Assessing Quality in Volcanic Ash Soils

47USDA Forest Service Proceedings RMRS-P-44. 2007

In: Page-Dumroese, Deborah; Miller, Richard; Mital, Jim; McDaniel, Paul; Miller, Dan, tech. eds. 2007. Volcanic-Ash-Derived Forest Soils of the Inland Northwest: Properties and Implications for Management and Restoration. 9-10 November 2005; Coeur d’Alene, ID. Proceedings RMRS-P-44; Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station.

Terry L. Craigg, Soil Scientist, Deschutes National Forest, Sisters OR. Steven W. Howes, Regional Soil Scientist, U.S. Department of Agriculture, Forest Service, Pacific Northwest Region, Portland, OR.

AbstractForestmanagersmustunderstandhowchangesinsoilqualityresultingfromprojectimplementationaffectlong-termproductivityandwatershedhealth.Volcanicashsoilshaveuniquepropertiesthataffecttheirqualityandfunction;andwhichmaywarrantsoilqualitystandardsandassessmenttech-niquesthataredifferentfromothersoils.Wediscusstheconceptofsoilqualityandhowitmaybealteredbyactivity-inducedsoildisturbances.Apracticalapplicationoftheuseofsoildisturbancecategoriesandphysicalsoilindicesusedtoperformoperationalsoilmonitoringonanashcapsoilisalsodiscussed.

Introduction

Soil disturbance resulting from forest management activities is of concern to both landmanagersandthepublic.OnNationalForestSystemlandsintheNorthwestmanycurrentprojectsarebeingadministrativelyappealedorlitigatedbasedontheassumptiontheywillresultinsoildegradation.Someclaimthatwhenimplementinggrounddisturbingactivities,theForestServicecommonlyexceedsitsownstandardsintendedtomaintainsoilquality.There is aneed forobjectiveandaccuratemeasuresof change in soilqualitycausedbymanagementactivities.Assessingchange in soilquality isdifficult andcanbe influencedby a number of complicating factors. Properties of volcanic ash soils may also requiredevelopment and application of unique soil quality standards and assessment protocols.

Properties of Volcanic Ash Soils that May Affect Quality and Function

Many soils in the Inland Northwest have been influenced by ashfall deposits from theeruptionofMt.Mazama(nowCraterLake)aswellasotherCascadevolcanoes(HarwardandYoungblood1969).ThesedepositsarefoundinmostsoilsoftheColumbiaPlateauofcentralandeasternOregonandWashington.Soilsrangeintexturefrom“popcornpumice”

Assessing Quality in Volcanic Ash SoilsTerry L. Craigg and Steven W. Howes

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Craigg and Howes Assessing Quality in Volcanic Ash Soils

nearCraterLaketosandsandsiltsasdistancefromsourceincreases.Volcanicashsoilshavephysicalandchemicalpropertiesthataffecttheirqualityandfunction;andthatmakethemuniquewhencomparedtoothersoilparentmaterials.

AstudyconductedbyGeistandStrickler(1978)comparedsomephysicalandchemicalpropertiesofvolcanicashsoilstoresidualsoilsderivedfrombasaltintheBlueMountainsofnortheasternOregon.Insummary,volcanicashsoilshad:

• Lowerbulkdensity • Higherporosity • Weakerstructuraldevelopment • Lowercohesion • Lowercoarsefragment(>2mm)content

SuchpropertiesaredescribedindetailbyothersintheseProceedings.Thispaperfocusesonsoilswithadistinct“ashcap”orsurfacelayerofvolca-

nicashdeposits.Suchdepositsvaryfromafewcentimeterstooverameterindepthdependingondistance fromthesource, topography,windpatternsandsubsequenterosion.

Volcanicashsoilshavearelativelyuniformundisturbedsurfacesoilbulkden-sity(0.67)andadisproportionateconcentrationofnutrients(totalOM,N,K)intheupper30cmofthesoilprofile.Thesepropertiestendtomakevolcanicashsoilsmoresusceptibletosurface(sheet,rill,wind)erosionwhenexposed;andtodisplacementbyground-basedequipment.Theystoremorewaterbutyieldalargerproportionofitinthelowsuctionranges.Volcanicashsoilsalsoseemtocompacttothesamedegreeoverawide-rangeofmoisturecontents.

Pumicesoilshavemanyof thesamepropertiesasothervolcanicashsoils.Likeothervolcanicashsoilsthelowdensity,lackofcohesion,andconcentra-tionofsoilnutrientsintheuppersoilprofileofpumicesoils,makepumicesoilssusceptibletosoilcompactionandthelossofnutrientsthroughsoildisplacement.Becauseoftheirlowcohesionbetweenindividualpumiceparticles,compactioninpumicesoilscanresultfrombothcompressionandvibration.Pumicesoilsalsohavebeenfoundtobemostsusceptibletocompactionwhentheyareverydryoratornearsaturation.Soilmoisturecontentsbetweenthesetwoextremesap-peartoprovidesomecohesionbetweensoilparticlesreducingsusceptibilitytocompaction.Amountsofsoilorganicmatterisalsoimportantwhendeterminingtheirsusceptibilitytocompaction.

