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Response and Recovery of Water Yield and Timing, Stream Sediment, AbioticParameters, and Stream Chemistry Following Logging

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Long-TermResponseofaForestWatershedEcosystem:ClearcuttingintheSouthernAppalachiansWayneT.SwankandJacksonR.Webster

Printpublicationdate:2014PrintISBN-13:9780195370157PublishedtoOxfordScholarshipOnline:May2015DOI:10.1093/acprof:osobl/9780195370157.001.0001

ResponseandRecoveryofWaterYieldandTiming,StreamSediment,AbioticParameters,andStreamChemistryFollowingLogging

WayneT.SwankJenniferD.KnoeppJamesM.VoseStephanieN.LaseterJacksonR.Webster

DOI:10.1093/acprof:osobl/9780195370157.003.0003

AbstractandKeywords

In1977,Watershed7(WS7)attheCoweetaHydrologicLaboratorywasclearcutandloggedusingamobilecablesystemthatcouldaccesslogsupto300mfromaroadandsuspendthelogscompletelyabovethegroundfortransporttotheloggingdeck.Watershed(WS)2,a12.6-hawatershedadjacenttoWS7,servedastheexperimentalcontrol.Thischapter(1)summarizesandevaluatesthelong-termhydrologicandwaterqualityresponsestoforestmanagement;and(2)linksstreamresponseswithprocess


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levelresearchconductedwithinthewatershed.

Keywords:watershedecosystemanalysis,waterquality,forestmanagement,streamresponse,resourcesustainability

IntroductionWatershedecosystemanalysisprovidesascientificapproachtoquantifyingandintegratingresourceresponsestomanagement(HornbeckandSwank1992)andalsotoaddressissuesofresourcesustainability(Christensenetal.1996).ThephilosophicalcomponentsoftheresearchapproachatCoweetaare(1)thequantity,timing,andqualityofstreamflowprovidesanintegratedmeasureofecosystemresponsetolandmanagementpracticesand(2)responsetodisturbanceprovidesavaluabletoolforinterpretingecosystembehavior(SwankandCrossley1988).

Ourobjectivesinthischapterareto(1)summarizeandevaluatethelong-termhydrologicandwaterqualityresponsestoforestmanagementand(2)linkstreamresponseswithprocesslevelresearchconductedwithinthewatershed.

Thedetailsofgeneralandspecificforeststudysites,experimentaldesign,managementprescriptions,andnaturaldisturbancesspanningthe32-yearhistoryofthestudyatCoweetaaredescribedbySwankandWebsterinchapter1ofthisvolume.Briefly,a59-hasouth-facingmixedhardwoodcoveredwatershedwasclearcutandloggedin1977usingamobilecablesystemthatcouldaccesslogsupto300mfromaroadandsuspendthelogscompletelyabovethegroundfortransporttotheloggingdeck.Watershed(WS)2,a12.6-hawatershedadjacenttoWS7servedastheexperimentalcontrolforassessinghydrologicandwaterqualityresponsestothetreatmentonWS7.

(p.37) Hydrology

Methods

PrecipitationInputPrecipitationinputsweremeasuredusingstandardraingageslocatedwithintheCoweetabasin.Precipitationinputforeachwatershedwascalculatedusingestablishedrelationshipsbetweenspecificwatershedlocationswithinthebasinandindividualormultipleraingages.

StreamflowandAnnualWaterYieldWeusedthepairedorcontrolcatchmentmethodofanalysis(Hewlettetal.1969)toquantifytheeffectsofloggingtreatmentonthequantity,timing,andqualityofstreamflow.Inthismethod,therelationshipofstreamattributesbetweenreferenceandtreatedwatershedsforthecalibrationperiodisdeterminedbyregressionanalysiswhichincorporatesexperimentalcontrolforclimaticandbiologicalvariationswithinandbetweenyears.Thecalibrationperiodforhydrologicanalysisinthisstudyspanned11years,from1966to1976,withcontinuousmeasurementofdischargeusingsharp-crestedV-notchweirs(figure3.1).MeanannualdischargefromWS7duringthisperiodaveraged106cmandrangedfrom76to149cm.


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Figure3.1 Upstreamviewof90ºV-notchweirinstallationonWS7,November2012.(USDAForestServicephoto)

(p.38) Thecoefficientofdetermination(r2)fortotalannualflowbetweenWS7andWS2duringthecalibrationperiodwas0.99.Theerrorterm(p<0.05)forpredictedindividualannualflowsfortreatmentyearsaveraged±5cm.Regressionanalysisusingmonthlyflowdatawasusedtoquantifythewithin-yearchanges;r2duringthecalibrationperiodrangedfrom0.96to0.99.

EarlyPostharvestSlashInterceptionLossAnimportantcomponentofevapotranspirationinforestsisinterceptionloss,whichhasseldombeenstudiedafterlogging.Forestcanopiesinterceptandaltertheamountandchemistryofprecipitationaswaterpassesthroughfoliage(throughfall)orflowsdownthestem(stemflow).ClearcuttingandharvestonWS7removedthecanopystructureforseveralyearsandaddedalargequantityofwoodyresiduetotheforestfloor.Several(1yrand8yraftercutting)studieswereconductedtoquantifytheeffectsoninterceptionloss(thischapter)andonnutrientleaching(seeKnoeppetal.,chapter4,thisvolume).

