JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 100, NO. D7, PAGES 14,319-14,326, JULY 20, 1995 Carbon dioxide exchange in a peatland ecosystem N.J. Shurpali,S. B. Verma, and J. Kim Department of AgriculturalMeteorologyand Center for Laser-AnalyticalStudiesof Trace Gas Dynamics Universityof Nebraska,Lincoln T. J. Arkebauer Department of Agronomy,Institute of Agriculture and Natural Resources,Universityof Nebraska,Lincoln Abstract. Micrometeorologicalmeasurementsof carbon dioxide exchangewere made in an openpeatlandin north centralMinnesotaduringtwo growingseasons(1991 and 1992). The vegetationat the site was dominatedby Sphagnumpapillosum,Scheuchzeria palustris, and Chamaedaphnecalyculata.The objectiveof the studywas to examinethe diurnal and seasonalvariationsin canopyphotosynthesis (P) and developinformationon the net ecosystemCO2 exchange.The two seasonsprovidedcontrastingmicroclimaticconditions: as comparedwith 1991, the 1992 seasonwas significantlywetter and cooler. Canopy photosynthesis was sensitiveto changesin light, temperature,and moisturestress(as indicatedby water table depth and atmosphericvapor pressuredeficit). Under moderate conditions(temperature18-28øC,vapor pressuredeficit0.7-1.5 kPa, and water table near the surface)duringthe peak growthperiod, midday(averagedbetween1000-1400 hours) P values rangedfrom0.15to 0.24mgm-2 s-1. Underhigh-temperature (30ø-34øC) and moisturestress(water table 0.16-0.23 m below the surfaceand vapor2pressure deficit 2.21 3.0 kPa) conditions,middayP was reducedto about0.03-0.06 mg m- s- . There was a high degree of consistencyin the values of P under similar conditionsin the two seasons. Seasonallyintegratedvaluesof the daily net ecosystemCO2 exchangeindicatedthat the studysitewasa source of atmospheric CO2,releasing about71 g C m-2 overa 145-day period (May-October) in 1991.Over a similarperiod in 1992,however,this ecosystem was a sinkfor atmospheric CO2witha netaccumulation of about32 g C m-2. Theseresults are consistentwith previousinvestigationson CO2 exchangein other northern wetland sitesduring wet and dry periods. oxideflux in northernwetlands(Coyneand Kelley[1975], over a wet meadowtundra in Alaska, and Neumann et al. [1994], Peat in northern wetlands contains about one third of the above a raised open bog at Lake Kinosheoin the southern total world pool of carbon[e.g.,Miller et al., 1983; Gorham, Hudson Bay lowlands).Here, we report the resultsof a mi1991]. The carbon sink/sourcestrengthsin these important crometeorologicalstudy conducted during two contrasting ecosystems, however,are not well understood.Northern wet- growingseasons(mid-May to mid-October1991 and 1992) in landshavebeen thoughtto be a net sinkfor atmosphericCO2 a peatland in north central Minnesota.The primary objective in the past[e.g.,Oechelet al., 1993].How theseecosystems will of the studywasto examinethe diurnal and seasonalvariations respondto future climaticperturbationsis not known.Recent in canopyphotosynthesis and net ecosystemCO2 exchangeat studies[e.g., Tans et al., 1990; Oechelet al., 1993] suggesta this site. possibleshift in the carbonbalance of northern boreal and arctic ecosystems,from a sink to a source. However, these Materials and Methods resultscan not readily be generalizedto other regions. Introduction Some information is available on carbon dioxide fluxes in arctic and subarcticpeatlands[e.g., Coyneand Kelley, 1975; Billingset al., 1982;Armamentoand Menges,1986;Oecheland Billings,1992]. However,very'little is known about the carbon dioxideexchangein peatlandsin other areas.Also, mostof the prior studieshave used chambersand are limited to small areas. Micrometeorological techniques have been recommended for measurementof large-scalefluxes of mass and energy.These techniquesallow direct, continuous,and spatially integratedfluxesand causeminimal disturbanceto the microenvironmentbeing investigated.We know of only two short-term micrometeorologicalmeasurementsof carbon diCopyright1995 by the American GeophysicalUnion. Paper number 95JD01227. 