Whencompactiondoesoccur,soilstrength(asmeasuredbyresistancetopen-etration)hasbeenfoundtoincreaseexponentiallywithanincreaseinsoilbulkdensity(Chitwood1994).Incompactedsoilssoilstrengthhasalsobeenshowntoincreasequicklyasthesoildriesoutoverthegrowingseason(Craigg2006).Thusmeasurementsofsoilstrengthandsoilbulkdensityareprovidingdifferentinformationaboutchangesoccurringinthesoilwhencompactionoccurs.Sincethereisnotasimplelinearrelationshipbetweenmeasurementsofsoilstrengthandsoilbulkdensitywhensoilconditionschangeitwouldbedifficulttoreadilysubstituteonemeasurementfortheother.

Considerationoftheseproperties,alongwithash/pumicedepthandunderlyingsoilmaterial, is important indeveloping soilmanagementobjectives/prescrip-tions,inevaluatingpotentialimpactsofproposedactivities,andinestablishingmeaningfulsoilqualitystandards.

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Assessing Quality in Volcanic Ash Soils Craigg and Howes

Soil Disturbance and its Relationship to Soil Quality ________________Definitionsofsoilqualitydiffergreatlydependingonwhomoneasks.Most

wouldsaytheyrecognizeahighqualitysoil“when they see it”butwhenaskedtodefinesoilquality,eachwouldmostlikelyprovideadifferentanswerdependingontheirindividualobjectivesandpersonalbiases(Karlenandothers1997).Thequalityofanindividualsoilmayvaryfordifferingpurposes:ahighqualitysoilforproductionoftreesorforagemayhavelowqualityforproductionofcottonorwheat.Tosaythatonesoilisofhigherqualityor“better”thananotherisnotproperwithoutqualification.

Therearesomewhousetheterms“soilquality”and“soilhealth”interchange-ably(Warkentin1995).Eachsoilhasinherentphysical,chemical,andbiologicalpropertiesthataffectitsabilitytofunction asamediumforplantgrowth,toregulateandpartitionwaterflow,ortoserveasaneffectiveenvironmentalfilter.Whenanyoracombinationoftheseinherentfactorsisalteredtoapointwhereasoilcannolongerfunctionatitsmaximumpotentialforanyofthesepurposes,thenitsqualityorhealthissaidtobereducedorimpaired(LarsonandPierce1991).

Manyfactorsenterintomakingdeterminationsofsoilquality.Someofthesefactorscanbechangedbydisturbance (LarsonandPierce1991;Seyboldandothers1997;Grossmanandothers2001).Alterationofsoilproperties throughdisturbancemechanismsisnotnecessarilyharmful.Tillageisanextremeformofsoildisturbanceoftenusedtoimprovesoilqualityor“tilth”forproductionofagriculturalcrops.Inforestryapplications,sitepreparationtoimproveregenera-tionsuccessorsoilrestorationworkofteninvolvesoildisturbancetosomedegree(surface soilexposure, scalping, subsoiling).What sets theseapart fromotherformsofsoildisturbanceisthattheyareplannedandare,orshouldbe,carriedoutwithspecificobjectivesinmindunderprescribedconditions.

Managementactivitiesonforestandrangelandsoftenresultinvariousformsofdisturbancethatcanaltersoilfunction.Commonly,landmanagersareconcernedabouthowchangesinsoilqualityorfunctionresultingfromunplanned,activity-induceddisturbanceaffectthelong-termproductivityandhydrologicresponseofwatersheds.Inwildlandsituations,concernusuallyfocusesondisturbance-relateddeparturesfromnaturalsoilconditions(consideredtobemaximumpotentialorbestpossibleconditions),occurringaseitherlegacyimpactsorthosecausedbycurrentmanagementactivities.Understandably,therearedifferencesofopinionamong soil scientists and landmanagers aboutwhenandwhere “significant”thresholdsarereached.Thisiscomplicatedbythefactthatsoilsdifferintheirabilitytoresistandrecoverfromdisturbanceeffects(Seyboldandothers1999).

Disturbance may have negative, neutral, or even positive effects on a soildependingonitsinherentproperties.Meaningfulsoildisturbancestandardsorobjectivesmustbebasedonmeasuredanddocumentedrelationshipsbetweenthedegreeofsoildisturbanceandsubsequenttreegrowth,forageyield,orsedi-mentproduction.Studiesdesignedtodeterminetheserelationshipsarecommonlycarriedoutaspartofcontrolledandreplicatedresearchprojects.Thepaucityofsuchinformationhascausedproblemsindeterminingthresholdlevelsfor,ordefiningwhen,detrimentalsoildisturbanceexists;andindetermininghowmuchdisturbancecanbetoleratedonagivenareaoflandbeforeunacceptablechangesinsoilfunction(productivepotentialorhydrologicresponse)occur.Givennaturalvariabilityofsoilpropertiesacross the landscape,asinglesetofstandards forassessingdetrimentaldisturbanceseemsinappropriate.

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Defining Detrimental Soil Disturbance _____________________________Soildisturbanceexistsinanumberofforms.Inforest,range,andwatershed

managementapplications,itmostcommonlyoccursascompaction,displacement,andpuddlingresultingfromuseofground-basedharvestingandslashdisposalequipment.Theremovalofabovegroundorganicmatterisalsoaresultofthistypeofdisturbance.Soildisturbancecanalsooccurintheformofsheetandrillerosionorcharredsoilinintenselyburnedareas.