Thebasicexperimentaldesignofthefirststudywastheestablishmentofeighteen4x4mplotsat9locationsinWS7,whichwerestratifiedtoproportionallyrepresenttheforesttypesbeforeclearcutting.One2x2mplotwasnestedinonecornerofeach4x4mplot;thus,therewereatotalof36plots.Inthefirstyearaftercutting,coarsewood(CW;i.e.,logsandbrancheswithdiameters≥5cm)wasmeasuredforenddiametersandlengthofCWlyingwithineach4x4mplottocalculatevolumeandsurfacearea.Disksweretakenfromrepresentativelogstodeterminewooddensityandmass.CWsamplingwasrepeatedinyears6,7,and11inalong-termstudyofwooddecompositiononWS7(Mattsonetal.1987;seealsoMattsonandSwank,chapter7,thisvolume).

Finewood(branchesandstems<5cmindiameter)wassampledinthe2x2mplotsnestedinthecornerofeachCWplot.Theseplotswereselectedtorepresenttherangeofslashdominatedbystems,“brush,”ormixedslash,all<5cmindiameter.Woodwasalsosampledontheseplotstoestimatesurfaceareaandbiomassoffinewood.


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Throughfallwascollectedbyinserting15x200cmV-shapedaluminumtroughsbeneaththeslash,attachedto19-Lpolypropylenecollectionjugs(seefigure4.1,inchapter4,thisvolume).Sampleswerecollectedonastormeventbasisandvolumesusedtoestimateinterceptionlossandleachingofnutrientsfromslash.

LaterPostharvestForestInterceptionLossAdetailedstudyofinterceptionloss,atmosphericdeposition,andfoliageleachingwasconductedonWS7whentheregeneratingforestwas8yearsold(Potteretal.1991;Potter1992)(figure3.2).InterceptionfindingsarereportedhereandcanopynutrientfluxesarereportedbyKnoeppetal.(seechapter4,thisvolume).Three20x20mplotswerelocatednearthemiddleofWS7inachestnutoak(Quercusprinus)community.A10-mtowerwaslocatedinthemiddleofthestudysiteandinstrumentedtocollectincidentrainfallanddryparticulateinputstothecanopy.(p.39)

Figure3.2 WS7eightyearsafterclearcutting.(USDAForestServicephoto)

Thirtytroughs(1.0x0.1m)wererandomlyplacedinthethreeplotstocollectthroughfall;stemflowwasmeasuredinnine1x2mplots(Potteretal.1991).Datawerecollectedonastorm-eventbasisandincluded20storms,14duringthegrowingseasonand6duringthedormantseason,throughouttheperiodJuly1984throughAugust1986.

ResultsandDiscussion

InitialWaterYieldandInterceptionResponsesIn1978,thefirstfullwateryear(MaytoApril)followinglogging,streamflowincreased26.5cm,orabout28%abovetheflowexpectediftheforesthadnotbeencut(figure3.3).Insubsequentyears,annualdischargeincreasesdeclinedatarateof5to7cmperyearuntilthefifthyearaftercutting,whenannualflowwasjust4cmabovepretreatmentlevels.Thereafter,changesinflowwerenotsignificant(p>0.05)anddischargefluctuatedaroundexpectedbaselinevalues.

ThepatternofinitialresponseandearlyrecoveryofannualstreamflowafterclearcuttingWS7areconsistentwithotherforestcuttingexperimentsatCoweeta(Swanketal.1988)


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andinotherlocationsoftheAppalachianregionoftheUnitedStates(seeAdamsandKochendenfer,chapter12andHornbecketal.,chapter13,thisvolume).Wateryieldincreasesaretypicallygreatestinthefirstyearaftercuttingbecausetranspirationismostreducedduetominimalleafareaindex(LAI).Insubsequentyears,assproutsandseedlingsregrow,LAIandtranspirationincrease(seeBoringetal.,chapter2,thisvolume)resultinginalogarithmicdeclineinstreamflowoverthefirstsixyearsofsuccession.

(p.40)

Figure3.3 AnnualdeviationsinstreamflowonWS7priortoandfollowingclearcuttingandcommercialloggingatCoweetaHydrologicLaboratory,1967–2012.*Denotessignificantchange;p<0.05).

Loggingslashinterceptionwasmeasuredforatotalof36storms,rangingfrom5to12mmfromDecember1977throughApril1979.Interceptionloss(precipitationminusthroughfall)wasalinearfunctionofrainfallamountforalltypesofslash.Statisticalanalysesofregressionslopesshowednosignificantdifferencesamongtheslashtypes<5cm,soalldatafromthesmallmaterialwerecombinedintheregressions.However,theregressionslopeofthecoarsewood(logsandbranches≥5cm)wasdifferentfromalloftheothertypesofslashandresultedinadifferentregressionequation.

Linearregressionsforestimatinginterceptionlossfromslashare:

(Eq.1)

(Eq.2)

whereILisinterceptionlossandPisprecipitation,bothinmm.