0148-0227/95/95JD-01227505.00 Site The study site, referred to as the Bog Lake Peatland, is locatedin the ChippewaNational Forest,adjacentto the Marcell ExperimentalForest (47ø32'N,93ø28'W)in north central Minnesota. The vegetationis dominatedby Sphagnumpapillosum, a carpet-forming moss species.The most common emergentspeciesare Scheuchzeria palustris(a triseededarrow grass), Carex spp. (sedges), and Chamaedaphnecalyculata (leather leaf). Other prevalentspeciesincludeRhynchospora alba (beak-rush),Sararacenia purpurea(pitcherplant), and 1to 3-m tall, widely scatteredLarix laricina (tamarack). The organicsoil of the peatland consistsprimarily of Sphagnumderivedpeat. The surfaceconsistsof a pattern of hummocks (microhillsof 0.15-0.55 m) and hollows(microvalleys).The site provided at least 250-300 m of upwind fetch of open 14,319 14,320 SHURPALI ET AL: CO2 EXCHANGE IN A PEATLAND ECOSYSTEM callyactiveradiation(Rp). Surface temperature wasmeasured by an infrared thermometer(althoughthe infrared thermometer waspointedtowardan area dominatedby moss,the measuredvalueswere assumedto representthe overallvegetation temperature).The daily water table positionwas recorded usinga recordingwell installednear the instrumentmast. The dailywater table depthwasmeasuredrelative to an "average" 0ø __ -- 300m. hollow ,,,,• 1O( : Tower ,, lOOm 1 Peatland Forest surface referenced at 415.84-m altitude from the mean sea level. Peat temperature(at 0.1-m depth relative to an averagehollow surface)was alsomonitored. To evaluate the overall performanceof the flux measurement systemat a micrometeorologicalstudy site, it is worthwhile to investigatethe tower footprint,examinethe closureof the surfaceenergybudgetcomponents,and developinformation on aerodynamiccharacteristics,such as the drag coefficient. A footprint analysis[e.g., Gash, 1986; Scheuppet al., 1990] indicated that about 90% of the measuredflux at a heightof 2.5 m (underneutraland unstableconditions)should be from the peatland. Resultson the surfaceenergybudget closureand drag coefficientfor this site, includedin the work by Vermaet al. [1992], are typicalof observationson agricultural cropsand prairie vegetationin reasonablyflat terrain. Soil Surface CO2 Measurements Figure 1. Map showingthe area upwind of the instrument tower. peatlandfrom the instrumenttower in the SSW throughNNE (2000-390ø) directions(Figure 1). Micrometeorological Measurements A closedgas exchangesystem[Norman et al., 1992] consisting of a 0.75 L dark chamber, attached to an infrared gas analyzer (model LI-6200, LI-COR Inc., Lincoln, Nebraska), was used to measuresoil surfaceCO2 flux (Fs). Details of these measurementsare reported by Kim and Verma [1992]. An empirical relationshipof the following type, developed from thesemeasurements,was usedin this study: Fs= [(b• + b2W)/(b•+ 0.4b2)]a3a[4 (r-•ø)/•ø• 0 -< W-< 0.4m Fluxes of CO2, latent and sensibleheat, and momentum were made employingthe eddy correlation technique.The (1) eddycorrelationsystemconsistedof a closedpath, differential, nondispersive, infrared CO2 gasanalyzer(modelLI-6251, LI- where W is the water table depth (in meters), T is peat temCOR Inc., Lincoln, Nebraska), one-dimensionalsonic ane- perature (in degreesCelsius)at 0.10-m depth, b • is a nondimometers (Campbell Scientific, Logan, Utah), a threedimensional sonic anemometer (Kaijo Denki Co., Tokyo, Japan), fine-wire thermocouplesand krypton hygrometers (Campbell Scientific,Logan, Utah). The three-dimensional sonic anemometer and a fine-wire thermocouple were mounted at a height of 3.5 m abovethe peat surface.The rest of the instruments were mounted on a horizontal boom at a height of 2.5 m abovethe peat surface.Further detailsof the eddy correlationinstrumentsand installationcan be found in the worksby Verma[1990],Vermaet al. [1992],andSuykerand Verma [1993]. The CO2 sensorwas calibrated twice on each day of measurement.Eddy fluxeswere obtained from covariancescomputed over 30-min averagingperiods.The CO2 flux was corrected for the variation in air density due to the transfer of water vapor followingthe method of Webbet al. [1980]. The useof metal intake tubingto draw air samplesthroughthe CO2 sensoreliminatedthe need for the densitycorrectionterm due to heat transfer[e.g.,LeuningandMoncrieff,1990].Covariance values were corrected for the effects of tube attenuation [Suykerand Verma, 1993] and spatial separationof sensors [Moore,1986].The combinedeffect(tube attenuationand spatial sensorseparation)wasof the orderof 10% for CO2fluxfor daytime conditions. Supportingmeasurementsincludedair