Effectsofsoildisturbanceonsiteproductivityorhydrologicfunctionofwa-tershedsisdependentonitsdegree,extent,distribution,andduration(Froehlich1976;SniderandMiller1985;Claytonandothers1987).Degree refers to theamountofchangeinaparticularsoilpropertysuchasporosity,bulkdensity,orstrengthandthedepthtowhichthatchangeoccurs.Extent referstotheamountoflandsurfaceoccupiedbythatchangeexpressedasapercentage of a specified area.Distributionofsoildisturbancewithinamanagementareamaylikelybemoreimportantthantheactualestimatedextent.Disturbancecanoccurassmallevenly-distributedpolygons,orinlargepolygonsinoneorafewlocations.Dura-tionisthelengthoftimedisturbanceeffectspersist.Forexample,compactioneffectshavebeenmeasuredover30yearsfollowingtimberharvestactivitiesonsomesoils(Froehlichandothers1985).

Degreeanddurationofeffectsarelargelydeterminedbyinherentsoilproper-tiesthatinfluenceresistanceto,andabilitytorecoverfrom,disturbance.Extent,distribution,and,insomeinstances,degreeofdisturbancecanbecontrolledbyimposingmanagementconstraintssuchaslimitingseasonofoperation,spacingofskidroadsandtrails,andnumberofequipmentpasses.

Achallengehasbeentoestablishmeaningfulsoilqualitystandardsgiventheinherentvariabilityofsoilsacrossthelandscape.Simplystatingasinglethresholdbeyondwhichdetrimentalsoilconditionsarethoughttoexistmaynotbeenough.Mostforestlandmanagersdonotwishtodamagetheirproductivebaseorcauseimpairedwatershedfunction.However,theyalsodonotwanttounnecessarilylimitmanagementopportunities.

Current Status and Needs ________________________________________Soilqualitycanbeimproved,maintained,ordegradedbyforestmanagement

activities.Inordertoidentifyandquantifysuchchanges,ascientificallyrigorousprotocolisrequired.Itmustquantifytypesandamountsofsoildisturbanceswithinaspecifiedarea;andidentifyareasofdisturbanceconsideredtobedetrimental.Protocolsmustbe:

• Statisticallyvalid • Relativelyeasytoimplement • Costeffective • Usableatdifferentscales • Abletodetermineifsoilqualityisbeingmaintained,improvedordegraded.

Numerousmethodsexistforsamplingsoildisturbanceinbothoperationalandresearchsettings.Methodsdifferwithrespecttosamplingobjectives,soilvariablesconsidered,andassessmentprotocols.Inconsistentapplicationofsoildisturbancemeasurementtechniquesacrossavarietyoflandownershipshasled,insomecases,tounreliableandincomparableresults.Moreeffectiveandefficient(cost,

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utilityofdata) soildisturbanceassessmentandmonitoringprogramscouldbeachievedthroughuseofcommonsoildisturbanceclassdefinitionsandconsistentuseofstatisticallyreliablesamplingprotocols(Curranandothers2005).

Definedsoildisturbancecategoriesmustberelatedtophysicalindicesofsoilqualityanddocumentedchangesinvegetationgrowthorhydrologicfunction.Adeterminationneedstobemadeif,infactoratwhatpoint,thesedisturbancecat-egoriesrepresent“detrimental”changesinsoilfunction.Inordertoevaluateimpactsofforestmanagementactivitiesonsoilquality,threedistinctlevelsofinformationareneeded.Thefirsttwoarecommonlygeneratedthroughagencyorcompanysoilmonitoringprograms.Thelastoneisprimarilyaresearchfunction.

Initialassessmentscanbeassimpleasdeterminingifspecifiedsoilconserva-tionorbestmanagementpractices(BMP)havebeenimplementedasplannedorifcontractual/legalrequirementsrelatingtosoildisturbancehavebeenmet.Thisisoftenreferredtoascomplianceorimplementationmonitoring.

After the initial compliance monitoring, soil disturbance assessment andmonitoringprojectsarecommonlydonetoevaluateeffectivenessofmanagementpractices.Theseassessmentsevaluatepre-determined,usuallyprovisional,soildisturbancestandardsorobjectivesbasedonbestavailableknowledge.Thisiscommonlyreferredtoasoperationaloreffectivenessmonitoring.

Soildisturbancestandardsorobjectivesmustalsobe testedorvalidated todetermineiftheyresultinunacceptablereductionsinproductivityorwatershedfunction,areappropriateforlocalsiteconditions,orifadjustmentsareneeded.Thisprocess isusually referred toasvalidationmonitoring.Validation isbestaccomplishedinaresearchenvironmentundercarefullycontrolledconditions.Ideally, research would be completed before standards were developed andimplemented.

Cost-effectiveapproachesareneededforoperationalsoildisturbanceassess-ments or monitoring that provides statistically valid and scientifically relevantdata.Monitoringmustmeetspecifiedquality-controlstandardsandcontributetoregionalstrategicdatabasesrelatingtreegrowthorhydrologicfunctiontodefinedclassesofdisturbance.Ourcurrentabilitytocompareresultsfromoperationaland research studies requiresmore commonor standardized soil disturbancedefinitionsandassessmentprotocols.

Tobecost-effective,operationalsoildisturbancemonitoringprotocolsmustmeet several criteria. Protocols (1) must provide scientifically and technicallysoundinformation,(2)mustbereliable,pertinent,andobtainedwithminimuminvestments,(3)mustbeclearlycommunicatedandunderstoodbyallaffectedparties,and (4)mustbeconsistentlyandefficiently implemented (Curranandothers2005).