EstimatesofannualinterceptionlossusingEqs.1and2werederivedfortheperiod

IL (CWD) = −0.44 + 0.2098 (P) ; = 0.70r2

IL (otherslash) = −0.82 + 0.1516 (P) ; = 0.47r2


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September1977–August1978;stormprecipitationwasmeasuredmonthlyonagagelocatedonWS7andadjustedforeachmonthbasedonpreviouslydevelopedseasonalweightingfactors(Swiftetal.1988).Precipitationforthe12-monthperiodtotaled1,731mm,withamonthlyrangeof22to265mmdeliveredin76stormsovertheyear.InterceptionlossforCWwasestimatedtobe324mm,or18.7%ofprecipitation,comparedtoanestimatedinterceptionlossof197mm,or11.4%ofprecipitationforsmallerslash.Acombinedwoodinterceptionlosswasderivedbyweightingtheamountofthetwoslashtypesmeasuredonthe36plotsbasedonwoodsurfacearea.Themeanwoodsurfaceareaperplotareawas0.448m2/m2forCWand0.898m2/m2forsmallerslash,foratotalwoodsurfaceareaof(p.41) 1.346m2/m2.Usingtheappropriateweightingfactors,annualinterceptionlossforwoodwasestimatedtobe239mm,or13.8%ofprecipitation.

Similarstudiesofinterceptionbyloggingresiduearenotavailableforcomparisonwiththesefindings.However,HelveyandPatric(1988)reportedanannualinterceptionlossformaturehardwoodsatCoweetaof250mm,or13%totalprecipitation,including3%litterinterceptionloss.Thus,theearlypostharvestinginterceptioncomponentofthehydrologicbudgetonWS7wasnotsubstantiallyalteredandthefirstseveralyearsofannualwateryieldincreasesmeasuredaftercuttingandharvest(figure3.3)weremainlyduetoreductionsintranspiration.Thereareseveralfactorscontributingtotheinitialhighinterceptionloss.First,onlysawlogswereharvestedandtheremainingvegetationwascutaspartofthesitepreparationtreatment,whichcontributedtoahighloadingofwoodyresidue.Secondly,WS7isasouth-facingslope;previousresearch(Swift1972)foundradiationavailableforevapotranspirationisgreateronsouth-thannorth-facingslopes,animportantfactorthatwasincludedinamodelusedtopredictwateryieldresponseonWS7(table3.1).

LaterPostharvestForestInterceptionLossIntheregenerating8-year-oldforest(Potteretal.1991;Potter1992),datawerecombinedforallstorms.Throughfallandstemflowwereestimatedtobe83.3%and5.8%ofprecipitation,respectively,foranestimatedinterceptionlossof10.9%ofprecipitation.ThesewaterfluxesfortheregeneratingstandweresimilarinmagnitudetothosethatwereconcurrentlymeasuredintheadjacentreferencehardwoodforestonWS2(Swanketal.1992).

TherecoveryofcanopyinterceptionlossearlyinsuccessioncanbemainlyattributedtoarapidrecoveryofleafareaonWS7(seeBoringetal.,chapter2,thisvolume).Thecontributionsofinterceptionlosstowatershedevaporotranspiration(Et)andstreamflowrecoveryisadynamicprocess.Theimportanceofinterceptionlossfromwoodmeasuredthefirstyearaftercuttingdeclinedovertimewithdecomposition.However,11yearsaftercutting,thequantityofCWwasstillsignificant(seeMattsonandSwank,chapter7,thisvolume),whichcontributedto

Table3.1Comparisonofannualobservedvs.predictedincreaseinwateryieldfollowingclearcuttingonCoweetaWS7.


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Yearfollowingclearcutting

Increasedpredictedbymodel(cm)

ObservedIncrease(cm)

1 25 262 17 203 12 174 8 125 6 46 3 4Total 71 83Source:Swanketal.(1988).

(p.42) waterstorageandinterceptionlossfromtheforestfloor.Overthesametimeperiod,canopyinterceptionlosswasreturningtoprecuttinglevels.Thus,duringthefirstdecadeofregrowth,thecombinedinterceptionlossfromwoodandtheregrowingcanopyprobablyexceededthatforamaturehardwoodforestandmayhavecontributedtosomeofthevariabilityinannualwateryield(figure3.3).

Longer-TermWaterYieldResponsesThesignificantincreaseinstreamflowin1992,whentheforestwas15yearsold(figure3.3)hasbeenattributedtoareductioninbothstemdensityandLAIassociatedwithcompetitionandself-thinning(stemexclusionstage)ofrapidgrowingcoppicevegetation(Swanketal2001;Elliottetal.1997),ahighmortalityrateofRobiniapseudoacaciacausedbystemborers(seeBoringetal,chapter2,thisvolume),declineofdogwood(Cornusflorida)duetoadisease,dogwoodanthracnose(Chellemietal.1992),andlossofabundantAmericanchestnut(Castaneadentata)sproutsduetothechestnutblight(seeBoringetal.,chapter2,thisvolume).Similarpatternsofchangesinstandstructure,wateruse,andstreamflowhavebeenfoundinotherclearcuttingexperimentsatCoweeta(SwiftandSwank1981).

