accessedat:https://doi.org/10.7267/N9G15XS6.
ceptsintoriskassessments.
Allotherdataarepubliclyavailablewithsources
listedinthemanuscriptorsupplementalmaterial.
Funding:Thisworkwassupportedbyfunding
fromtheNationalScienceFoundation(http://www.
nsf.gov),CoupledHumanandNaturalSystems
Program(CHH-1013296),theUnitedStates
DepartmentofAgriculture,ForestService,Pacific
PLOSONE|DOI:10.1371/journal.pone.0172867 March3,2017 1/28
Networkanalysisofwildfiretransmission
NorthwestResearchStation(http://www.fs.fed.us/ Introduction
pnw/)andtheJointFireScienceProgram(http://
AsfireeventsinthewesternUSandelsewherehavebecomeincreasinglylargeanddestructive,
www.firescience.gov/)grant(14-1-01-22toAA).
Thefundershadnoroleinstudydesign,data developinglong-termsocialandecologicalstrategiestomitigateimpactsremainsachallenge
collectionandanalysis,decisiontopublish,or [1].Awiderangeofsolutionshavebeenadvocatedinrecentliterature,includingchanging
preparationofthemanuscript. existingsuppressionpoliciestoallowmorenaturalignitionssothatwildfireseventually
Competinginterests:Theauthorshavedeclared becomeself-limiting[2],improvingtheefficiencyofprogramstotreatandreducewildland
thatnocompetinginterestsexist. fuels[3],changingmanagementcultureandattitudestowardsfire[4],andincorporatingnew
analyticaltoolsforwildfirescenarioplanningtoimprovethecouplingofhumanandbiophysi-
calsubsystems[1,5].Thewiderangingdiscussionsreinforcethefactthatmanaginglong-term
riskfromlarge,highlyuncertainwildfireeventsisacomplexsocioecologicalproblemthatwill
requirerapidadjustmentsinexistingriskgovernancesystemsinachangingclimate.Byrisk
governance,wemeantheprocessbywhichauthorityisexercisedanddecisionsaretaken
withinsocialandinstitutionalenvironmentstoidentify,assess,manage,andcommunicate
risk[6,7].Existingriskgovernanceischallengedbythefine-graineddiversityoflandtenures
andsocioecologicalsettings(e.g.fireregime,attitudetowardsfire)relativetothescaleoflarge
fireevents[8,9].
Perspectivesonwildfiremanagementareoftendeeplydivisiveamongpublicandprivate
entities,andaconsistentperceptionofriskimpedesthedevelopmentofsocialorganizations
thatareneededtoaligngoalswithrespecttowildfiresandtheirimpacts[9,10].Forinstance,
inthewesternUS,statewildfiremanagementagencieshaveapublicmandatetoprotectpri-
vatelandsfromwildfire,whilefederalpolicyonadjacentfire-adaptedforestsencourages
increaseduseofprescribedandmanagedwildfiretoreducefuelsandwildfirerisk[11].The
increaseduseofwildandprescribedfireasafuelmanagementstrategyonpublicforests[12]
inparticularhasandwillcontinuetogeneratefiremanagementconflictsamongstate,federal,
andprivatelandjurisdictions.Anotherexampleisthemanagementofprivateindustrialtim-
berlands,wherethelackoffinancialincentivestoremovesurfacefuelsgeneratedfromharvest-
ingcanexacerbatewildfirerisktosurroundingparcels[13–15].Atthecommunityscale,
residentslivingnearfire-adaptedpublicforestsmayrecognizethebenefitsfrompoliciesfor
increaseduseofprescribedandnaturalfiretorestorefireresiliency,butobjecttosmokeand
otherbyproductsthathavedeleteriouseffectsonairquality[14].Severalprescribedfireshave
escapedcontrolmeasuresand,insomecases,resultedincatastrophiclossesandsubsequent
tighteningoffiremanagementregulations[16–18].Riskgovernanceconflictsamonglarge
landowners,publiclandagencies,andcommunitieswillbecomemoreacuteintheAmerican
Westwhereurbanizationandamenityproductionincreasinglycompetewithtraditionalland
uses.Atthesametime,individualwildfireeventsincreasinglyintersectparcelswithlandown-
ershavingdiverseattitudes,firemanagementpoliciesandriskgovernancesystems,and
changingcurrentfiremanagementpoliciesisslowedbysubstantial“institutionalmomentum”
[14,15].