USDA Forest Service Approach to Operational Soil Monitoring _______InForestServiceRegion6,initialassessmentsofactivity-inducedchangesin

soilqualityaretypicallymadethroughastratificationprocessinwhichvisuallyrecognizablesoildisturbancecategoriesanddisturbancesthatcanbeeasilyde-tectedbyprobingthesoilaredescribedandquantifiedwithinanactivityarea.Thisiscommonlyaccomplishedusingapointgridsystemincombinationwithpointobservationsalongaseriesofrandomlyorientedtransects.ThisprocessisdescribedindetailbyHowesandothers(1983).

Morerecently,pointobservationsarebeingreplacedbyvisualsoildisturbancecategories, developed and used to describe areas with a combination of soil



disturbancesthatrepeatacrossanactivityareaandwhicharelikelylargeenoughinextentanddistributiontoaffectsoilfunction.Visualcategoriesalsoaccountforthefactthatvariousformsofdisturbancearenotmutuallyexclusive.Forex-ample,compactionanddisplacementoftenoccurtogether.AninterimprotocolofthisprocessthatisbasedonproceduresdescribedbyScottandothers(1979)iscurrentlybeingusedonnationalforestsintheBlueMountains.

Theprocessofquantifyingdifferentsoildisturbancecategoriesacrossaland-scapeprovidesarelativelyrapidandinexpensivemethodforquantifyingdiffer-enttypesofsoildisturbancesoverlargeacreagesandisrecognizedbytheFSasanessentialstepinthesoilassessmentprocess.This process, however, is often stopped at this point and the incorrect assumption made that any soil distur-bance is detrimental.

Toavoidthispotentialproblem,visualsoildisturbancecategoriesmustbefurtherevaluatedtodetermineiftheydo,infact,adequatelyrepresentdetrimentalsoilconditions.Thefirststepinevaluatingadisturbancecategoryistoidentifythesoilfunction(s)thatmaybenegativelyaffectedbyasoildisturbance.Forexample,ifsurfacesoilshavebeendisplaced,asoil’sabilitytosupplynutrientsmaybereduced.Incompactedskidroadsandlandings,changesinsoilresistancetorootpenetrationaswellasairandwatermovementinthesoilmaybethemostimpor-tantconsideration.Identificationofsoilfunction(s)affectedbydisturbanceallowsfortheselectionofthemostappropriatesoilindicatorswhichcanthenbeusedtoassessachangeintheabilityofthesoiltofunction.Caremustbetakenwhendevelopingandinterpretingsuchindices.Environmentalfactorssuchasmacro-andmicro-climatecanmaskchangesinsoilfunctionduetodisturbance.

Soilindicatorsorindicesaresoilparametersthatcanbequantitativelymea-suredusingstandardizedmethodsandcomparedbetweendisturbedandundis-turbedsites.OnNationalForestSystemlandsintheNorthwest,soildisturbanceisconsideredtobedetrimentalwhenasoilindexiseitherhigherorlowerthanadefinedthreshold.

Development and Application of Forest Service Soil Quality Standards ______________________________________________________

TheUSDAForestServiceisdirectedbylawtosustainproductivityatcurrentorenhancedlevelsandtoprotectandimprovethesoilsforwhichitisresponsible.Recognizingthisresponsibility,thePacificNorthwestRegion(R6)issuedaForestServiceManualSupplementdealingwithsoilproductivityprotection in1979.Ithasbeen revisedanumberof times since then (current versionFSM2520.R-6Supplement2500.98-1,EffectiveAugust24,1998).Thisdirectionspecifiesthresholdvalues(degree)forestimatingwhendetrimentalsoildisturbanceoccursandalsosetsarealimits(extent)fordetrimentalsoildisturbanceonanactivityareabasis.MostnationalforestswithinR6haveincorporatedthesevalues,withminormodifications,asstandardsintheirforestlandandresourcemanagementplans.OtherForestServiceRegionssubsequentlydevelopedsimilarpolicyanddirection (Powersandothers1998).Thresholds forestimatingdetrimentalsoilcompaction,displacement,puddling,andseverelyburnedsoilsweredevelopedbasedonapplicableresearchresultsavailableatthetime.

Standards emphasize both observable and measurable soil characteristicsthat field personnel can use to monitor effectiveness of activities in meeting

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soilmanagementobjectives.Allformsofdetrimentalsoildisturbance,includingpermanentfeaturesofthetransportationsystemsuchasroadsandlandings,arelimitedinextenttonomorethan20percentofanactivityarea.Inactivityareasthatexceed thisextent limiteitherasaresultofpreviousorcurrentactivities,restorationplansmustbeinplacebeforenewprojectsareimplemented.These standards are applied to all soils regardless of their inherent properties and ability to resist or recover from disturbance. Theonly special considerationgivenvolcanicashsoilsisanallowable20percentincreaseinsoilbulkdensitybeforetheyareconsidereddetrimentallycompacted.Thereasonforthisistheirrelativelylowundisturbedbulkdensitieswhencomparedtosoilsdevelopedinotherparentmaterials.

Application of Operational Soil Monitoring Techniques _____________Thefollowingisanexampleoftheuseofanoperationalsoilmonitoringproce-

durethatwasusedtoevaluatetheeffectsofaforestthinningoperationonthesoilresource.Theprocedureillustratestheapplicationofsoildisturbancecategoriesandphysicalsoilindices.

Overview

Duringthewinterof2005,theSistersRangerDistrictoftheDeschutesNationalForest in central Oregon implemented a thinning project designed to reducehazardousfuelsinpinestandswithinthewildlandurbaninterface.ATimberjackcut-to-lengthharvesterandaTimberjackforwarderwereusedtoremovetrees12inchesindiameterandsmaller,resultinginastandwithirregulartreespacingofabout20x20ft.