Canopyopeningscreatedduringthestemexclusionstageofsuccessionwereshort-lived;andby1994to1995,17yearsaftercutting,thestandbasalareawas23m2/ha,whichissimilartothe25m2/habasalareaoftheoriginalforest(Elliottetal.1997)andLAIalsoincreasedtoprecuttinglevels(seeBoringetal.,chapter2,thisvolume).IntheensuingdecadeafterLAIstabilized,therehasbeenapatternofannualstreamflowreductionsthatfrequentlyexceed2.5cm(figure3.3).WehypothesizethathigherEtfortheregrowingversusmatureforestisrelatedtohighertranspirationlossassociatedwithmajorshiftsinspeciescomposition.Forexample,therewereverylargeincreasesinbasalareaofLiriodendrontulipifera,Acerrubrum,andRobiniapseudoacaciainthesuccessionalforestandanequallylargedeclineforcombinedCaryaandQuercusspp.(seeBoringetal.,chapter2,thisvolume).ThishypothesisissupportedbyrecentphysiologicalresearchatCoweetathathasshownlargedifferencesincanopytranspirationratesamonghardwoodspecies.Specifically,diffuseporousspecies,suchas


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yellowpopularandredmaple,havemuchhighertranspirationratescomparedtooakandhickoryspecies(Fordetal.2010;Fordetal.2011).PersistentdecreasesinannualwateryieldwerealsoobservedatHubbardBrookfollowingaharvestandattributedtohighertranspirationratesforringporousearlysuccessionspeciescomparedtomaturenorthernhardwoodforests(Hornbecketal.1977;seealsoHornbecketal.,chapter13,thisvolume).

WaterYieldModelOneoftheoriginalobjectivesofthecuttingexperimentonWS7wastoobtainwateryieldresponsedataforasouth-facingwatershed.PrevioussynthesesofwatershedexperimentsintheAppalachianHighlandsPhysiographicregion(DouglassandSwank1972,1975)establishedempiricalequationsbetweenfirst-yearwateryield(p.43)increasesasafunctionofpercentbasalareacutandaninsolationindex(energyvariablerelatedtoslopeaspect)foracatchment.Inaddition,anotherempiricalequationwasdeveloped(DouglassandSwank1975)forpredictingwateryieldincreasesforanyyearfollowingharvestuntilstreamflowreturnstobaselinelevels.Themodelscontainlittledatafromsouth-facingclearcutwatershedswithnaturalforestsuccession.However,predictionsfromthesemodelsgenerallyshowedgoodagreementwithannualwateryieldresponsesmeasuredonsouth-facingWS7(table3.1).Thefirstyearaftercutting,streamflowincreasedabout26cmcomparedto25cmpredictedbythemodel.Subsequently,inthenext2years,predictedincreasesweresubstantiallybelowobservedincreases.TheseyearscoincidedwiththewettestyearonrecordandoneofthedriestyearsatCoweeta.Intheensuing3years,predictionswereincloseagreementwithobservedincreasesinwateryieldandthetotalchangepredictedforthe6-yearperiodwaswithin17%oftheobservedchange.

Intra-AnnualWaterYieldDuringthefirst3postcuttingyears,themonthlydistributionsofwateryieldincreases(figure3.4)weresimilartootherlow-elevationcuttingexperimentsatCoweeta(Swanketal.1988).Flowincreasesoccurredineverymonth,withthesmallestamountsinthespring(AprilandMay),atthesametimethatsoilmoistureinanundisturbedforestisusuallyfullyrecharged.Concurrentwiththegrowingseason,streamflowincreasesbecomelargerduetoreducedEtonWS7.SubstantialflowincreasescontinuedintothelatefallandwintermonthsandpartiallyreflectthelagbetweenwhenreducedEtoccursonthecutwatershedandwhenthewatersavingsreachtheweirduringtheperiodofhighprecipitation(figure3.4).


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Figure3.4 Meanmonthlychangesinwateryieldduringthefirst4postcuttingyearsonCoweetaWS7.(ModifiedfromSwanketal.,1982)

(p.44) Ofnotableimportanceisthattwoofthelargestflowincreases(1.6cm,or28%increase)occurredinSeptemberandOctoberwhenflowsarenormallylowandwaterdemandsarehigh.

StormHydrographResponsesDetailedanalysisofeightstormhydrographparameterswasconductedforWS7usingpretreatmentstormdataonWS7regressedagainstthesameparametersasthereferencecatchment(WS2).Theanalysisuseddatafor75storms(≥2cm)fromthefirst4yearsaftertreatment,whichencompassedtheperiodofmaximumwateryieldincrease(Swanketal.2001).

Followingharvest,statisticallysignificantchangesinregressioninterceptsandslopeswerefoundforallstormparametersexcepttimetostormpeak(table3.2).Thelargestincreasesoccurredinpeakflowrates(15%)andinitialflowrates(14%);thelatterisduetoelevatedratesofbaseflowfromthewatershed.Quickflow(stormflow)volumeincreased10%,whichwasassociatedwitha10%increaseinrecessiontime.Takencollectively,thehydrographresponsesareconsideredtobeofminorimportancetodownstreamflooding.Forexample,inthefirst4yearsaftercuttingandharvest,theaverageprecipitationstorm,≥2cm,increasedthequickflowvolumebyonly0.03cm,or2.43m3/ha,andthepeakflowrateby1.7m3/ha.

