owl in the central and southern sierra nevada, california, usa … · 2017. 3. 16. · et al....

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Historical Forest Conditions within the Range of the Pacific Fisher and SpottedOwl in the Central and Southern Sierra Nevada, California, USAAuthor(s): Chad T. Hanson and Dennis C. OdionSource: Natural Areas Journal, 36(1):8-19.Published By: Natural Areas AssociationDOI: http://dx.doi.org/10.3375/043.036.0106URL: http://www.bioone.org/doi/full/10.3375/043.036.0106

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� Natural Areas Journal Volume 36 (1), 2016

ABSTRACT:Thereissignificantdebateaboutrestorationtargetsforponderosapine(Pinus ponderosa)andmixed-coniferforestsintheSierraNevada,California,USA.Ononesidearerecommendationstocreatebothextensiveopenandpark-likepine forests, and to reducehigh-severityfireoccurrencebymechanicalthinningofforests.Theserecommendationsdrivecurrentmanagement.Ontheothersidearerecommendationstomanagelandscapesforbothdense,oldforest,andcomplexearly-seralforestthatiscreatedbybothhigh-severityandmoderate-severityfirescharacteristicofhistoricalfireregimes.OurresearchsuggeststhatthelatterapproachmaybestmaintainforestassociatedwithtwoimperiledspeciesthataretopmanagementconcernsoffederalagenciesintheSierraNevada:theCaliforniaSpot-tedOwl(Strix occidentalis occidentalis)andthePacificFisher(Pekania pennanti).WeusedspatiallyextensiveUSForestServiceforestsurveydatafrom1910and1911,andsynthesizedresearchfromotherpartsofthisregionforcomparison,toassessreferenceconditionsinlow/mid-elevationSierraNevadaforests.We found that historical ponderosa pine and mixed-conifer forests had a mixed-severity fireregime,withanaverageof26%high-severityfireeffects,andvariedmorewidelyinspeciescomposi-tionanddensitythansuggestedbypreviousresearch.Ourfindingsarecontrarytootherreportsusingavery small subset (~6%)of theavailabledata from these same1910and1911surveys.Therefore,wesuggest thathistorical referenceconditionsof forests in theSierraNevadarangeof thesespeciesarenotlikethatreportedpreviouslyinotherstudies,andthatmixed-severityfire,andforestsdefinedby strong contrasts and dynamic natural processes, were characteristic of historical ponderosa pineandmixed-coniferforestsofthewesternSierraNevada.Ouranalysisindicatesthatmanagingforbothdense,old forests, andprotectingcomplexearly-seral forest createdbyhigh-severityfire,will likelyadvanceconservationandrecoveryoftheSpottedOwlandPacificFisher,whilecurrentmanagementdirectionmayexacerbatethreats.

Index terms:mixed-conifer,mixed-severityfire,PacificFisher,ponderosapine,SpottedOwl

INTRODUCTION

Historicalfireregimesofmixed-coniferandponderosapine(Pinus ponderosa Dougl.exLaws)forestsofthenorthernSierraNevadaandsouthernCascadesinCaliforniahavebeendescribedasmixed-severity,andoftenincludedsubstantialportionsofhigh-sever-ity fire effects (Leiberg 1902; Show andKotok1924,1925;BeatyandTaylor2001;BekkerandTaylor2001,2010;Odionetal.2014).However, thewesternslopeofthecentralandsouthernSierraNevadahasbeen lessexamined in this regard.SomerecentarticleshavehypothesizedthatSierraNevadaponderosapineandmixed-coniferforestshadlow/moderate-severityfirere-gimes historically (Stephens et al. 2013;Fulé et al. 2014), and a modeling studyassumedverylowhigh-severityfirelevelsinhistoricalforests(Malleketal.2013).Incontrast,ananalysisofUSGeneralLandOffice surveys from the mid/late-1800sfoundamixed-severityfireregimeandawiderangeinforestdensityandcomposi-tion(Baker2014).

Tworecentstudies,usingasubsetofthe1911 portion of the “1910/1911 ForestSurvey” conducted in this area (USFS1910-1911), described generally openforest conditions hypothesized to have

been maintained by frequent, low-sever-ityfire,andnotedthattherewasnoclearevidenceofhigh-severityfireoccurrence(Scholl and Taylor 2010; Collins et al.2011). These results were from 17 and28plots,respectively(combined,~6%ofthe available data), in partially overlap-pingstudyareasof2125haand4000haonthewesternedgeofYosemiteNationalPark(inanareathatwas,in1911,partofStanislausNationalForest);thesubsetofforestselectedwasnotastatisticalsampleof the landscape. Mechanical thinningacrossmuchoftheSierraNevadahasbeensuggestedbysome to restore theseopenconditions, prevent higher-severity firefromaffectingdense,oldconiferforests,andincreasethepinecomponent(StephensandRuth2005;Northetal.2009;Collinsetal.2011;Schelleretal.2011).

However, two imperiled wildlife speciesnativetothisarea,theCaliforniaSpottedOwl (Strix occidentalis occidentalis deVesey) and the Pacific Fisher (Pekania pennant Rhoads),arestronglyassociatedwith dense, mature/old forest with highcanopycoverandabundantunderstorytreesandsnagsfornesting/roosting(forSpottedOwls) and denning/resting (for Fishers)(Verneretal.1992;Zielinskietal.2005,2006;Purcell et al. 2009;Underwoodet

Natural Areas Journal 36:8–19

R E S E A R C H A R T I C L E

3Correspondingauthor:[email protected];(530)273-9290

•

Historical Forest Conditions within the Range of the Pacific Fisher and Spotted Owl in the Central and Southern Sierra Nevada, California,

USA

Chad T. Hanson1, 3

1EarthIslandInstitute2150AllstonWay,Suite#460

Berkeley,CA94704

Dennis C. Odion2

2EarthResearchInstituteUniversityofCalifornia

SantaBarbara,CA93106-and-

EnvironmentalStudiesDepartmentSouthernOregonUniversity

Ashland,Oregon,97520

•

Volume 36 (1), 2016 Natural Areas Journal �

al.2010).Theforestsselected(selection,asmanifestedbypreference—i.e.,dispro-portionaluse relative toabundance:Halletal.1997)bythesespeciestendtohaveasubstantialcomponentofponderosapine(Pinus ponderosa)andsugarpine(Pinus lambertiana Dougl.),butareoftendomi-natedbywhitefir(Abies concolor Gordon&Glend.)andincense-cedar(Calocedrus decurrens Torr.)(Underwoodetal.2010).Recently, Fisher den sites were mostlyfound to occur in white fir and incense-cedar (R. Sweitzer, wildlife ecologist,The Great Basin Institute, unpubl. data:http://snamp.cnr.berkeley.edu/static/docu-ments/2011/10/28/SNAMP_2011_An-nual_Meeting_Presentation_AM.pdf).