Theinterdependenceofriskbetweendifferentlandownersandparcelswithinpublicland
isrecognizedinnewerUS“alllands”wildfirepolicy(e.g.USCohesiveWildfireManagement
Strategy[11]),andmanagementinitiativesincreasinglyemphasizecollaborative,cross-
boundarysolutionstofiremanagement[19,20].Herewedefinerelevantboundariesasthose
delineatingnotonlylandownership,butalsothepotentialtomanageforestsandfuelsby
mechanicalmeans(henceforthlandtenures).However,mappingthescaleofrisk,quantify-
ingrisktransmissionandmeasuringwildfireconnectivityandinterdependenceacrossland
tenuresarelargelyunexploreddomains.Thestudyofrisktransmissioniswidelydiscussedin
thepropagationofinfectiousdiseaseinhumans,plants,andanimalpopulations[21]where,
forinstance,oneorganismtransmitsinfectiousdiseasetoanother.Inthecaseofwildfire,the
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originsandextentofthedisturbancearewelldefinedspatially(i.e.mappedwithinafew
meters),comparedtoothernaturaldisturbances(e.g.earthquakesandhurricanes),andits
propagationisaffectedbyfuelloadingonindividualparcels[22].Afewstudieshaveexam-
inedwildfirerisktransmissionfrompubliclandstothewildlandurbaninterface[23–26],but
havenotexaminedcommunityspecificwildfiretransmissiononlarge,mixed-tenureland-
scapes.Forinstance,howiswildfirerisksharedandaretherehotpotsofrisktransmission
associatedwithspecificlandtenuresandlocations?
Tounderstandtheimportanceofrisktransmissionandhelpcreateassessmentmethods
thatcanadvanceconceptsforcross-boundaryriskgovernance,westudiedtheexchangeof
wildfireexposureamonglargelandtenures(federalmanaged,federalwilderness,private
industrialforest,andstatelands),and56adjacentcommunitiesona3.3millionhafireprone
areaofthewesternUS.Weusethetermexposureratherthanrisksincewelimitouranalyses
toareaburnedratherthanpredictingexpectedloss[27].Sincemostareaburnedbylargewild-
fireeventsisunderextremeburningconditions,wecanassumethatmeasuresofburnedarea
translatetohighseveritywildfireandlossofvaluesonburnedparcels.Weusedsimulation
modelingandnetworkanalysistoexaminefivespecificquestions:(1)towhatextentdolarge
tractsoffederal,state,andprivatelandstransmitwildfireexposuretoeachother;(2)dosome
landtenurestransmitmorewildfireexposurethantheyreceive;(3)howmanyuniqueland
tenurescontributetothewildfireexposureoftypicalwesternUScommunities;(4)whatisthe
relativeinfluenceofhumanversusnaturalignitionsinthetransmissionofwildfireexposure;
and(5)aretheresubstantialdifferencesinfireexposureamongthedifferentlandtenuresas
describedbyfireintensityandlikelihood?
Ourstudysetsthestageforimprovingcross-boundarycommunity,collaborative,and
nationalforestplanning[19,20,28,29],withanalysesthatcanhelpidentifyconflictsand
opportunitiestoachievefederalwildlandfirepolicy,includingfireadaptedcommunities,fire
resilientlandscapes,andwildfireresponse.Specificallywefillagapinlocal,regional,and
state-scaleriskassessmentmethods[27,30,31]usedforcollaborativeplanningbyprovidinga
frameworktoquantifycross-boundaryexposureanddefinethegeographicscaleofriskto
communities.Thisinformationcanbeusedtominimizescalemismatchesbetweenlandten-
uresandriskplanningatthecommunityandlandscapescales.