Stormhydrographresponsestoharvestarepartlydependentupon(1)themagnitudeandmethodofloggingandassociatedroaddisturbanceand(2)theinherentresponsivenessofthewatershedtoprecipitationeventsintheabsenceof

Table3.2Stormhydrographparametersandchangesduringthefirst4yearsfollowingclearcuttingandloggingonWS7.Parameter Meanfor

treatmentwatershed

Significanceofregressioncoefficients

Percentchangeinparameteraftertreatmentformeanstorm


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WS7 WS2 Intercept SlopeInitialflowrate(m3s−1km−2)

0.037 0.27 ** ** 14

Peakflowrate(m3s−1km−2)

0.136 0.127 ** ** 15

Timetopeak(h)

8.0 8.0 NSb NS 0

Totalquickflowvolume(cm)

0.32 0.40 ** ** 10

Quickflowafterpeak(cm)

0.08 0.09 ** ** 6

Quickflowafterpeak(cm)

0.24 0.31 * NS 11

Quickflowduration(h)

27.6 28.3 ** * 5

Recessiontime(h)

20.0 21.0 ** NS 10

aDerivedfromdifferencebetweenvaluepredictedfromcalibrationregressionandmeasuredvalue.

(b)Nonsignificant.

(*)p<0.05.

(**)p<0.01.

Source:ModifiedfromSwanketal.(1982).

(p.45) disturbance.Inherentresponsivenessisdrivenbyavarietyofphysicalfactorssuchaswatershedsize,soildepth,slopeandtopographiccomplexity,andinfiltrationrates.Theresponsefactor(meanannualquickflow/meanannualprecipitation)forWS7wasverylow(0.04),whichaccountsforsomeofthesmallchangesinstormhydrographparameters.Furthermore,thelowdensityofloggingroads,minimaldisturbanceofthesurfacesoilbycablelogging(seeSwankandWebster,chapter1,thisvolume),andcarefuldesignofroads(Swift1988)alsolimitedchangesinstormflowonWS7.

AbioticResponsestoClearcutting

SoilMoistureandTemperature

Regenerationcuttingcanproducesignificantchangesinthemicroenvironmentoftheforestfloorandsoilthatinturnregulateecosystemprocesses,suchasdecomposition;microbialactivity;nutrientcycles;andthegermination,sprouting,survival,andgrowthofvegetation.BeginninginAugust1977,studieswereinitiatedonWS7toevaluatetheeffectsofharvestingonsoilmoistureandtemperature.Soilmoisturewasmeasuredat


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biweeklyintervalsintheO1andO2(organic)litterlayersoftheforestfloorand0to10-cmand10–30-cmdepthsinthesoilonWS7andalsoWS2,theadjacentcontrolwatershed(SwankandVose1988).Inthefirstautumn(Aug–Nov)afterharvest,littermoisturewas20%to30%belowthatfoundonWS2(table3.3).Inthesubsequentwinterquarter,littermoisturewassimilaronbothwatersheds.However,intheensuingyear(1978),littermoisturewasconsistently30%to50%lowerintheclearcutcomparedtoWS2.Incontrast,surfacesoilmoisture(0–10cm);increasedmorethan11%thefirstyearaftercutting(table3.3).Deeperinthesoilprofile,moistureincreasesweresmallorshowednochanges.

Soiltemperaturesweremeasuredatthelitter-soilinterfaceonWS7duringtheprecutyear(1976)priortoclearcuttingandinthefirstgrowingseasonaftercuttingandlogging(SwankandVose1988).Meanmonthlytemperatureswere7°Cto10°CaboveprecutlevelsintheperiodMaythroughOctoberinthefirstyearaftercutting.Meanmonthlymaximumsoiltemperaturesshowedlargeincreaseswithvaluesbeing10°Cto35°Caboveprecutlevelsanddailymaximumtemperaturesfrequentlyexceeded45°Cduringthisperiod.Insubsequentyears,increasesinsurfacesoiltemperaturesweremoderatedbyshadefromregeneration.

StreamChemistry

Methods

Streamchemistrymeasurementsbeganinlate1971onbothWS7andWS2.Weeklygrabsampleshavebeencollectedatafixedlocationjustabovetheweirfromeachwatershedsince1971,andflowproportionalsampleswerealsocollectedintheperiod1975–1981.SolutedeterminationsincludeNO3−,NH4+,SO4−2,PO4−3,Cl−,and(p.46)

Table3.3Quarterlyforestfloorandsoilmoistureaverages(percent)formixedhardwoodforestsduringthefirstyearafterclearcutting(WS7)andforthereferencewatershed(WS2).Treatmentanddepth

YearandQuater

Aug–Nov1977

Dec–Mar1977–78

Apr–Jul1978

Aug–Nov1978

Watercontent(percentbyweight)ClearcutO1 70 124 88 51O2 95 170 97 580to<10cm 46 70 55 3110to30cm 36 40 34 23ControlO1 92 121 120 105O2 126 211 120 109


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0to<10cm 35 59 36 2810to30cm 32 40 29 24Note:O1=includesfreshorslightlydecomposedorganicmaterials.

O2=includesintermediateandhighlydecomposedorganicmaterials.

Source:ModifiedfromSwankandVose(1988).

basecations(Ca+2,Mg+2,K+,Na+)usingestablishedanalyticalmethodsatCoweeta(Brownetal.2009).Comparisonof3-yearaverageannualexportofsolutescalculatedfromweeklygrabsamplesversusflow-proportionalsamplesshowedgoodagreementformostsolutes(SwankandWaide1988).Annualchangeinexportofeachsoluteduetotreatmentthefirst6yearswasestimatedfrompretreatmentregressionsofmonthlyexportsbetweenWS7andWS2.Relationshipsofmonthlyexportsbetweenthetwocatchmentsweregood(r2values≥0.92)formostions.