Moreover, a radiotelemetry study in Se-quoia National Forest found that, whendistancefromnestandroostsiteswastakenintoaccount,CaliforniaSpottedOwls inthe McNally fire area selected unloggedhigh-severityfireareas—aconditionmoststronglydistinguishedbyhighsnagbasalareaandshrubcover—forforaging(Bondet al. 2009). A radiotelemetry study ofNorthernSpottedOwls(Strix occidentalis caurina Merriam)byClark(2007:Figure6.2)foundthat,afterfire,theowlsforagedinunloggedmoderate- andhigh-severityfire areas occurring in pre-fire nesting/roosting/foraging habitat (generally, pre-firedense,oldforest)morethanexpectedbaseduponabundance,andforagedinpost-fireloggedareaslessthanexpected.Theynotedthatthelimitedforagingdocumentedwithinpost-fire loggingunitswasgener-ally in logging-exclusion zones, such asriparianareas.Roberts(2008)found60%higher reproduction of California Spot-tedOwlsinunloggedmixed-severityfireareasthaninunburnedforests,andLeeetal. (2012)foundthatmixed-severityfire,averaging 32% high-severity fire effects,did not reduce California Spotted Owloccupancyinastudyof41territories.Interritorieswithmostly high-severityfire,63% were occupied 5–7 years post-fire(Leeetal.2012).Similarly,PacificFish-ersselectedmixed-severityfireareasoverunburnedforestwhennearfireedges,andthefrequencyofdetections(scat)indense,oldforestthatexperiencedmoderate/high-severity fire was nearly identical to thedetectionfrequencyindense,oldunburned

forest (Hanson 2013). The proportionof high-severity fire effects at detectionlocationswithinfireboundarieswasalsosignificantlyhigher thanatrandomloca-tionswithinfires(Hanson2013).

Theassociationwithmixed-severityfirear-easbythesetwoimperiledwildlifespeciesislikelytiedtotheenhancedsmallmammalprey base in complex early-seral forestsrich with snags, downed logs, montanechaparral, and dense pockets of naturalconifer regeneration (Bond et al. 2013;Hanson 2013). Mechanical thinning hasbeenfoundtoadverselyaffectoccupancyforCaliforniaSpottedOwls(SeamansandGutiérrez 2007). Owl populations havebeendeclininginresearchstudyareasman-agedwithextensive thinningonnationalforests (Conner et al. 2013; Tempel andGutiérrez 2013), while they have beenstableorincreasingonprotectednationalparklands(Conneretal.2013).Likewise,Fishers have been found to avoid areaswithin200mofmechanicallythinnedareasintheSierraNevada(Garner2013).

Thus, there isaneedtobetterdeterminethe historical reference forest conditionsin the Sierra Nevada in general, and theforests inhabited by California SpottedOwlsandPacificFishersinparticular,toresolvetheinconsistencybetweenhabitatcurrently used and theoretical historicalconditions.Ontheonehand,thesespeciesselectdense(Zielinskietal.2006;SeamansandGutiérrez2007;Purcelletal.2009),fir/cedar-dominatedforests(Underwoodetal.2010),andforageinunloggedhigh-se-verityfireareas(Bondetal.2009;Bondetal.2013;Hanson2013,2015).Ontheotherhand,somestudiesconductedatrelativelysmallspatialscalesindicatethathistoricalforestswerefairlyopenandsparse,withonlylower-severityfireeffects(SchollandTaylor2010;Collinsetal.2011),andwerecommonlydominatedbyponderosapineandJeffreypine,evenatmiddleelevations(Collinsetal.2011).

The1910/1911ForestSurvey(USFS1910–1911)coveredacombinedareaof65,296hainthecentralandsouthernmostportionsoftheStanislausNationalForest(includinga small area that isnow includedwithinYosemiteNationalPark),andincludeddata

onunderstorystructure(conifers<15.2-cmdbh,andshrubcover),aswellasfireef-fects,andspeciescomposition.Thus,thereis an opportunity to investigate a muchlargerportionofthehistoricallandscapeinponderosapineandmixed-coniferforestsofthecentralandsouthernSierraNevadathan in previous studies and to compareresultswithspatiallylimitedassessmentswithinthesamelandscape.The1910/1911surveys also allow an assessment of theextentofhigh-severityfireandvegetationconditions associated with potential forhigh-severityfire,suchasshrubcoverandconiferregeneration(Leiberg1902;ShowandKotok1924;Brownetal.2003;Odionetal.2010),inunloggedforestsgenerallybeforetheeffectsoffiresuppression.

Our goal was to describe the historicalfireregimeandforestconditionsoverthewholelandscapecoveredinthe1910/1911ForestSurveydataandcomparethesefind-ings with those obtained from analyzingselect, spatially limited areas within thislandscape.Specificobjectivesinthisstudywere to: (1) determine whether forestsdominated by ponderosa pine had lowerlevels of high-severity fire than fir/cedarforests;(2)estimatethehigh-severityfirerotation interval; (3) determine whetherforests dominated by ponderosa pinehadlowerunderstory(shrubsandconiferseedlings/saplings) density than fir/cedarforests,andwhetherponderosapinepreva-lencechangedonwetter/coolernortherlyslopes or at higher elevations within themixed-conifer zone; and (4) determinewhether live conifer density varied withdifferinglevelsofhigh-severityfire.