Methods
Studyarea
Thestudyareawaspartofthe“Forests,People,Fire”project[5]andspans3.3millionhain
centralOregon,alongtheeastslopeoftheCascaderange(Fig1A).Thephysiographicgradi-
ents,forestvegetation,climate,andpublicmanagementpracticesregardingwildfiremanage-
mentresemblethenationalforeststhroughoutthewesternUS,andaredescribedindetail
elsewhere[5].ThestudyareaincludesthetownsofRedmond,BendandKlamathFallsand
manyothersmallruralcommunities.Elevationrangesbetween500and3240mandthetopo-
graphic-moisturegradientfromwesttoeasthasastronginfluenceonthedistributionof
majorecologicalzones.Higherelevationregionstothewestaredominatedbymountainhem-
lock(Tsuga mertensiana)andfirandsprucesubalpineforest,whilemidelevationcoolerslopes
arecharacterizedbymoistmixedconiferpatchesofDouglas-fir(Pseudotsugamenziesii),grand
fir(Abiesgrandis),whitefir(Abiesconcolor)sometimesintermingledwithlodgepolepine
(Pinuscontorta),andponderosapine(Pinusponderosa)atlowerelevations.Highelevationpla-
teausontheeastsidearedominatedbyponderosapinemixedwithsemi-aridjuniper(Juni-
perusoccidentalis)woodlandsandaridshrublands.About30%ofthestudyareaisdrymixed
conifer,whilearidareasandjuniperwoodlandsrepresent19%.Standsofmoistmixedconifer
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Fig1.Studyareamapsoflandtenuresandcommunities.(A)Landtenures(definedbyownershipandmanagementpractice),towns,andmajor
highways.SeeTable1forlandtenuredescriptions.(B)CommunitiesasdefinedbyUSCensusdataoncorecommunityareaandsurroundingSILVIS
wildlandurbaninterface(WUI)polygons.Communityboundaries(lightgraypolygons)representthecombinationofthecorecommunityareaand
associatedWUI.
doi:10.1371/journal.pone.0172867.g001
forests,ponderosapineandlodgepolepineaccountfor9%,14%and5%ofthestudyarea,
respectively.Alpineandhighelevationnon-forestedareasandnon-vegetatedareasrepresent
10%and13%oflanduse,respectively.Theareaisnotedforextensivecontiguousstandsof
lowdensityponderosapineoldgrowththatweremaintainedhistoricallywithperiodicnatural
fire[32].Manyoftheseforestshavedevelopeddenseunderstoryconditionsandladderfuels
largelyduetopoliciesthatprioritizesuppressionofnaturalfires[28]andlimitharvestingto
preserveoldforesthabitatforwildlifeandrecreationalamenities.Recentlargefiresincludethe
B&BComplexin2003(36,733ha),PoleCreekin2012(10,844ha)andSunnySidein2013
(21,448ha).Thestudyareahasalsoexperiencedrapidpopulationgrowthandexpansionof
urbaninterface/intermixaroundseveralcoreareasincludingBend,Redmond,Sisters,and
KlamathFalls,contributingtoagrowingconflictbetweenlandmanagementobjectivesofpub-
licforestsandamenityinterestsofnewerresidents.
Landtenureandcommunitydata
Themajorlandownershipsinthestudyareaarethefederalgovernment(66%,28%protected),
ConfederatedTribesofWarmSprings(native,8%),andprivateindustrial(7%)(Fig1A,
Table1).Privateindustriallandsaregenerallymanagedforwoodproductsandareharvested
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Networkanalysisofwildfiretransmission
Table1. Majorlandtenuresandtransmittedwildfireestimatedwithwildfiresimulation.