ResponsetoTreatment

Streamchemistryresponsestotreatmentwererelativelysmall(table3.4)asdescribedinanearlieranalysis(Swanketal.2001).IncreasesinexportofPO4,K,Ca,andMginthefirstfullyearfollowingloggingarepartiallyrelatedtoreleasefromthefertilizerappliedtoroads.LackofsignificantNO3responsewasdueinparttosedimentdenitrificationthatdepletedNO3beforeitreachedtheweir(SwankandCaskey1982).DenitrificationinBigHurricaneBranch(WS7)wasremeasuredin2004aspartofalargeregionalstudyin49streamswithvaryingland-usecategories(Mulhollandetal.2009).Theyfoundmeasureablebutlowerratesofdenitrificationthanthosefoundin1977bySwankandCaskey(1982).However,differencesinbothmethodsandthesupplyofNO3ofthetwostudieslimitdirectquantitativecomparisons.

ThemagnitudeofnutrientexportisdeterminedbybothchangesinsoluteconcentrationsandincreasesindischargeresultingfromreducedEtfollowingcutting.Thefirsttwoyearsaftercutting,annualflowincreasedanaverageof23.5cm/y,but(p.47)

Table3.4Annualchangesinstreamflowandsolutesfollowingclearcuttingandlogging(posttreatment–pretreatment)onWS7.Timesincetreatment(May–Aprilwateryear)

Flow(cm)

Increaseordecreaseinstreamflowandsoluteexport(kgha−1)a

NO3-N

NH4-N

PO4-P

K Na Ca Mg SO4-S

Cl

First4months 0.5 0.01 <0.01

0.01 0.43 0.42 0.24 0.26 0.39 0.68

Firstfullyear 26.5 0.26 0.03 0.04 1.98 1.37 2.60 0.96 0.27 1.13Year2 20.5 1.12 <

0.010.01 1.95 2.22 2.51 1.15 −0.08 1.62


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Year3 17.3 1.27 0.05 0.02 2.40 2.68 3.16 1.42 0.39 2.08Year4 11.9 0.25 0.15 0.02 0.80 1.07 1.63 0.46 0.31 0.59Year5 4.3 0.28 0.01 <

0.010.52 0.13 1.19 0.18 0.04 0.10

Year6 4.1 0.62 0.06 <0.01

0.73 0.69 0.89 0.42 −0.06 0.33

(a)Annualincreaseordecreasederivedfromsumofdeviationsusingmonthlycalibrationregressions.

Source:ModifiedfromSwanketal.(2001).

maximumconcentrationsofmostsolutesdidnotoccuruntilthethirdyearwhenwateryieldonWS7wasstillmorethan17cmabovepretreatmentlevels(table3.1).Bythesixthyearofpostdisturbance,streamflowwasnearpretreatmentlevelsandsoluteexportsalsoappearedtobeapproachingpretreatmentlevels.Thelonger-termresponsesofmostsolutesshowedasimilarpattern.Forexample,small(5µeq/L)increasesinCaconcentrationswereobservedafterloggingbutlaterreturnedtoexpectedpretreatmentlevels(figure3.5).Similarly,followingtheinitialincreaseinKconcentrations,interannualconcentrationsofKafter1983werehighlyvariable,andtherewerenoconsistentdifferencesinconcentrationsbetweenWS7andWS2(figure3.5).ThesamepatternwastrueforMg(figure3.6).TherewaslittlechangeinSO4concentrationsonWS7aftercutting,butbeginningin1989therehasbeenaconsistentdeclineinSO4concentrationsonbothWS7andreferenceWS2(figure3.6).ConcentrationsofNH4andPO4werelowandalmostidenticalforWS7andWS2duringtheentireperiodofrecord.Incontrast,thelong-termrecordforNO3showedinteresting,significantdynamicsfollowingcuttingandforestsuccession(figure3.7).

TheinitialincreaseinNO3concentrationsonWS7wasattributedtoincreasesinsoilNpoolsandconcentrationsthefirstthreeyearsafterlogging(Waideetal.1988;seealsoKnoeppetal.,chapter4,thisvolume).DeclineinstreamNO3concentrationsuntilabout1987areassociatedwiththerapidsequestrationandstorageofnutrientsinsuccessionalvegetation(Boringetal.1988;seealsoBoringetal.,chapter2,thisvolume).However,majorshiftsininternalecosystemNcyclingareevidentinalarge,sustainedpulseofNO3tothestreamfromabout1987through1997(figure3.7).MeanannualpeakNO3concentrations20yearsafterdisturbanceareaboutdoublethevaluesintheearlypostharvestyears.Thereafter,NO3concentrationsdeclinedabout8µeq/Lovera5-yearperiod,followedbyanotherincreaseduringtheensuing5yearsthatreachedamaximumof12µeq/Lin2008(figure3.7).

(p.48)


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Figure3.5 MeanannualconcentrationsofCa(a)andK(b)instreamwaterofWS7andWS2duringcalibration(1971–1976),treatmentactivities(1976–1977),andpostharvestperiod(1978–2010).