METHODS

Study Sites

The1910portionofthestudyareacom-prised 9328 ha of ponderosa pine andmixed-coniferforestinthecentralportionoftheStanislausNationalForest,ranginginelevationfrom1221to2416m(Figure1). The 1910 survey included data onlogging history, fire effects, and coniferspecies composition. The 1911 portionofthestudyareacomprised55,968haofponderosapineandmixed-coniferforestin

10 Natural Areas Journal Volume 36 (1), 2016

Figure 1. The 1910 and 1911 portions of the USFS forest inventory study area, western slope of the central and southern Sierra Nevada, California, USA.

Volume 36 (1), 2016 Natural Areas Journal 11

thesouthernendoftheStanislausNationalForest,includingthewesternedgeofwhatisnowYosemiteNationalPark(Figure1).Plot locations with complete surveys inthe 1911 study area ranged from 725 to1989 m in elevation. Plot data includedinformationonlogginghistory,fireeffects,coniferspeciescomposition,percentshrubcover,anddensityofconiferseedlingsandsaplings.Another distinct portion of theStanislaus National Forest was surveyedin 1912, but there were no notes aboutprevious logging history so we did notinclude this portion of the survey in theanalysis. At the lower elevations of thestudyarea,theforestisdominatedbypon-derosapine,mixedwithCaliforniablackoak (Quercus kelloggii Newb.), canyonlive oak (Quercus chrysolepsis Liebm.),andscatteredgraypine(Pinus sabiniana Dougl. ex D. Don.); at the intermediateelevations, the forest is composed of amixture of ponderosa pine, Jeffrey pine(Pinus jeffreyi Grev.&Balf.),sugarpine,incense-cedar, Douglas-fir (Pseudotsuga menziesii Franco),andwhitefir,withsomeCaliforniablackoakandcanyonliveoak;andat thehigherelevationsof the studyarea,itiscomposedofwhitefir,incense-cedar,sugarpine,ponderosapine,Jeffreypine, a small amount of red fir (Abies magnifica Andr. Murray) and lodgepolepine(Pinus contorta Louden).

Surveys

Surveyswereconductedattwoscales:1.6-hectare(ha)plots(belttransects)througheachofthe16.2-hasubsectionssurveyed;and 259.1-ha (640-acre) sections.At theplot scale, the 1910/1911 Forest Surveyrecordedtimbervolumebyconiferspecies,and also estimated percent shrub cover(exceptinthe1910portion,whichdidnotrecord shrub cover extent in subsectionswithmerchantabletimber).Onlyplotswithmerchantabletimberweresurveyedinthestudyareain1910and1911.

At the 259.1-ha section scale, surveyorsrecorded average conifer seedling andsaplingdensityinforestedportionsofeachsection in the 1911 portion, but not the1910portion.Saplingsweretrees<15-cmdbh.Seedlingswerenotexplicitlydefined,intermsofsize,butthetypicaldefinition

then,andnow,forseedlingsisatreelessthanbreastheight (i.e., less than1.37minheight).Inthe259.1-hasectionsurveys,forsubsectionswithnomerchantabletim-ber,surveyorsrecordedcovertype,notingwhether subsections were comprised ofmontanechaparralandearly-seralconiferregeneration, or other cover (e.g., rockoutcroppings), and made explicit notesoffireeffectsinconiferforest,aswellaswhetheranypreviouslogginghadoccurredonagivensection.Therewerevery fewsections in which logging had occurredinthisareaasof1910and1911,andweexcluded these from the analysis (whichalsoexcludedsubsectionsand,therefore,plots,inwhichlogginghadoccurred).Inboth portions of the study area, not allsectionsweresurveyed.Insomesections,surveyorsdidnotrecorddataforallsubsec-tions,ordidnotrecordfireeffectsonsomesubsections.Weexcludedtheseincompletesectionsfromouranalysis.

Historical Fire Regimes

Forthepurposesofthisstudy,wedividedtheforestintothreestrata:ponderosapine(>85%ponderosapine,bytimbervolume);mixed-conifer/pine (50–85% ponderosapine); and mixed-conifer/fir (10–49%ponderosapine).

Weassessedhigh-severityfireoccurrenceinthestudyareabaseduponfieldnotesinthe1910/1911ForestSurvey.Thisdetermi-nationwasmadebasedonevidence,orlackofevidence,ofhigh-severityfireacrossen-tire16.2-hasubsections(surveyorsdidnotmakerecordsoffireeffectsatfinerspatialscalesthanthis)ineachofthethreeforeststrata.The1910/1911ForestSurveyfieldnotescontainedarequiredfieldonsurveydatasheetspertaining tofireeffects,andsurveyors included specific notes on fireseverity,orlackoffireevidence.

To verify that any subsections recordedin1910/1911asconifer forests inwhichhigh-severity fire occurred actuallyrepresented potential conifer forest, wequantified the extent to which any suchareas have regenerated back to coniferforestinrecenttimes.ForthisweusedtheCaliforniaWildlifeHabitatRelationships(CWHR) vegetation database (www.dfg.

ca.gov/biogeodata/cwhr/)tocalculatetheproportionof thehistoricalhigh-severityfire area that is currently vegetated withconiferforest.

Weonlyrecordedareasasforeststhatexpe-riencedhigh-severityfireif:(1)surveyorsexplicitly included notes about ‘severe’fireinconiferforest;and(2)theaffectedsubsections were recorded as having nomerchantable timber at the time of thesurveys,withsurveyorsnotingdominanceby brush and conifer seedlings/saplings.Wecomparedtheproportionofplotswithno merchantable timber, resulting fromhigh-severityfire effects in theseconiferforests, among the three conifer foreststrata, using a Chi-square test for trendsinbinomialproportions(Rosner2000)todeterminewhetherthisproportionchangedwith increasing proportion of ponderosapine.Wealsoevaluatedtheresultsinthecontextofotherpreviousstudiesexaminingtheproportionofhigh-severityfireintheseforesttypes(inhistoricaldatasetsandcur-rentstudieseitheranalyzinghistoricaldataorusingstandstructureoragetoestimatepasthigh-severityfireeffects)within theSierraNevadamanagementregion,whichconsistsoftheSierraNevada,theportionofthesouthernCascadesinCalifornia,andtheModocPlateau(Milleretal.2012).