Landtenure Description Area(100 Total Totalfire Annualareaburned(hayr-1)
ha) (%) (hayr-1)
Human Natural
NonTF TF-IN TF-OUT NonTF TF-IN TF-OUT
BLM BureauofLandManagement 2050 6 1377 233 406 410 283 456 465
Community Corecommunities+wildlandurbaninterface 4338 13 2580 496 905 826 352 828 698
NF-M Nationalforest,managed(e.g.generalforest, 9350 29 4559 1139 679 764 1791 950 1094
timberproduction)
NF-P Nationalforest,protected(e.g.biodiversity 9280 28 3996 1056 709 777 1429 801 1085
reserves,wilderness)
NPS NationalParkService 390 1 72 4 9 2 37 23 24
Native NativeAmerican(ConfederatedTribesof 2460 8 6706 3738 525 487 2000 443 351
WarmSprings)
ODF OregonDepartmentofForestry 180 1 66 5 36 21 4 21 16
OtherFED Otherfederalland(e.g.DepartmentofEnergy) 290 1 423 59 138 124 53 172 93
Private Non-industrialprivate 1450 4 829 70 337 304 50 371 223
Private Corporateforestsmanagedforwoodproducts, 2140 7 882 299 269 314 106 208 175
Industrial harvestedonregularrotation
USFWS USFishandWildlifeService 300 1 129 25 37 34 19 47 57
Totalfire=nonTF+TF-IN.NonTF=theannualareaofnon-transmittedfire(self-burning).TF-IN=theannualareaburnedfromincomingfiresignitedon
otherlandtenures.TF-OUT=theannualareaburnedonotherlandtenuresbyfiresignitedlocally.Additionaldescriptionsofthecalculationsfortransmitted
fireareinthemethodssection.
doi:10.1371/journal.pone.0172867.t001
onaregularrotationtomaximizerevenue.TheBureauofLandManagement(BLM)andnon-
industrialprivateownersownroughly6%and4%ofthearearespectively.Thelatterarerela-
tivelysmalltracksoflandrangingfrom5to100ha.Oregonstateforestedlandsaccountfor
1%ofthestudyarea.FederallandincludestheDeschutesandFremont-WinemaNationalFor-
ests,CraterLakeNationalPark(NPS),BLMandareasmanagedbytheUSFishandWildlife
Service(USFWS).Asmallportion(1%)ofthelandscapeincludesminorfederalownerssuch
astheDepartmentofEnergyandDepartmentofDefense.Theownershipdatawerecombined
withfederallandmanagementdatatocreatealandtenuredatasetusingdatasourcesfromthe
“Forest,People,Fire”study[5].Nationalforestlandswerepartitionedbasedonlandand
resourcemanagementplans(e.g.[33])intoprotectedareas(variousconservationandbiodi-
versityreserves)versusthoseavailableformechanicalfueltreatments.Allotherfederalland
withtheexceptionofnationalparklandwasconsideredavailableformechanicalfueltreat-
ment.Theseparationofmanagedversusnon-managedwasthenincorporatedintotheland
tenureclassification.
Inadditiontothelandtenureslistedabove,wecreatedacommunityclassbasedoncore
communitiesandsurroundingsparselypopulatedwildlandurbaninterface(WUI)areas.We
definedcorecommunitieswithinthestudyareaaccordingtotheUSCensusdataoncommu-
nityboundaries[34].Fifty-sixcommunitieswereidentifiedinthesedatarepresenting233,000
peopleand118,000structures.WethenattachedsurroundingSILVISwildlandurbaninterface
(WUI)[35]polygonsoutsidethecorecommunityboundarybasedondistancetotheclosest
communitycore.WeremovedSILVISpolygonsthatwere:(1)classifiedasuninhabited,(2)
classifiedaswater,or(3)<0.1hainsize.Thecriteriaforremovingpolygonsconservedeven
thelowestdensityWUIareasadjacenttonationalforests.AllotherWUIpolygonswere
assignedtoacommunityandeachWUIpolygonwasattributedwithhousingunitdensity
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(hereafterreferredtoasstructures)andarea(ha).Theresultingcommunitylayerconsistedof
boththecoreareadefinedintheUSCensusandtheadjacentWUI(Fig1B).