Projectobjectivesincluded:

• ReduceriskofwildfiretothenearbycommunityofBlackButteRanch. • Promoteresidualtreegrowthandmovestandtowardlargerdiametertrees. • Produceabiomassproductthatcouldbeusedtohelpoffsetthecostofthe

thinningoperation.

ThinningoperationsoccurredduringFebruaryandMarchof2005.Duringmuchofthistime,thesurface2to4inchesofsoilwasfrozen.Therewere,however,timeswhentemperaturesroseandsoilsweremoistbutnotfrozen.Severalsnowstormsalsooccurredduringthisperiod,depositingfromafewinchestooverafootofsnow.Soilconditionswerefavorableformostofthetwomonthperiodduringwhichoperationsoccurred.

Soilfunctionspotentiallyaffectedbytheprojectincluded:

• Abilityofthesoiltosupplywaterandnutrientstoresidualtrees. • Abilityofthesoiltoabsorbandtransmitwater. • Ability of the soil to resist other detrimental soil impacts including

displacement,puddling,severeburning,andsurfaceerosion.

Mitigationmeasuresused to reducepotentiallydetrimental soil impacts in-cludedoperatingoverfrozenground,snow,andslash;useofharvestequipmentdesignedtohavelowgroundpressure;designatingspacingofequipmenttrails;andhandpilingslash.



Forestmanagerswishedtoknowifthesepracticeswereeffectiveinmeetingsoilqualityobjectivesonthisandothersimilarprojects.Inthiscase,managerswished toknow if soildisturbance resulting from theabovesuiteofpracticesexceeded20percentoftheactivityarea.Theyalsowishedtoknowifdifferentcategoriesofdisturbancecontaineddetrimentalsoilimpactsasdefinedin(FSM2520.R-6Supplement2500.98-1).

Project Area Soils

SoilsintheprojectareaweremappedaspartoftheSoilSurveyoftheUpperDeschutesRiverArea,Oregon(NRCS2002).TheyaredescribedastheSisters-Yapoahcomplex0-15percentslopes.Soilsareclassifiedas:

Sisterssoilseries–Ashyoverloamy,mixed,frigidHumicVitrixerandsYapoahsoilseries–Ashy-skeletal,frigidHumicVitrixerands

Harvester/Forwarder Transportation System

TheTimberjackcut-to-lengthharvesterusedinthisoperationwasequippedwithacuttingheadmountedona30footboom.Thisallowedtheharvestertocutandprocessmaterialswhilemakingparallelpassesacrosstheharvestunitataspacingofapproximately60feet.Harvestedmaterialsarepositionedsotheycanbereachedfromalternateharvestertrailsbytheforwardermachine.Thisresultsintwotypesoftrailswithintheharvestunit(1)thosethatbeendrivenacrossonlyonetimebytheharvester(ghosttrails)and(2)trailsthathavebeendrivenacrossbytheharvesterfollowedbytheforwarder(harvester-forwardertrails).Becausetreesarelimbedandtoppedatthetimeoffellingtherearenolandingswithintheharvestunit.Oncetheforwarderhascollectedharvestedmaterialstheyarepilednexttoahaulroadpriortoloadingonlogtrucks.

Description of Visual Soil Disturbance Categories

Acombinationofvisualobservationsandprobingofthesoilwithatilespadeormetalrodwasusedtoidentifyanddescribefourcategoriesofsoildisturbancewithinactivityareas.Thesecategoriesare:

Condition Class 1:Undisturbedstate,natural. • Noevidenceofpastequipmentoperation. • Nowheeltracksordepressionsevident. • Litteranddufflayersintact. • Nosoildisplacementevident.

Condition Class 2:Trailsusedbytheharvestermachineonly.Ghost trails. • TwotracktrailscreatedbyonepassofaTimberjack cut-to-lengthharvester. • Faintwheeltrackswithaslightdepressionevidentandare<4inchesdeep. • Litteranddufflayersintact. • Surface soil has not been displaced and shows minimal mixing with

subsoil.

Condition Class 3:Trailsusedbybothharvesterandforwarder. • TwotracktrailscreatedbyoneormorepassesofaTimberjackcut-to-length

harvesterandoneormorepassesofaTimberjackforwarder.



• Wheel tracksevidentandare4 to6 inchesdeepwith theexceptionofareaswhereoperatorwasabletolaydownenoughslashtomitigatesoilimpacts.

• Litteranddufflayersarepartiallyintactormissing.

Condition Class 4:Skidtrailsfrompreviousentries.Allwerere-usedduringcurrententry. • Oldskidtrailscreatedintheearlypartof20thcenturywhenselectivehar-

vestoccurred. • Trailsappeartohavehighlevelsofsoilcompactionacrosstheentiretrail. • Evidenceoftopsoilremoval.

Soil Functions Affected by Soil Disturbances

Based on visual observations described above, it was determined that soilcompactionwasthemostprevalentformofsoildisturbancewithintheharvestedarea.Whencompactionoccurs,therearealterationsofbasicsoilpropertiessuchassoildensity,totalporevolume,poresizedistribution,macroporecontinuity,andsoilstrength(GreacenandSands1980).Soilsvaryintheirsusceptibilitytocompaction(Seyboldandothers1999).Onceasoilbecomescompacted,theconditioncanpersistfordecades(Froehlichandothers1985).This,inturn,af-fectssoilfunction.