Next,weestimated thehigh-severityfirerotation interval (years divided by theproportionofforestareaaffectedbyhigh-severityfire)intheseforests.Werestrictedthisanalysistoforestsofthewestsideofthe central and southern Sierra Nevada(southoftheMiddleForkoftheAmericanRiver)thathadnotbeenpreviouslylogged(WildernessAreas, Inventoried RoadlessAreas,NationalParks,andWildandScenicRiverCorridors) toavoidanypotentiallyconfoundingeffectofpasttimberremoval.Sincethesurveyorsforthe1910/1911For-estSurveyrecordedhigh-severityfireareasasnonmerchantableifdominanttreeswere<31-cmdbh,weusedUSForestServiceForestInventoryandAnalysis(FIA)datatodeterminethetimerequiredfordomi-nantandcodominanttreestoreach31-cmdbh (rounding up to the nearest decade,tobe conservative), and alsodeterminedwhether therewas a correlationbetweendiameterandagetocheckthevalidityof


thisapproach.FIArecordsvariousmetricsin fixed plots, with approximately oneplot for every 2400 ha (http://www.fia.fs.fed.us/tools-data/). The high-severityfire rotation interval was defined as thetimeperiodinyears, t (here, thenumberofyearsfortreestoreach31-cmdbhafterstandinitiation),dividedbytheproportionof forest experiencing high-severity fire,ph (here, the proportion of subsectionswithhigh-severityfire andno remainingmerchantabletimber):t / ph.

Historical Forest Structure and Composition

Todeterminewhethershrubcoverchangedwith increasing proportion of ponderosapine,weusedaChi-squaretestfortrendsinbinomialproportions(Rosner2000)tocompare theproportionof259.1-ha sec-tions(withinmerchantableconiferforestasof1911surveys)aboveandbelowthemean percent shrub cover in all coniferforest. Similarly, to determine whetherconifer seedling/sapling density changedwith increasing proportion of ponderosapine,weagainusedaChi-squaretestfortrends in binomial proportions to com-pare the proportion of 259.1-ha sections(withinmerchantableconiferforestasof1911surveys)aboveandbelowthemeanconifer seedling/sapling stems/ha in allconiferforest.

WeusedaChi-square2×2contingencytest (Rosner2000) todeterminewhethersoutherly slopes had a greater relativeabundanceofpines(Pinus spp.)thandidnortherlyslopes.Weconductedtheanaly-siswithintheelevationbandanalyzedinCollinsetal.(2011):1460–2130m,wherepines, firs and cedar tend to intermix intoday’s forests.FollowingUnderwoodetal. (2010),northerlyslopesweredefinedtorepresentthecoolerandwetteraspects,centered on an aspect of 45° (north andeast), and southerly slopes were definedtorepresentthewarmeranddrieraspects,centeredonanaspectof225°(southandwest).Northerlyslopeshad>50%oftheareaintheplotonnorth-andeast-facingslopes,whilesoutherlyslopeshad>50%oftheareaintheplotonsouth-andwest-facing slopes. Plots were categorized as

primarily pine (ponderosa, Jeffrey, andsugarpine,withoccasionalsmallamountsoflodgepolepineathigherelevations)if>50% of the conifer timber volume wascomprisedofPinusspecies,andwerecat-egorizedasprimarilynon-pineif<50%oftheconifertimbervolumewascomprisedofPinusspecies(whitefir,incense-cedar,Douglas-fir, and some red fir at higherelevations).

We used a Spearman’s rank correlationtest (Rosner2000) todeterminewhetherthehigherelevations(in50-mincrements)correlated to higher proportions of non-pine conifers in terms of timber volume(boardfeet).Werestrictedthisanalysistothesameelevationband(1460–2130m)inmixed-coniferforests.

Last,weconductedanaposteriorianalysisusingat-testfortwoindependentsampleswithunequalvariances(Rosner2000) todetermine whether the mean plot-scaletimbervolumewasdifferentbetweenthe1910and1911portionsofthestudyarea.Theformerhadnoevidenceoflargehigh-severityfirepatches(i.e.,coveringentiresubsections),whereasthelatterhadexten-sive evidence of such high-severity fire.Werestrictedthisanalysis to theoverlapinelevationbetween the twoportionsofthestudyarea:1221–1989m.

RESULTS

Historical Fire Regimes

Therewere166subsectionsavailableforthisanalysisinthe1910portion,and480subsections in the 1911 portion, whichyieldedatotalof646plots.The1910/1911ForestSurveyfielddatacontainedextensiveevidencethatamixed-severityfireregimein ponderosa pine and mixed-coniferforests occurred on the western slope ofthe central/southern Sierra Nevada priorto modern fire suppression, as describedbelow.Surveyorsoftenexplicitlyrecordedin their data forms (in the required fieldregardingfireeffects)that“severe”firehadkilled all of the timber in many subsec-tionswithinagivensectionsuchthatthereweremanyareasin1910/1911thatwerecharacterizedbybrushandyoung(nonmer-

chantable)coniferregeneration.Withinthe7773haof forestwithcomplete surveys(representing480plots)inthe55,968-ha1911 portion of the study area, 6640 hawererecordedaseitherconiferforest,orearly-seralconditionsresultingfromhigh-severityfireinconiferforest(i.e.,excludingmontanechaparralareasthatdidnotclearlyresultfromhigh-severityfire).Withinthis6640-haarea,37%(2429ha)wasrecordedashavingbeen affectedbyhigh-severityfire,suchthattherewasnomerchantabletimberinthesubsections(Figure1).Noneofthesubsectionsinthe2688-haforestedarea with complete surveys in the 1910portion of the study area were affectedby high-severity fire (Figure 1) acrossthe forest subsections (i.e.,at least somemerchantable timber remained in everyforest subsection).All areas included inthisanalysiswereunlogged,accordingtofieldnotes.