Wildfiresimulation
Analyzingcross-boundaryexposurefromwildfiresrequiresspatialinformationonignition
locationsandfireperimeters.Broadgeographicpatternsofcross-boundaryexposurehave
beenpreviouslydescribedwithhistoricalfiredatabasesforlargeareas(e.g.westernUS,Fig8
inAgeretal.[36]).However,dataonhistoricallargefires,includingperimeters,areonlyavail-
ableatbestforthepast20–30yearsformuchofUS,andthusareinsufficienttomapandpre-
dictfuturepatternsofpotentialwildfiretransmissionatthescaleconsideredinthisstudy.A
solutioniswildfiresimulation,whichisnowawidelyusedpracticeforbothtacticalandstrate-
gicwildfiremanagementintheUSandelsewhere[37–40].MonteCarlosimulationsofmany
fires(>100,000)usinghistoricalweatherandcalibratedmodelscanbeusedtogeneratesuffi-
cientfiresamplestomapfinescaletransmissionamongparcelson>2millionhawithdesktop
computers.MostpractitionersintheUSusetheminimumtraveltime(MTT)algorithm[41]
thatcanreplicatelargefireperimetersandfireintensitypatterns[37,42–46].Weusedaver-
sionofMTTencapsulatedinFConstMTT,acommandlineversionofFlamMap[41].Fire
simulationsusedaspatiotemporalfirepredictionsystemusingthemethodsofPreisleretal.
[47]anddescribedindetailelsewhere(S1Appendix).Thespatiotemporalignitionprediction
modelwasbuiltusing18yearsofignitiondata[48]tocalculate:(1)theprobabilityofafireina
givenpixelasafunctionofdailyenergyreleasecomponent(ERC),location(X,Ycoordinate)
andday-of-year;and(2)theexpectedfiresizeforlocationswheretheabove-mentionedproba-
bilityequalsoneasafunctionofdailyERCandlocation.Weseparatedhumancausedigni-
tionsfromnaturalcausedignitions(S1Appendix).
Weexecutedthespatiotemporalignitionmodeltogenerate3000fireseasonsforatotalof
63,736simulatedfires,sufficienttosaturatethelandscapewithwildfireandburneverypixel
atleast10times.Thefirelistsgeneratedbythespatiotemporalignitionmodelusedby
FConstMTTspecifyignitionlocation,day-of-year,year,cause,ERC,windspeed,winddirec-
tion,fuelmoisture,burnperiodandexpectedfiresizeofeachfiresimulated.
Burningconditionsassociatedwitheachignitionweresampledfromhistoricaldaysbased
ondailyERC.Fuelmoistureforeachfuelsizeclassandfuelmodelwereaveragedoverhistori-
calconditions(1987–2011)foreachvalueofERC,whilewindspeedanddirectionwerebased
ondailyERCbutrestrictedtogustobservationsindaysofthehistoricalrecord(1994–2011)
wherefiresexceeded500hainsize.AllweatherdatawereobtainedfromtheLavaButte
RemoteAutomatedWeatherStation(RAWS,[49]).Weestimatedtheburnperiod(required
byMTT)toachieveaspecificfiresizeusingarelationshipbetweenburntimeandfiresize
builtfromcontrolledsimulations(S1Appendix).Simulatedwildfireperimetersboreaclose
resemblancetohistoricalfireswithinthestudyarea[26].Examplelargefireperimetersfrom
thesimulationsshowthepotentialimpactsintermsofareaandstructuresaffected(Fig2).