Anumberofresearchershavemeasuredreductionsinsiteproductivityattributedtosoilcompaction(Froehlichandothers1986;CochranandBrock1985;HelmsandHipken1986).Effectsofsoilcompactiononsiteproductivity,however,arenotuniversallynegative.Gomezandothers(2002)lookedatarangeofforestsoiltypesinCaliforniaandfoundcompactiontobedetrimental,neutral,orbeneficialdependingonsoiltextureandwaterregime.

Quantifying Visual Soil Disturbance within an Activity Area

Toassistwithquantifying theamountof soilcompaction,a recordingpen-etrometerwasusedtodeterminetheaveragewidthofcompactedsoilsacrossboththeghosttrailsandtheharvester-forwardertrails(figs.1and2).Athresholdlevelfortheincreasedsoilstrengthof2.5MPawasusedtoidentifycompactedareas.Soilstrengthsbeyondthislevelwereconsideredtobehighenoughtobegintoinhibitplantrootgrowth.Basedonresultsharvesterghosttrailswereinitiallyconsideredtoconsistoftwodetrimentallycompactedtracks,eachapproximatelythreefeetinwidth.Harvesterforwardertrailswereconsideredtoconsistoftwodetrimentallycompactedtracks,eachapproximatelyfourfeetwide.

Arandomlyorientedsquaregridwithonegridintersectioneverytwoacreswasnextoverlainontheactivityareaandusedtolocatesamplepointsfordetermin-ingarealextentofsoildisturbancecategories(fig.3).Ateachofthegridinter-sections,arandomlyoriented,100-fttransectwasestablished.Thefourdefineddisturbancecategoriesweremeasuredalongeachtransectandlengthsoccupiedbyeachcategoryrecorded.Ameanforeachcategorywasthencomputedfortheentireactivityarea.Samplinginthismannerensuredanunbiased,representativesample.Itwasdeterminedthat17percentoftheactivityareawasoccupiedbysoildisturbance(totalofcategories2-4).A95percentconfidenceintervalaroundthisestimatewascalculatedtobeplusorminus2percent.

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Figure 1—Average soil resistance measured for the 10 to 2� cm soil depth. Measurements were made perpendicular to the harvester ghost trails.

Figure 2—Average soil resistance measured for the 10 to 2� cm soil depth. Measurements were made perpendicular to the harvester forwarder trails.

Use of Soil Indices to Interpret Visual Soil Disturbance Categories

ThreephysicalsoilindicesusedbytheForestServicetoassesschangesinsoilphysicalpropertiesattributedtocompactionareincreasesinsoilbulkdensity,increases in soil strength (resistance topenetration),andchanges in soilporesizedistribution.

Soil Bulk Density

Soilbulkdensityisdefinedasthemassperunitvolumeofsoilandrepresentstheratioofthemassofsolidstothetotalorbulkvolumeofthesoil(SoilSurveyStaff1996).ThedirectmeasurementofanincreaseinsoilbulkdensityfromsoilcompactionrequiresaminimumamountofsamplingequipmentandisthemostcommonlymademeasurementofsoilcompactionusedbytheForestService.Physical factors that affect soil bulk density and compactability include soil



Figure 3—Example of a pre-established grid (randomly oriented) with transects oriented along random azimuths.

particlesizesanddensity,organicmattercontent,andmineralogy(Howardandothers1981).

Soilbulkdensitysampleswerecollectedinthespringoftheyearusingaham-mer-drivensoilcoresampler.Table1showssoilbulkdensitiesreportedforboththewholesoilandfor thefinefractionofsoil (soilparticles<2mmremoved).DeterminingsoilBDbasedonthefinefractionofsoilresultsinalowerBDvaluecomparedtothatdeterminedonawholesoilbasis.Thisisduetothehigherweighttovolumeratioofsoilcoarsefragmentsinthesoilcore,comparedtoanequalvolumeofsoilmaterial.Whendeterminedonawholesoilbasis,soilcompactionresultedinasignificantincreaseinsoilbulkdensityinboththeghosttrailsandtheforwardertrails.Soilbulkdensitiesdeterminedforthefinefractionsofsoilweresignificantlygreaterintheforwardertrailsbutnotintheghosttrails.

Table 1—Soil bulk density measurements determined based on the whole soil (including soil coarse fragments >2mm) and based on the fine fraction of soil.

% Volume Soil Soil Bulk Density Soil Bulk DensitySoil Condition Class Coarse Fragments Whole Soil (Mg/m3) Fine Fraction Soil (Mg/m3)

1 1 0.9�a 0.92a 2 2 1.02b 1.00ab � 2 1.0�b 1.0�b Columns within a soil bulk density analysis method are not significant at p = 0.05 if followed by the same letter. n = 3



Inthiscase,fewcoarsefragmentswereencounteredandtherewaslittledif-ferenceinsoilBDbetweenresultsobtainedusingcoreswithandwithoutcoarsefragments.Iftherehadbeenmorecoarsefragmentsincoresoralargevariationincoarse fragmentsbetweencores, itwouldhavehadagreater influenceoncalculatedsoilbulkdensitiesandonthewayinwhichthesoilfunctions.Tocor-rectlyinterpretsoilbulkdensitymeasurements,itisnecessarytoreportsoilbulkdensitiesforwholesoilandthefinefractionofsoil.Itisalsonecessarytoknowtheamountsofcoarsefragmentsinthesoilcores.