Wheretherewasnoclearevidenceofpasthigh-severityfireinsubsectionsvegetatedbymontanechaparral,surveyorsrecordedcovertypeandevidenceofpreviousfires,butdidnotdescribesuchpastfireasbe-ing severe fire that killed the merchant-able timber across entire subsections. Inaddition,surveyorsdidnotrecorddatainsectionswithoutmerchantabletimber,sothere may have been some larger areasof high-severity fire—covering one ormoreentire259.1-hasections—thatwerenot recorded in the 1910/1911 ForestSurvey(Figure1).Therefore,our resultsregardinghigh-severityfireoccurrencearelikelyconservative.Subsectionsrecordedin1910/1911ashavingmontanechapar-ral from high-severity fire in previouslymatureconiferforestcurrentlyhave62%coniferforestcover(oak/shrubsdominatetheremainder).

Our results were not consistent with thehypothesisthatlowerlevelsofhigh-sever-ityfirecharacterizedforestsdominatedbyponderosapineintheSierraNevada.Wefound no evidence of a significant trendtowarddecreasinghigh-severityfirepro-portionas theamountofponderosapineinstandsincreased(χ2=2.45,P=0.294,df=2,n=410plots).High-severityfireproportionsinthe1911surveyareawere67%,22%,and48%inmixed-conifer/fir,

Volume 36 (1), 2016 Natural Areas Journal 13

mixed-conifer/pine, and ponderosa pineforests, respectively, and averaged 37%overall.Weusedonlythe1911portionforthis analysis because: (1) it was unclearhow differing high-severity fire levels inthethreeforeststratacouldbeassessedinthe1910portionwhentherewasnohigh-severityfirerecordedacrossanyindividualsubsectioninthatportionofthestudyarea;and(2)onlyaportionoftheelevationbandsof the 1910 and 1911 parts of the studyareaoverlapped,andanevencomparisonusingtheoverlappingportionoftheeleva-tionbandwouldhaveeliminatedmostofthehigh-severityfirerecordsinponderosa

pineforest,underminingtheanalysis.Thecombinedproportionofrecordedhigh-se-verityfireeffects,includingboththe1910and1911portionsofthestudyarea,was26%.Theseresultsarewithintherangeofthosefrompreviousstudiesofmixed-co-niferandponderosa/Jeffrey-pineforestsinotherportionsoftheUSFSSierraNevadamanagementregion(Table1).

Therewasasignificantcorrelationbetweenthediameterandageofdominant/codomi-nanttrees(r=0.74,t=41.94,P<0.001,df = 1439, n = 1441 trees) (Figure 2).Basedonthisregression,thetimerequired

fromstandinitiationto31-cmdbhwas60years.Thehigh-severityfirearea(2429ha)dividedbytotalforestarea(1910and1911)withcompletesurveys(9328ha)yieldedaproportionof0.26.Thus,theestimatedhigh-severityfirerotationintervalinpon-derosapineandmixed-coniferforestsinthestudyareawas60/0.26=231years.

Historical Forest Structure and Composition

Wefoundnotrendindecreasedshrubcoverastheamountofponderosapineinstands

Study Location High-severity fire proportion

Leiberg (1902) Northern Sierra Nevada >8% (ponderosa pine) and

>20% (mixed-conifer) a

Show and Kotok (1925) California, including the Sierra 14% b

Nevada

Minnich et al. (2000) Northern Baja California, Mexico 16% c

Beaty and Taylor (2001) Southern Cascades 18–70%, depending on slope aspect

Bekker and Taylor (2001) Southern Cascades 52–63%

Beaty and Taylor (2008) Eastern Sierra Nevada 7–36%, depending on slope position

Baker (2012) Eastern Cascades, Oregon 16% (ponderosa pine) and 23%

(mixed-conifer)

Baker (2014) Western Sierra Nevada 31–39%

a Hanson (2007) used digitized versions of Leiberg’s mapping of high-severity fire areas in unlogged forests (areas mapped

as logged were excluded), overlaid with forest type, to determine high-severity fire percentages in patches >32.4 ha. Because

these percentages do not include patches <32.4 ha, they are underestimates (Hanson 2007). Leiberg (1902) explicitly noted

that if high-severity fire patches <32.4 ha were included the amount of high-severity fire would be “considerably increased”

(Leiberg 1902: p. 41). Mallek et al. (2013: Table 5) referenced this study with regard to 8% high-severity fire effects.

However, this figure was based only on the narrowest subset of high-severity fire effects: areas with 100% tree mortality

(Leiberg 1902: p. 41). Leiberg specifically mapped areas of 75–100% timber volume mortality (in patches >32.4 ha),

corresponding to high-severity fire areas, and such areas comprised a much larger acreage than the areas of 100% mortality

(Leiberg 1902: Plate VII; Hanson 2007). b Show and Kotok (1925: p. 3, footnote 2) noted that one acre out of every seven, on average, was montane chaparral habitat

resulting from previous high-severity fire. Mallek et al. (2013: Table 5) referenced this study with regard to 5% high-severity

fire effects in ponderosa pine-dominated forests. However, the reference to this proportion (5%) was in regard to the percent

of timber volume typically killed in “surface fire,” not crown fire, and even for typical fires the authors estimated an ultimate

timber volume mortality level of 14% (Show and Kotok 1925: p. 3).c Mallek et al. (2013: Table 5) referenced this study with regard to 4–8% high-severity fire effects. However, Minnich et al.

(2000) used “stand-replacement” fire, defined as 90–100% mortality of overstory trees, as the highest fire severity category,

which comprised 16% of all fire effects. The standard, and most currently accepted, definition of high-severity fire is RdNBR

values (from satellite imagery) >641 (Miller and Thode 2007), and this corresponds to approximately 70% basal area

mortality, based upon field validation plots (Hanson et al. 2010: Table 1). Thus, the 16% figure from Minnich et al. (2000) is

likely an underestimate of the high-severity fire proportion.