WeobtainedsurfaceandcanopyfuelforwildfiresimulationsfromthenationalLANDFIRE
dataset[50],aswellaselevation,slope,aspect,fuelmodel[51],canopycover,canopybase
height,canopyheight,andcanopybulkdensity[52].LANDFIREisastandardizedfueldataset
availablefortheconterminousUSandwidelyusedforwildfiremodelingandresearchinthe
US[50,53].
FConstMTTsimulationswereperformedat90mundertheconditionsspecifiedinthefire-
lists,andgenerated:(1)fireperimeters,(2)annualburnprobabilities,and(3)conditional
burnprobabilitiesfor200.5mflamelengthcategories.Theburnprobability(BP)foragiven
intensityclassistheratioofthenumberoftimesapixelburnedtothetotalnumberoffires
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Fig2.Simulatedfireperimetersandignitionlocationsandassociatedlandtenuremap.Perimetersandignitionlocationsof(A)sixofthelargest
firesbasedonareaburned,and(B)twoofthemostdestructivefiresbasedonexposuretothemostnumberofstructures.Perimeterswereextractedfrom
over63,736simulatedfiressampledfromdistributionsmodeledfromhistoricalignitionlocationandfiresize.SeeTable1forlandtenuredescriptions.
doi:10.1371/journal.pone.0172867.g002
simulatedandestimatestheannualburnprobabilityateachpixel.Theconditionalburnprob-
abilitiesareoutputasagriddedpointfileandestimatetheprobabilityofafireatagiveninten-
sity.Aprobabilityweightedaverageconditionalflamelength(CFL)wascalculatedfromthe
conditionalburnprobabilitiestomeasuretheexpectedflamelengthgivenapixelburns.
Wildfiretransmissionbetweenmajorlandtenures
Wecalculatedtransmittedwildfireexposureusingmethodsdevelopedinourpreviousstudies
[54,55].Specifically,fireperimeteroutputsandignitionlocationswereintersectedwiththe
landtenureandcommunitymap(Fig1).Wethencross-tabulatedtotalareaburnedineach
landclassandcommunitybyignitionsourcetoderivetheamountofincoming(TF-IN),out-
going(TF-OUT),andnon-transmittedfire(NonTF).Theresultingdatawerethenusedto
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buildtransmissionnetworkstoanalyzetheconnectivityamongspecificlandtenuresandinto
communities.NetworkconstructionandanalysisweredoneinR[56]usingtheigraphpackage
[57].
Networksarecomprisedofnodesandedges.Inthisstudynodescorrespondedtolandten-
uresandcommunitieswhiletheedgesrepresenteddirectionaltransferoffire(i.e.directed
weightednetwork).Nodesrepresentedmultiplepolygonsforagivenlanddesignation;hence
theedgesmeasuretheaggregatetransmissionofallthepolygonsofonelandtenuretothe
polygonsofotherlandtenures.Networkswereanalyzedattwo‘scales’:thescaleoflargeland
tenuresand,thescaleofindividualcommunities.Weseparatedtransmissionoriginatingfrom
humanandnaturalignitionswithineachnetwork.Abriefoverviewofnetworkterminology
usedinthisanalysiscanbefoundinTable2.
Wecalculatedwholenetworkandnode-specificmeasurespertainingtothefrequencyand
strengthoflinkages.Thenode-degreemeasuresthenumberoflinkagesforeachnodeand
indicateshowcentralanodeiswithinthenetworkandisofteninterpretedasanindicatorof
connectivityandinfluence[58].Inthecaseofwildfire,nodedegreerepresentsthenumberof
landownersthatcontributewildfiretoaspecificparcel,andtheoverallpotentialcomplexityof
managingandresolvingcross-boundaryexposureissuesforparticularcommunitiesorparcels
ofland.Networkdensitydescribesthenumberofnodespresentinthenetworkcomparedto
themaximumpossible,whichrepresentstheoverallconnectednessinthenetwork.Wealso
calculatedthenumberoflinkagestransmittingfireintothelandtenure(in-degree),andthe
numberoflinkagesthelandtenurewastransmittingfiretoothernodes(out-degree).Node
degreeisthetotalnumberoflinkagesforanode(landtenure),whilein-degreeandout-degree
arethenumberofincomingoroutgoinglinkages.Networkswereconstructedandanalyzedat
twoscalesofaggregation.Thecoarser-scalegroupingdescribedtransmissionbetweenlarger
landownerswithcommunitiesaggregatedintoasinglelandtenure.Thefiner-scalegrouping
includedthesamelargelandtenuresandindividualcommunities.