ForestServicesoilqualitystandardsidentifyachangeinsoilbulkdensityof20percentormoreaboveundisturbedlevelsasathresholdfordeterminingdetrimentalcompactioninAndisols(soilsderivedfromvolcanicash).Whencalculatedonbothawholesoilandfinefractionofsoilbasis,nomeasuredincreasesinsoilbulkdensityexceededthe20percentrelativeincreasethreshold.Therefore,noneofthesoilbulkdensitychangesinanyofthevisualsoildisturbancecategoriesmettheForestServicedefinitionofadetrimentallycompactedsoiltable2.

Soil Strength

Soilstrengthdescribesthesoilhardnessorthesoilsresistancetopenetration.Soilprobesandspadescanbeusedtodetectchangesinsoilstrengthresultingfromsoilcompaction.Thistechniquecanbequantifiedbyusingarecordingsoilpenetrometer.Measuredsoilstrengthscanvarydependingonsoilparticlesizedistributionandshape,clayandorganicmattercontent(ByrdandCassel1980).Withinasoiltypechangesinsoilwatercontentandstructurecanalsoaffectsoilstrength(Gerard1965).

Changesinsoilstrengthresultingfromsoilcompactionweremeasuredwithindifferentdisturbancecategoriesusingarecordingsoilconepenetrometer,whichiscapableofrecording,atpredeterminedintervals, theforcerequiredtopushaconeintotheground(Millerandothers2001).Measurementsweremadeinearlyspring,shortlyafterharvestoperationswerecomplete.Fieldmeasurementsofgravimetricsoilmoisture,basedonthefinefractionofsoil,rangedfrom0.19to0.22g/gandwereslightlylessthanthe0.23g/gestimateofsoilfieldcapacitythatwasobtainedfromsoilcoresamplesusedtodeterminesoilporesizedistri-bution.Thepenetrometerwassettorecordsoilresistanceat1.5cmincrementsbetween0and60cmsoildepth.ReadingswerethendownloadedtoaMicrosoftExcelSpreadsheetforanalysis.

Table 2—Absolute and relative percentage increase in soil bulk density as a result of soil compaction.

Absolute Increase Relative % Soil Bulk Density Soil Bulk in Soil Bulk Increase in Soil Condition Class Determination Method Density (Mg/m3) Density (Mg/m3) Soil Bulk Density

1 Whole Soil 0.9� 2 Whole Soil 1.02 0.09 10 � Whole Soil 1.0� 0.1� 1�

1 Fine Fraction 0.92 2 Fine Fraction 1.00 0.0� 9 � Fine Fraction 1.0� 0.14 1�



Soiltextureandseasonalchangesinsoilmoisturecangreatlyinfluencesoilstrengthmeasurements.Resistancetopenetrationhasbeenshowntoincreaseforbothuncompactedandcompactedsoilsas theydryoutover thegrowingseason.Theincreaseinresistancetopenetrationwithdecreasingsoilmoisture,however,tendstooccurmorerapidlyincompactedsoilsthaninuncompactedsoils(Craigg2006).Therefore,soilmoisturerelationshipsshouldbeconsideredwheninterpretingsoilresistancemeasurements.

Measurementsofsoilresistanceintheundisturbedareasgraduallyincreasedwithsoildepthreachingaresistanceof1MPainthefirst10cmandafinalsoilresistanceof2MPaatapproximately60cmsoildepth.Intheforwardertrailssoilresistanceincreasedmuchfaster,reachingaresistanceof3MPawithinthefirst10cmsoildepthandretainingthathighstrengththroughoutthe60cmdepth.Increasesinsoilresistanceintheghosttrailswerehigherthantheundisturbedbutlessthantheforwardertrails(fig.4).Duringthethinningoperationeffortsweremadetolimbharvestedtreesontheequipmenttrailsandthendriveontheslashmattomitigatesoilcompaction.Soilpenetrometermeasurementsindicatedthattheslashmatdidhaveatleastsomeaffectonmitigatingsoilcompaction(fig.5).

Currently,ForestServiceRegion6doesnothaveasoilqualityindexthresh-oldforincreasesinsoilresistanceresultingfromcompaction.Researchresultssuggestthatsoilresistancesof2MPaandgreatercanbegintoaffectplantrootgrowth(Siegel-Issemandothers2005).Thustheseresultssuggestdifferencesinsoilfunctionbetweenthedifferentsoilconditionclasses,inparticulartheabilityofrootstopenetratecompactedsoils.

Figure 4—Soil strength measured in the spring of 200� for different soil disturbance classes. Horizontal bars indicate one standard error. n = 30.



Soil Pore Size Distribution

Soilporespaceaccountsforapproximatelyhalfofthetotalvolumeofmineralsoil.Changesintotalsoilporosityandporesizedistributionthatmayresultfromsoilcompactioncanaffectsoilfunctionsbyalteringairandwaterflowintoandthroughthesoil.Changesinsoilporositycanalsoaffecttheamountsofwaterstorageinthesoil(Scott2000).

Measuringafullrangeofsoilporesizesrequiresrelativelysophisticatedlabora-toryequipmentcomparedtothatneededformeasuringsoilbulkdensityorsoilstrength.Althoughtotalsoilporositycanbeestimatedfrombulkdensityandsoilparticledensity,thiscalculationprovidesnoinformationaboutporesizedistri-butionchangesoccurringinthesoil.Tobetterunderstandchangesinsoilporesizedistributionresultingfromsoilcompaction,arelativelysimplemethodwasusedwhichdoesnotrequiretheuseofsophisticatedlaboratoryequipment.Thistechniquemeasuresmainlytheporositychangesoccurringinthelargersoilporesizes(soilpores>30μm).