temperature,humidity, horizontalwind speed,wind direction,and photosyntheti- mensional constant, b2 isa constant withunitsof m- •, a3 isthe soil surfaceCO2 flux at 10øC(g m-2 d-•), and a4 is the temperatureQ•o coefficient.The soil surfaceCO2 flux was about5-15 g m-2 d-1 duringmidseason in 1991.In 1992,when the water table was higher and air temperature substantially cooler (see the next sectionfor details), the midseasonsoil CO2fluxwassmaller(4-8 g m-2 d-•). Thesevaluesof Fs includethe contributionfrom mossdark respiration.We estimated this contributionby making soil CO2 flux measurements at locationswhere the mosswasclipped.Resultsindicatedthat the moss dark respiration during the midseasonwas about 1.5-3.0g m-2 d-• in 1991and1992. Dark Respiration Measurements Leaf level measurementsat the study site were used to estimatethe valuesof dark respirationof Scheuchzeria palustris and Chamaedaphne calyculata.The datawere expressedby the followingrelationship' dark respiration(vascularplants) = R• exp {E(Ti- 25)/[298R(Tl + 273)]}L (2) whereR} is the rate of darkrespiration in/xmolm-2 s-1 at 25øC(1 tzmolm-2 s-1 = 0.044mgm-2 s-i), E istheactivation energyin J mol-•, and T1 is the leaf temperature in degrees Celsius.Sincethesevascularplantsgrow in closeproximityto SHURPALI ET AL: CO 2 EXCHANGE IN A PEATLAND moss, T/wasapproximated bythemoss surface temperature measured by an infrared thermometer discussedabove. R (8.314J øK-1mo1-1)isthegasconstant. ThevaluesofR• and E wereestimated to be 3.0/.rmolm-2 s-1 and42,884J mo1-1 for Scheuchzeria palustris and1.9/.rmolm-2 s-1 and27,552J mo1-1for Chamaedaphne calyculata, respectively. The termL representsthe leaf area index of the vascularplants.The leaf area was measuredusinga hand-heldleaf area meter (model LI-2000, LI-COR Inc., Lincoln, Nebraska).The value of L reachedup to 0.6. This value was approximatelyequallypartitionedbetweenthe two dominantvascularspecies.Mossdark respirationwas estimatedas describedin the previoussubsec- ECOSYSTEM 14,321 - 120 1991 A 0.10_ •'• MAY I JUN I JUL I AUG •1SEP •1OCT80• • -40 -0.20- . [•. , • I1.1, , tion. . E 0 120 Calculation of Canopy Photosynthesis 1992 During daytime, F c (the atmosphericflux measuredwith eddycorrelationsensors)is the sumof the net uptakeof CO2 by the vegetation(or canopyphotosynthesis, P) and the soil surfaceCO2 flux due to microbialrespiration: B O. lO_ _ g so [ • o _ Fc(day) = P + (1 - a)Rs(day) (3) \ -40 E The termRsincludesonlythe contributionfrom CO2 evolution • due to microbialactivityin the soilandroot respiration.We used • -0.10_ an averagevalue of 0.7 for the term a, whichis the fractionof Rs dueto rootrespiration [e.g.,Billings etal., 1977].Equation(3) was -0.200 usedto computecanopyphotosynthesis. On somedays,Fc data MAY JUN I JUL • AUG SEP I OCT 120 150 180 210 240 270 300 duringthe earlymorningandlate afternoonhourswere missing. The missing F cvalueswereestimated by a canopyphotosynthesisDay of Year lightresponse relationship (5). Figure 3. Seasonaldistributionsof precipitation and water At night,F c is the sumof R s and the dark respiration(Rd) tabledepthin (a) 1991and (b) 1992.Water tableAe•pthis the of the aboveground vegetation(vascularplantsand moss): distanceof the water table from an averagehollbw surface (referencedat 415.84m from the meansealevel).The negative Fc(night) = Rs(night)+ Rd(night) (4) value of water table depth indicatesthe water table position _ l.,I,_l.l.I ,,....Ill.I, .... below the surface. Most of our CO2 flux data are from daylighthours. Many nightswere quite calm and did not allow accurateeddy flux measurements.The missingnighttime F c values were estiThetotalpreciPltation andaverage temperature mated usingthe measurements of R s and R d in (4). A com- andcooler. during May-October in 1991 were 452 mm and 14.9øC, while in parisonof calculatedand measuredvalues,duringthe periods when the nighttimeeddycorrelationmeasurements were avail- 1992 thesevalueswere 642 mm and 13.4øC,respectively.The 30-year(normal) total precipitationand averagetemperature able, suggests that this approachis reasonable(Figure 2). for the May-October period are 553 mm and 13.6øC. Results The water table fluctuated ';vithin the first 0.10 m below an and Discussion averagehollow surfacefrom late May until the third week of Microclimatic Conditions July in 1991 (Figure 3a). There was a sustaineddrop in the The 1991 and 1992 seasonshad contrastingmicroclimatic water table (from 0.08 to 0.23 m belowthe surface)duringlate conditions.As comparedwith 1991,the 1992seasonwaswetter Julyuntil the end of August.During the remainderof the 1991 season, the water table fluctuated between 0.14 and 0.22 m below the surface.The water table was generally above the surfaceduring most of the 1992 season,exceptfor a brief dry spell during late May to mid-June,when the water table declinedfrom 0.03 to 0.20 m below the surface(Figure 3b). 