Table2. Organizationofriskgovernanceandwildfiretransmissionintonetworks.
Function Network Networkscale Riskgovernanceimplications
representation
Spatialdefinitionoflandscapeparcelin Node Node Defineslandscapeorganizationandthescaleformeasuringwildfire
termsoflandtenure transmissionandsharedrisk
Numberofwildfirelinkagesamongland Nodedegree Node Potentialcomplexityformanagingfireexposuretransmittedtoand
tenures fromsurroundinglandtenures
Wildfireexchangebetweenapairofland Directededge Node Magnitudeofrisksharingamongtwolandtenures
tenures
Numberofincomingwildfirelinkagesfora Nodein-degree Node Numberofsurroundinglandtenuresthatcontributetolocal
landtenure exposure
Numberofoutgoingwildfirelinkagesfora Nodeout-degree Node Numberofsurroundinglandtenuresthatreceivefirefromthelocal
landtenure landtenure
Overallwildfireconnectivityamongland Networkdensity Network Measurestheproportionofobservedwildfireconnectionsamong
tenures landtenurestothetotalpossible
Theimportanceofaparticularlandtenure Centrality Node/network Prioritylandtenureforriskmitigation
inthenetwork
Networkwidestatisticsdescribelandscape-scalepropertiesrelatedtowildfireconnectivityandsharingofwildfireexposureamonglandtenures.
Communitiesareconsideredaspecificlandtenurecategoryinthetableforsimplicity.Nodestatisticsaredescriptorsofindividuallandtenureexposureto
wildfireintermsofamount,numberofsources,andoverallconnectivitytootherlandtenures.Thecombinedanalysesofnodeandnetwork-widestatistics
canfacilitatecommunityandlandscapeplanningbyrevealingthescaleandconnectivityofwildfiredisturbances.
doi:10.1371/journal.pone.0172867.t002
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Housingexposureandcommunityfireshedcharacterization
Communitywildfireexposureoccurredwheresimulatedfirescoincidedwithdeveloped
areas(i.e.thecorecommunityandthesurroundingwildlandurbaninterface).Weinter-
sectedfireperimetersgeneratedfromthefiresimulationswithcommunitypolygonsand
thencalculatedthenumberofstructuresineachcommunitypotentiallyaffected.Thestruc-
tureestimateswerecalculatedastheproductoftheproportionofareaofeachcommunity
polygonburnedandthestructurecountforthatpolygon.Structuresweregroupedbycom-
munity(n=56)allowingasingleignitionpointtocreatewildfireexposureinmultiple
communities.
Weusefireshedstoexplorethespatialscaleofcommunityfireexposurewithinthestudy
area.Wedefinecommunityfireshedsastheareasthatarelikelytotransmitwildfiretocom-
munities[36].Usingthesimulationoutputsdescribedabovewecreatedacontinuous
smoothedsurfaceofstructureexposureforeachcommunityusingthelogistickrigingalgo-
rithmintheRgstatpackageinR3.1.1[56,59].Krigingisbasedonasemi-variogramthatcap-
turesobservedspatialdependencebetweenpoints[60,61].Sincethekrigingalgorithmis
generalizableweusedittocaptureoccurrencedataasabinarylogitfunction.