Table 1. Historical high-severity fire proportions in mixed-conifer and ponderosa/Jeffrey-pine forests in various portions of the Sierra Nevada management region of California (Sierra Nevada, southern Cascades in California, and Modoc Plateau) and nearby regions.

1� Natural Areas Journal Volume 36 (1), 2016

increased(χ2=1.13,P=0.568,df=2,n=30sections)inthe1911portionofthestudyarea(asdiscussedaboveintheMethods,surveyorsdidnotrecordshrubcoverinthe1910portionofthestudyarea).Shrubcovercomprised40%,29%,and75%oftheareain mixed-conifer/fir, mixed-conifer/pine,and ponderosa pine forests, respectively,andtheseforeststratacombinedhad34%shrubcoverinmerchantableconiferforest(range=6–80%).

Similarly,wefoundnotrendindecreasedconifer seedling/sapling density as theamount of ponderosa pine in stands in-creased in the1911portionof the studyarea (χ2 = 0.13, P = 0.937, df = 2, n =30 sections) (as discussed above in theMethods, surveyorsdidnot recordmeanconifer seedling/sapling density in the1910 portion of the study area). Coniferseedlingdensityaveraged673/ha(range=37–4081/ha),andsaplingdensityaveraged164/ha (range = 7–1386/ha) in the threeconiferstratacombined.

Inmixed-coniferforests,at1460–2130minelevation,therewasasignificantrelation-shipbetweenaspectandpinedominance,wheresouth-facingslopeshadmorethantwiceasmany(59versus26)pine-domi-natedplots(χ2=6.31,P=0.012,df=1,n=162plots).However,contrarytoourexpectations, plots dominated by fir andcedar did not show an effect of aspect,with approximately the samenumberonsoutherlyslopesasonnortherlyslopes(40onsoutherlyslopes,and37onnortherlyslopes).Overall (southerly andnortherlyslopescombined),pineandfir/cedarplotswerenearlyevenlysplitinmixed-coniferforests,at52%and48%,respectively.

The proportion of timber volume com-prisedbypinedecreasedsignificantlywithincreasing elevation within the mixed-conifer zone, 1450–2150 m (rs = -0.82,P < 0.001, df = 12, n = 14; elevationincrementsof50m).Dominanceshiftedfrom primarily pine to primarily fir andcedaratapproximately1850mineleva-tion (Figure 3). Our review of historical

data sets (including modern analyses ofhistoricaldata)inthemixed-coniferzoneofthewesternSierraNevadaalsoindicatesahighproportionofsurveyedareasweredominated by non-pine conifers, mostlywhitefirandincense-cedar(Table2).

Finally, within the overlap of elevationbetweenthe1910and1911portionsofthestudyarea(1221–1989m),themeantimbervolume (board feet/ha) was significantlyhigherinthe1910portion(75,141bf/ha,SD=42,466,n=105plots),whichdidnothave evidence of extensive high-severityfire,thaninthe1911portion(31,658bf/ha,SD=19,735,n=103plots),whichhadsubstantialoccurrenceofhigh-severityfire(t=9.50,P<0.001,df=148).

DISCUSSION

Basedonevaluationofallforestinventorydataconducted in1910and1911by theUS Forest Service, we found consider-able evidence for substantial portions oflargeareasaffectedbyhigh-severityfires

Figure 2. Diameter (cm dbh) and age (years) of dominant and codominant trees in ponderosa/Jeffrey-pine and mixed-conifer forests of the west side of the central and southern Sierra Nevada in unlogged forests.

Volume 36 (1), 2016 Natural Areas Journal 1�

(Figure 1). Moreover, it was clear thatmixed-severity fire regimes were char-acteristic of ponderosa pine and mixed-coniferforestsofthewesternslopeofthecentralandsouthernSierraNevadabeforefire suppression. Therefore, our findingswerecontrarytohypothesesarticulatedinNorthetal.(2009),Collinsetal.(2011),Fuléetal.(2014),Stephensetal.(2013),andrecentmodeling(Malleketal.2013)thathigh-severityfirewas relatively rare

in forest in this mountain range beforefiresuppression,andrangedfrom4–13%(Mallek et al. 2013). Some other recentstudies,overlappingportionsofourstudyarea, also reported low levels of histori-calhigh-severityfire(Collinsetal.2015;HarrisandTaylor2015),butwerebasedupon 1911 survey forms (Form 321a)pertainingonlytostandsofmerchantabletimber,anddidnot include theabundantrecordsofmerchantabletimberstandsen-

tirelykilledbyhigh-severityfirethatwerecontainedondifferentforms(Form322)inthesame1911StanislausNationalForestdata set.Another recent study, Stephensetal.(2015),alsoused1911surveyformspertainingonlytostandsofmerchantabletimber,anddidsoinanarea(asmallareaof the Greenhorn Mountains, about 235kmsouthofour studyarea)where1911surveyorssimplydidnotaddresstheissueofstandscompletelykilledbyfireintheir

Study Location Composition (trees/ha)

Leiberg (1902) Western Sierra, northern In four unlogged watersheds:1

35% pine, 63% non-pine conifer, 3% oak

Cooper (1906) Western Sierra, northern

761–1218 m elevation 50% pine, 40% non-pine conifer, 10% oak


Western Sierra, southern

1218 m elevation 61% pine, 34% non-pine conifer, 5% oak


Hodge (1906) Western Sierra, northern



Western Sierra, central

1523 m elevation 26% pine, 74% non-pine conifer

Western Sierra, southern



~1800–2000 m elevation 24% pine, 76% non-pine conifer

Stephens (2000)2

Central/northern Sierra

“average” mixed-conifer 51% pine, 43% non-pine conifer, 6% oak

“large” mixed-conifer 20% pine, 80% non-pine conifer

North et al. (2007) Southern Sierra

1900–2600 m elevation 49% pine, 51% non-pine conifer

Lydersen et al. (2013) Central Sierra

1740–1805 m elevation 17–25% pine, 75–83% non-pine conifer

Baker (2014) Northern Sierra 30% pine, 38% non-pine conifer, 29% oak

2% other species

Southern Sierra 46% pine, 28% non-pine conifer, 22% oak,

3% other species

1 Feather River, South Fork, between Lexington Hill and Strawberry Valley; Yuba River; American River, North Fork;

American River, Middle Fork.2 These data were synthesized from plot data gathered by George B. Sudworth of the US Geological Survey in 1899

(Stephens 2000).