AcomplexcombinationofecologicalprocessescontributetothemagnitudeandtemporaldynamicsinstreamNO3.AcceleratedNO3releasetothestreamcoincideswithstemexclusionin1992,andthus,someoftheNO3losswasprobablyduetothereduceduptake.However,thelargestcontributortoincreasedNavailabilitywasextensiveblacklocustmortalityduetolocustborerinfestation,whichisasimilarresponseobservedinanotherearlysuccessionwatershed(WS6)atCoweetawithlargelocustinfestationsandmortality(SwankandWaide1988).Blacklocustisasymbioticnitrogenfixer;inthe4-year-oldlocuststandsonWS7,fixationwasestimatedtobe30kgNha−1yr−1whilefixationcatchmentwidewasestimatedat10kgNha−1yr−1(BoringandSwank1984).Moreover,blacklocustisknowntoaccumulatelargequantitiesofNinfoliage,roots,branches,andstems(BoringandSwank1984).DecompositionoftheN-richorganicmatterfromthedeadtrees(330stems/ha)couldbeamajorsourceofstreamNO3laterinsuccession(figure3.7).(p.49)


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Figure3.6 MeanannualconcentrationsofSO4(a)andMg(b)instreamwaterofWS7andWS2duringcalibration(1971–1976),treatmentactivities(1976–1977),andpostharvestperiod(1978–2010).

Figure3.7 MeanannualconcentrationsofNO3instreamwaterofWS7andWS2duringcalibration(1971–1978),treatmentactivities(1976–1977),andpostharvestperiod(1978–2010).

(p.50) Decomposingloggingresidue(seeMattsonandSwank,chapter7,thisvolume)isalsoapotentialsourceoflong-termstreamNO3enrichmentthatappearsinthestream.OtherpossiblereasonsforincreasedstreamNO3concentrationsareelevatedratesofsoilNmineralizationandnitrificationandreductioninthesoilC/Nratio.


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MeasurementoftheC/Nratioshowednosignificantchangeoverthe18-yearperiodfollowingcutting(KnoeppandSwank1997).However,long-termassessmentofsoilNtransformationsshowcontinuedincreaseinNavailabilityinsurfacesoils20yearsfollowingharvest(seeKnoeppetal.,chapter4,thisvolume).

ThestreamNO3responsesobservedonWS7demonstratethevalueoflong-termstudiesinforestecosystemsandtheprocessesregulatingsystemresponses.Forexample,thefirst10-yeartrendinconcentrationssuggestedthatNO3concentrationshadreturnedtonearpretreatmentlevels.However,inthesubsequent30-yearperiod,concentrationsshowedverylargeincreasesanddecreasesthatgreatlyexceededtheinitialresponses.Thus,earlyterminationofthestudyandassociatedconclusionswouldhavebeenincompleteandpartlyinaccurate.

Theimportanceoftheinteractionbetweensuccessionalvegetation(e.g.,blacklocust)andinsectinfestationsonstreamNO3isclearlyevidentonWS7;however,thisdoesnotexplainallofthetemporalvariationinstreamNO3.Forexample,interannualmagnitudeandvariabilityofstreamNO3concentrationarealsorelatedtohydrologicvariables.AnnualstreamflowonWS7,rangedfrom45to130cm/yfrom1990to2009,andexplained36%oftheannualvariationinstreamNO3concentrationsfollowingcutting(figure3.8).

Takencollectively,nutrientlossesonWS7shouldnothaveanadverseimpactonthesustainabilityandgrowthofthesuccessionalforest.Atmosphericdepositionofnutrientsexceededtheelevatedlossesofnutrientsinmostyearsofthestudy(Swanketal.2001).Moreover,thehighN-fixationratesofblacklocustandavailabilityofNtoothertreespeciescanbeviewedasabenefittotreegrowthandforesthealth.Furtherdiscussionontherelevanceoffindingstomanagementandecologicalvaluesisfoundinchapter14ofthisvolume.

Figure3.8 TherelationshipbetweenmeanannualNO3-NconcentrationsandannualstreamflowonCoweetaWS7overa20-yearperiod.

(p.51) Sediment

Methods


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TheeffectsofthemanagementpracticesonsoillosstostreamsonWS7weredeterminedatperiodicintervalsbymeasuringsedimentaccumulationsintheweirpondingbasinandalsoontheapproachapronofthepondingbasinofbothWS7andWS2.Thisapproachdoesnotaccountforsuspendedsedimentthatpassesacrosstheweirblade;therefore,totalsedimentexportwasunderestimated.Sedimentvolumeswereestimatedbymeasuringsedimentelevationsalongpermanenttransectswithatransitandlevelrodbeforeandaftercleaningthepondingbasinandapproachaprons.Bulksampleswerecollectedateachelevationmeasurementandprocessedtoestimatedryweight.Apretreatmentcalibrationregressionequationofperiodicsedimentlossovera2-yearperiodbetweenWS7andWS2wasderived(r2=0.91)toestimatechangesinsedimentlossduetomanagement.

SoillossesonsubdrainageswithinWS7werealsomeasuredtoseparateandquantifysedimentsourcesduetoroadsversuslogging.DischargeandsoilexportweresampledwithanHflumeandaCoshoctonwheelusingproceduresdescribedbyDouglassandVanLear(1983).Oneinstallationwaslocatedinaperennialstreambelowthemiddleloggingroad(figure3.9)andthreeinstallationswerelocatedabovetheinfluenceofloggingroadstoevaluateeffectsofcuttingandloggingonly.