Intactsoilcoresformeasuringsoilporesizedistributionwerecollectedusingthesamehammer-drivencoresamplerusedtosamplesoilbulkdensity.Follow-ingcollectionofintactsoilcores,soilporesizedistributionwasdeterminedus-ingamodificationofthewaterdesorptionmethoddescribedin(DanielsonandSutherland1986).Briefly,ahangingwatercolumnwascreatedbyconnectingaBuchnerfunnelwithaporousceramicplateinthebottomtoapproximately3metersofplastictubingwithaburetteconnectedtotheotherend.Asoilcorewas thenplaced in the funnelandsaturatedwithwaterbyraising theburette

Figure 5—Soil strength measured in the spring of 200� for different soil dis-turbance classes with measurements on harvester forwarder trails separated depending upon whether or not slash was present. Horizontal bars indicate one standard error. n = 30 (undisturbed and ghost trails)n = 15 (forwarder trails)



andcreatingaslightheadofwater.Aseriesofsuctionswerenextappliedtothesaturatedsoilcorebyraisingthefunnelaboveapredeterminedmarkonthebu-rette.Volumesofwaterremovedfromthesoilcoreswererecordedforaseriesofappliedsuctionsandtheequivalentradiusofthelargestsoilporesfilledwithwaterdeterminedbythecapillaryequation(Scott2000).

Resultsindicatedashiftinsoilporesizedistributionfromlargerporesizesormacropores(soilpores>30μm)tosmallerporesizes(soilpores<30μm)whensoilsbecamecompacted(fig.6).Whilesoilcompactiondidresultinasignificantdecreaseinmacroporesandasignificantincreaseinsmallersoilporesizes,therewasnotasignificantchangeintotalsoilporositybetweenthedifferentdisturbancecategories(table3).AlthoughstandardshavebeenestablishedinForestServiceRegion6formacroporespacereductionsinresidualsoils,nonehaveyetbeenestablishedforash-derivedsoils.

Figure 6—Shift in soil pore size distributions for different soil disturbance classes following soil compaction.

Table 3—Changes in soil pore size distribution and total soil porosity resulting from soil compaction.

Pore volume Pore volume Total pore volumeSoil condition Class >30µm (cm3/cm3) <30µm (cm3/cm3) (cm3/cm3)

1 0.��a 0.24a 0.�9a 2 0.29b 0.27b 0.��a � 0.2�b 0.�0c 0.��a n = 3 Columns within a pore size class are not significant at p = 0.05 if followed by the same letter.



Interpretation of Results

Extentofsoildisturbancewithintheprojectareawasinitiallydeterminedbyfirstdescribingandquantifyingvisualsoildisturbancecategoriesandthenmea-suringasuiteofindiceswithineachcategory.Soilindexmeasurementsthatweremadewithindisturbedareaswerethencomparedtothosemeasurementsmadeinundisturbedareas.Degreeofchangeinspecifiedsoilindiceswasnextcomparedtodefinedthresholdsfordeterminingifthechangeisconsidered“detrimental”or“undesirable.”

AllmeasuredincreasesinsoilbulkdensitywerebelowthresholdvaluesdefinedbythePacificNorthwestRegionforsoilsderivedfromvolcanicash.Althoughin-creasesinsoilstrengthweremeasuredforghosttrailsandforharvester-forwardertrails,nothresholdvalueorsoilqualitystandardhasyetbeenestablishedinthePacificNorthwestRegionforsoilstrength.Reductionsinmacroporespacewerealsomeasured in ghost trails and forharvester-forwarder trails. Forest ServiceRegion6doesnotcurrentlyhaveasoilqualityindexthresholdestablishedformacroporespacereductionsinash-derivedsoils.Soilstrengthandreductionsinmacroporespaceappeartobereliableandeasilymeasurableindicesofsoilfunc-tion.Theyalsoprovideadditionalinformationaboutchangesoccurringinthesoilwhencompactionoccurs.Validationworkneedstobecompletedtodetermineappropriatethresholdsforeachoftheseindices.

Summary ______________________________________________________Soilqualitycanbedefined inanumberofwaysdependingonone’spoint

of view. For forest practitioners, management-induced changes in soil qualityareofconcernwhensiteproductivity ismeasurably reducedorwaterqualityisimpaired.Soildisturbancecanbeaccuratelydescribedandquantifiedusingconsistentvisualcategoriesandsound,cost-effectivesamplingprotocols.Assess-mentsofsoilqualitymustrelatevisualdisturbancecategoriestochangesintheabilityofthesoiltoprovidenutrientsortoabsorbandtransmitwater.Anumberofphysicalsoilindices(soilbulkdensity,soilstrength,soilporesizedistribution)areavailabletohelpdefinesuchrelationships.

Soilsvaryacrossthelandscapeandresponddifferentlytomanagementprac-tices.Caremustbetakenwhenmakingsoilqualityassessmentstoselectthemostappropriate soil indices, to followproper samplingprotocols, and to interpretresultscorrectly.Finally,withoutknowingeffectsofsoildisturbanceonsitepro-ductivityandsedimentyield,meaningfulassessmentsofsoilqualitycannotbemade.Validationofsuchcauseandeffectrelationshipsisastepintheprocessthatisoftennevercompleted.

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Timberjack cut-to-length harvester

Timberjack forwarder machine



Harvester forwarder trail

Quantification of soil disturbance categories using randomly oriented 100 foot transects.



Measuringsoilresistanceusingtherecordingsoilpenetrometer.

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