Table 2. Historical conifer species composition in ponderosa pine and mixed-conifer forests of the Sierra Nevada.


surveynotes.However,immediatelyafterthose 1911 surveys, US Forest Servicesurveyors issued a report noting that, inthe 1870s, that area of the GreenhornMountainsexperienceda“widespreadanddestructive fire” that burned for monthsandkilledmuchofthetimberinthatarea(Childs1912).

Inouranalysis,wefoundarangeof7–63%high-severity fire, based upon historicalsources and reconstructions, and mostvalues were ~15–30% (Table 1). In the1910/1911 study area, the proportion ofhigh-severity fire averaged 26%, includ-ing both the 1910 and 1911 portions,consistent with other data in this region(Table1).Also,ourestimatedhigh-sever-ityfirerotationintervalof231yearswasconsistentwithsomeresearch(HansonandOdion2014),andmoderatelyshorterthanrotations inother research(Baker2014).Furthermore,therewasahighcorrelationbetweentreediameterandage,whichwascontrarytothehypothesisadvancedbyFuléetal.(2014).AlthoughMilleretal.(2012)hypothesized thathigh-severityfire rota-tionsintherangeof200–250yearsmight

betooshorttomaintainoldforestonthelandscape,dataonstandagedistributionsresulting from fire indicate that, with a200-yearhigh-severityfirerotation,~25%oftheforestwouldbe100–199yearsold,~17% would be 200–299 years old, and~16%wouldbe>300yearsold,unlikethedistributionfora200-year-rotationclearcutloggingcycle,whichwouldresultinzerostands over 200 years old (e.g., see Cyretal.2009).

The forests of the study area also hadhigh variability in both density and spe-ciescomposition,withadenserstructureassociated with old forest areas that hadnotexperiencedhigh-severityfireforalongtime.Pinewasgenerallydominantatlowerelevations within mixed-conifer forests,although substantial proportions of non-pineconifers stilloccurred there;firandcedardominatedmixed-coniferforestsatupperelevations(>1850m)atthislatitude(Figure3).Thiswasalsoconsistentwithotherhistoricaldatasetsfortheseforests(Table 2). Understories were also highlyvariable, but were generally dense, withshrub cover averaging 34% and conifer

seedling/saplingdensityaveraging837/hawithinforestedareas.

Spatial scale was an important issue inour results. Had we analyzed only thesmaller1910portionofourstudyarea,orasmallsubsetofthe1911portion,ashadbeendonepreviously (SchollandTaylor2010;Collinsetal.2011),wemightalsohave reached overly narrow conclusionsthatwouldnothavecorresponded to thefull range of variability of historical fireeffects, forest structure, and tree speciescomposition.Curiously,wefoundexplicitnotes about extensive high-severity fireeffectsthatweremadeby1911surveyorsaboutthesubsectionsanalyzedbyCollinsetal.(2011)(USFS1910–1911,e.g.,T2S,R20E, Sections 4 and 5), yet they drewtheoppositeconclusion.Ourresultsalsounderscoretheepisodicnatureofhistoricalhigh-severityfire,suchthatoneportionofthestudyarea(1911)evidencesextensivehigh-severityfireoccurrencewhileanother(1910) does not at a particular point intime (Figure 1). Thus, data from largerlandscapes are essential, and landscape-levelinferencesshouldnotbeextrapolated

Figure 3. Proportions of pine and non-pine conifers with increasing elevation in mixed-conifer forests from the 1910/1911 survey data, western slope of the central and southern Sierra Nevada, California, USA.


fromspatiallylimitedstudies.

The 1910/1911 Forest Survey data didnot support the hypothesis of relativelyhomogeneous, open, pine-dominatedforestsandsparseunderstories thatweremaintainedbylow-severityfireacrossthisSierraNevadalandscape.Rather,thedatadescribe forests that were characterizedby strong contrasts and dynamic naturalprocesses.ThesehistoricalpatternswerealsoconsistentwiththewidelycontrastinghabitatassociationsreportedforCaliforniaSpottedOwlsandPacificFishersintermsofverydense,oldforestfornesting/roost-inganddenning/restinghabitat(Gutiérrezetal.1992;Zielinskietal.2006;Purcelletal.2009;Underwoodetal.2010;Hanson2015), and complex early-seral forest,createdbymoderate/high-severityfire,forforaging (Bond et al. 2009; Bond et al.2013;Hanson2013).Thesewere foreststhatcouldsustainnotonlythefullrangeofhabitatrequirementsforCaliforniaSpottedOwls andPacificFishers, but could alsoprovide abundant habitat for other rarespeciesthatarenarrowlyassociatedwithhigh-severityfireindense,oldconiferfor-est,suchastheBlack-backedWoodpecker(Picoides arcticus Swainson)(HansonandNorth 2008). Leiberg (1902: page 171)observed this juxtapositionofdense,oldforest and high-severity fire patches innumerous mixed-conifer forest locationsin the Sierra Nevada: “All the slopes ofDuncanCanyonfromitsheaddownshowthesamemarksoffire—deadtimber,denseundergrowth, stretches of chaparral, thinlines of trees or small groups rising outof the brush, and heavy blocks of forestsurroundedbychaparral.”