Figure3.9 OnefootH-FlumeandCoshoctonsampler(no.701)onWS7—oneoffoursuchinstallationsusedtoassesswaterqualityaboveandbelowroads,1976.(USDAForestServicephoto)

(p.52) Results

Sedimentyieldinthe2yearsofpretreatmentcalibrationfromWS7andWS2averaged230and135kgha−1yr−1respectively.Thesebaselinesedimentyieldsaresimilartothemeanvaluesforsmall,forestedcatchmentsintheeasternUnitedStatessummarizedbyPatricetal.(1984).Inmid-May1976,roadsinWS7werefertilizedandseededbutroadfillsandtherunningsurfacewereunsettledandwithoutgrassorgravelcover.ThethirdweekofMay1976,a16-cmstormoccurredandwasfollowedMay28byalargerstormof22cmwithintensitiesof7cm/h.ThesecondstormproducedthegreatestdischargeratesmeasuredonmostcatchmentsatCoweetaduringtheprevious62yearsoftheLaboratorygaginghistory.Thesetwoeventsgreatlyacceleratedsedimentyieldonboth


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WS7andWS2withanincreaseinsoillossinMayonWS7of1,470kgha−1yr−1(figure3.10).Roadsweretheprimarysourceofincreasedsedimentyieldasillustratedbysoillossmeasuredatoneofthegagingstationsinastreamimmediatelybelowaroadcrossingthemiddleofthecatchment(figure3.11).FollowingtheMaystorms,sedimentyieldatthestationwasnearly50tfrom0.086haofroadcontributingareacomprisedofroadbed,cut,andfill.FollowingroadstabilizationandminimumuseovertheperiodofJunethroughDecember1976,soillossfromtheroadwaslow,butitacceleratedagainduringthepeakofloggingactivities(figure3.10).Inensuingyears,soillossreturnedtobaselinelevels.Inthesametimeperiod,samplerslocatedaboveroads,whichwereonlyinfluencedbycuttingandyarding,onlycollectedsmallamountsofmaterialcomprisedmainlyoforganicmatter.

FollowingtheinitialpulseofsedimentfromtheMay1976storms,sedimentyieldshowedmuchdifferenttemporalpatternsattheweir(figure3.10)comparedtosedimentlossfromtheroads.Sedimentyieldremainedsubstantiallyelevated

Figure3.10 SedimentyieldmeasuredinthepondingbasinonWS7followingloggingcomparedtothesedimentyieldpredictedfromWS2,theadjacentreferencewatershed,overa35-yearperiod.

(p.53)

Figure3.11 Cumulativesedimentyieldmeasuredinafirst-orderstreambelowaloggingroadduringthefirst32monthsafterlogging.


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duringandafterloggingdisturbancesandcontinuedtohavesedimentlossesthatwerefrequentlyinexcessof500kgha−1yr−1aboveexpectedlossesovertheperiod1978—1985(figure3.10).AlargepulseofsedimentwasdeliveredtoweirsonbothWS2andWS7in1989inresponsetothewettestyearonrecordatCoweeta.SedimentyieldonWS7wasabout800kgha−1yr−1abovetheexpectedyield.In1991sedimentyieldonWS7returnedtopretreatmentlevels(234kgha−1yr−1)andremainedataboutthesamelevel(179kgha−1yr−1)basedonthreesampleperiodsinsubsequentyears(figure3.10).

Thelong-termsedimentyieldresponsesillustratethedelayorlagbetweenpulsedsedimentinputstoastreamandroutingofsedimentsthroughthewatershed.Soillossderivedfromroadswasverylowfollowingstabilizationwithgrassandgravelcover.Moreover,followinglogging,roadtravelwasminimal—roadswereonlyusedforaccesstoresearchsites.Also,basedonlong-termcross-sectionmeasurementsonthemainstreamonWS7,therehaveonlybeeninfrequentandminorinstancesofstreambankerosion(Websteretal.unpublisheddata).Thus,intheabsenceofsignificantadditionalsourcesofsedimenttoWS7streams,thelong-termsedimentincreasesobservedattheweironWS7wereduetoacontinuedreleaseofsedimentfromupstreamstorage,whichwasdepositedfromthreeroadcrossingsonperennialstreamsandfourcrossingsofintermittentstreamsonWS7intheMay1976storms.

Theuniqueconditionsthatproducedthesesedimentresponsesshouldberecognized,thatis,recordstormsoccurredattheprecisetimewhenroadswerefreshlyconstructedandwithoutvegetationcover,andthusmostvulnerabletoerosion.Itisalsoimportanttopointoutthatbestmanagementpracticeswereusedinharvesting(p.54)andinloggingroadlocationanddesign.Thelong-termeffectofthismanagementprescriptiononwater,soil,vegetationsustainabilityandhealth,andthestructureandfunctionofbenthicinvertebratesisfurtherdiscussedinaconcludingsynthesis(seeWebsteretal.,chapter14,thisvolume).

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Notes:

(*)Correspondingauthor:CoweetaHydrologicLaboratory,USDAForestService,3160CoweetaLabRoad,Otto,NC28763USA

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