Thekrigingsurfacesdescribethepredictedlikelihoodthatanignitionoccurringatagiven
pointwouldexposestructurestowildfirewithinthatcommunity(i.e.valuesvariedbetween0
and1).Toconvertthekrigingsurfacetoadiscreteboundary(i.e.thefireshed)weuseda
thresholdof0.1meanttocaptureignitionlocationswheretherewasatleastanominal(10%)
chanceofexposure.Toaccountforareasinthefireshedwhereignitionswererareorabsent
weremovedareaswheretheignitionpointdensitywasverylow(i.e.lessthan1.5e-4km-2).
Finally,communityfireshedswereintersectedwiththelandtenuremaptocalculatethe
fireshedareabylandtenure,andtheproportionofthestudyareathattransmitsfiretothe
WUI.Ourmethodfordelineatingfiresheds,ascomparedtoconvexhullsusedbyothers(e.g.
[62,63]),eliminatesoutlierignitionsthatrarelythreatencommunitiesandcontributetolarger
fireshedsizes.
Results
Historicalfireactivityandignitionpatterns
Analysisofhistoricalfiredataprovidedinsightsintotheseasonalityandspatialdistributionof
wildfiresandtherelativefrequenciesofhumanversusnaturalignitionsandresultingburned
area.Theresultsconfirmedthatbothhumanandnaturalignitionscontributedsubstantiallyto
areaburned,andthateachignitionsourcerequiredadifferentspatiotemporalmodelforthe
wildfiresimulations.About16,061haofwildfireburnedwithinthestudyareaeachyearannu-
allybasedon11,618ignitions(645ignitionsyr-1)occurringbetweenJanuary1992andDecem-
ber2009(Fig3).Lightningresultedinroughlythesameburnedareaashumancausedfires
(7,877haversus7,184hayr-1)althoughwithmoreignitions(6,379or354ignitionsyr-1versus
5,239,or291ignitionsyr-1).Lightningcausedignitions(hereafternatural)hadaslightly
smalleraveragefiresize(22ha)comparedtohuman(26ha).Ignitionlocationsfromhumans
andlightningdifferedsubstantially(Fig3).Humanignitionswereclusteredneartownsand
citieswithinthestudyarea,especiallyWarmSprings,LaPineandBend.Naturalignitions
werefrequentinthesouth-centralandsoutheastportionofthestudyareawherehumanigni-
tionswererelativelysparse.Temporalvariationinignitionswasalsopronounced(Fig4).Nat-
uralignitionspeakedinthemiddleofthesummermonths(Fig4C)whereashumanignitions
wererelativelymorefrequentinthespringandfall.
PLOSONE|DOI:10.1371/journal.pone.0172867 March3,2017 9/28
Networkanalysisofwildfiretransmission
Fig3.Spatialdistributionofhistoricalignitionsbyfirecause.(A)Humanand(B)naturalcausedignitionsinthestudyareabasedonhistoricalignition
databetween1992and2009[64],totaling11,618ignitions.
doi:10.1371/journal.pone.0172867.g003
Simulatedfireactivity
Wesummarizedsimulatedfireactivitytoexaminebroadtrendsinrelationtohistoricalfires.
Simulatedfiresburned21,650hayr-1:11,200hayr-1weretheresultofhumancausedignitions;
10,460hayr-1burnedduetonaturallycausedignitions.Firewasmostcommononnational
forests(8,554ha)andnativelands(6,710ha),whichtogetheraccountedfor72%ofallland
burned.Morethanone-thirdoftheareaburned(8,402ha,39%)resultedfromwildfireevents
thatspannedatleastoneboundary.Humancausedfiresburnedcomparativelymoreareaof
native,community,privateandprivateindustriallandtenures(Fig5A;Table1).Thisdiffer-
encewasmostpronouncedfornativelandsandleastfortheprivatelandtenures.Firesfrom
PLOSONE|DOI:10.1371/journal.pone.0172867 March3,2017 10/28