Inthemixed-coniferforestplotssurveyedbyHodge(1906),standswereconsistentlydominatedby small (<30.5-cmdbh) andsmall/medium-sizedtrees(<48.3-cmdbh),anddensityrangedfrom185to2322/ha;these trees were comprised primarily ofwhite fir and incense-cedar. Historicalbasalareaof treesalsorangedwidely inmixed-conifer forests, with mature/oldforestsoftenexceeding50m2/ha(Northetal.2007),andsometimesreachinglevels>100, or > 200 m2/ha (Stephens 2000;Lydersenetal.2013).Suchstandstructurelikelywouldhaveprovidedexcellentden-

ningandrestinghabitatforPacificFishersbasedonthehistoricalUSFSdatacollectedin1910/1911(Zielinskietal.2006;Purcelletal.2009;Lofrothetal.2010),aswellasnesting/roostinghabitat forCaliforniaSpottedOwls(Gutiérrezetal.1992).

Collinsetal.(2011)suggestthattherela-tivelylowforestdensitiesintheirspatiallylimited(i.e., lowsamplesize)studyareabeextrapolatedtothelargerSierraNevadalandscape,andthatextensivemechanicalthinningoperationscouldbeusedtogreatlyreduce forest density in order to restoreforestsandtopreventhigh-severityfireinthefuture.Givenouranalysisofallthedataavailableinthe1910/1911surveys,webe-lievethisisanunjustifiedrecommendationbecausethehistoricalconditionsindicatedthewidespreadpresenceofhigh-severityfire, mixed-severity fire landscapes, anda heterogeneous landscape comprised ofdifferentforestconditionswithhighstruc-turalandfloristicvariability.Moreover,thewidelycontrastinghabitatassociationsofCaliforniaSpottedOwlsandPacificFish-ersfornesting/roostinganddenning/rest-ing habitat, respectively, as compared toforaginghabitat,suggesttheCollinsetal.(2011) approachwould result inunnatu-rally homogenized forest conditions thatwouldleadtofurtherlossesofnotonlyhighqualitynesting/roostinghabitat(CaliforniaSpottedOwl)anddenning/restinghabitat(PacificFisher),butalsoforaginghabitatforbothspecies,whichischaracterizedbyhighsnagdensitiesandhighshrubcoverfollowingfire(Bondetal.2009;Hanson2013)—conditions that mechanical thin-ninggenerallyseekstoreduceorprevent.Thisisofsignificantconservationconcern,because thePacificFisher is acandidatefor listingunder theEndangeredSpeciesAct (ESA), and the California SpottedOwl isdeclining throughout the forestedregions of the Sierra Nevada that havebeen managed by extensive mechanicalthinning and fire suppression, while theowl is not declining in forests protectedfrom mechanical thinning and post-firelogging(Conneretal.2013;TempelandGutiérrez2013).Arecentanalysisfounda43%lossofCaliforniaSpottedOwloccu-pancyaftermechanicalthinningandgroupselection logging (Stephens et al. 2014).PacificFishersintheSierraNevadahave

beenfoundtoavoidmechanicallythinnedareas(Garner2013).

Moreover,high-severityfireremainsheav-ily suppressed in these forests currently,relativetoitsnatural,historicaloccurrence(OdionandHanson2013;HansonandOdi-on2014;Odionetal.2014).Someanalyses,reporting increases in fire severity, haveused vegetation data that post-dates thetimeseriesanalyzed(Malleketal.2013).However,thisapproachhasbeenfoundtocreatefalsetrendsofincreasingfiresever-ity,andthemostcomprehensiveanalysistodateindicatesfireseverityisnotincreasingintheSierraNevada(HansonandOdion2014).Thus,moreactivemanagedwildlandfire will be needed to effectively restorefiretotheseecosystems.

Concurrently,therateofoldforestgrowthgreatlyoutpacestherateofhigh-severityfireinoldforest(OdionandHanson2013).Consequently, old forest would remainheavily dominant on the landscape, spa-tially,evenifhigh-severityfirewasrestoredtothelevelsbeforefiresuppression(Odionand Hanson 2013). Complex early-seralforest created by high-severity fire sup-ports high levels of native biodiversityandmanywildlifespeciesthatarefoundpredominantly, or almost exclusively, inpost-fire landscapes (Hanson and North2008; Donato et al. 2009; Burnett et al.2010;Swansonetal.2011;DellaSalaetal.2014).Thus,restorationofmoreactivefireregimeswouldbenefitmanyspecies.

CONCLUSION

Ouranalysisunderscorestheimportanceofdatafromlargerspatialscaleswhenusinghistoricalsurveystodrawinferencesaboutlandscape-levelreferenceconditions,andalsohighlightstheproblemsthatcanresultduetoextrapolatingfromasmallnumberof plots (Collins et al. 2011; Lydersenet al. 2013) to infer desired landscape-level forest structure, composition, andfireregimes.OurresultsalsoindicatethatcurrentplansbytheUSForestServicetocreate,throughlogging,alandscapedomi-natedbyopenpineforestsmaintainedbylower-severityfirewouldresult innovel,

1� Natural Areas Journal Volume 36 (1), 2016

overlyhomogeneousconditionsthatcouldexacerbateriskstoCaliforniaSpottedOwlsandPacificFishers.

ACKNOWLEDGMENTS

Wethankthepeerreviewersandtheedi-torial staff of Natural Areas Journal fornumerous suggestions that improved themanuscriptsignificantly.WealsothankTimSinnottatGreenInfoNetworkfortheGISanalysistoproducethestudyareamap.

Chad Hanson, PhD, is the staff Ecologist and Director of the John Muir Project of Earth Island Institute. He has authored, and co-authored, papers published on topics as diverse as fire history, current fire pat-terns and trends, post-fire conifer response, and wildlife habitat selection in post-fire forests. Hanson focuses his research in the Sierra Nevada, with a particular emphasis on the Black-backed Woodpecker and the Pacific Fisher.

Dennis Odion, PhD, is a research ecolo-gist with the Department of Environmental Studies at Southern Oregon University and the Earth Research Institute at UC Santa Barbara. His research has focused on the role of variation in fire in shaping patterns of vegetation and biodiversity in forests and chaparral. He has studied mechanisms of nonnative species invasions and has au-thored, or coauthored, numerous papers on these subjects.

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