0'15 1 0.12 Canopy Photosynthesis 0.09- 0.060.03- + Sphagnumpapillosumand vascularplantssuchas Scheuchzeriapalustrisand Chamaedaphne calyculatawere the dominant speciescontributingto canopyphotosynthesis in this peatland. Chambermeasurementsof photosynthesis (T. J. Arkebauer, + manuscript in preparation, 1995)andaboveground drymatter 0 0 i i 0.03 0.06 ! 0.09 i 0.12 0.15 Computed Nighttime Fc = Rs (night) + Rd (night)(mgm samplingindicatedthat the relative contributionof Sphagnum papillosum to the totalcanopyPhOtosynthesis wasabout50% and the remainderwascontributedby vascularplants.Canopy Figure 2. Comparisonof measuredand calculatednighttime photosynthesis inthisecosystem i'saffected byfactors including F cß light, temperature,mossmoisture content, and atmospheric SHURPALI ETAL:CO2EXCHANGE IN A PEATLAND ECOSYSTE M 14,322 kPa)areshown inFigure 4b{anonlinear leastsquares fitof 0.25 + ++ % + + 4+ + + + + + +• •.+ + + +• •'"'•+ + + + + +•.•-'-• ++ +++ + + + +/•++ + + + thesedatato (5) •i½lded anR•2valueof 0.58).These.Oata clearlydepictthe •egects of high-temperature and moisture stress onphpto•ynthesis 6f thepeatland vegetation. Under these conditi6ns, thevalueofa• wasreduced to0.07mg./n-• s-•. Thecoefficient a:2 remained about thesame (0.0005 mg CO2per/xmolof,Rp). However,thevalueof the lightcorfi- pensation point(a3)wasmuch higher (81/xmol m-2 S-•).A similarobservation of increased lightcompensation pointin mosses withincrehsing temperature hasbeen reported byHarI I I I I I + -0.05 stress, the mosssurface tissueturnsbrown.andpresumably 0.25 contains a lower percentage ofphotosynthetically competent cells[Harleyet al., 1989].This maybe a resultof the gradual shrinkageof the Sphagnumprotoplastson removalof water •,• 0.20• fromphotosynthetically active cells[Clymo andHayward, 1982; 0.15- Kaiser, 1987; Tenhunen etal.,1992]. Vascular plants commonly .,• • + • + (manuscript in preparation,1995)founda markeddeclinein leaf photosynthesis for Scheuchzeria palustrisat leaf tempera- + ++ 0.05- O-0.05 havesomewhat higheroptimum temperatures. T. J.Arkebauer + 0.10+ '= leyetal. [1989].OechelandCollins[1976]reportedan optimum temperature range of 10ø-15øCfor net photosynthesisof mosses at tundra•sites. Undbr high-temperature and moisture I• 0 + 3d0 + + ++ i 6•}0 9•0 i + i I i2•00 15•0018b0 2100 Rp(gmol m'2s'l) turesabove30øCin a concurrentstudyat this,site.Limbachet al. [1982]noteda significant reductionin netphotosynthesis of threecommontundraplanttypes(Vaccinum vitis-idaea, Betula nana,andCarexaquatilis) at leaf temperatures above25øC. Temperaturesrangingover30øCwere foundto reducephotosynthesis of the vegetationin an openbog at Lake Kinosheo, Figure 4. Canopy photosynthesis asafunction oflight(Rp) nearOntario [Whiting, 1994]. Thehigh-temperature andmoisunder (a) moderateconditions(temperature•20ø-28øC,va- por pressuredeficit • 1.2-1.5 kPa, and water table 0.00-0.08 m belowthe surface)and (b) high-temperature (30ø-34øC) and moisture stress(water table 0.21 m below the surface and ture stress(Figure4b) thereforepresumably affectedthe photosyntheticratesof the vascularplantsalso. Diurnal variation. Shownin Figure5 are valuesof P and clearday(July23,1991:day205)withmodvaporpressure deficitbetween1.9and2.4 kPa).Data arefitted Rpfora mostly erate conditions. On this day, temperaturewas 20ø-24øC,the (nonlinearregression) with a rectangular hyperbolic relationwatertablewas0.08 m belowthe surface,andvaporpressure ship(seetext for details). deficitwas1.2-1.5kPa.ThevalueofP earlyin themorningwas near zero. It increasedwith increasinglight, and reacheda peakvalueof 0.23mg m-2 s-• at about1000hours.Photovapor pressuredeficit. Moss moisture content was not mea- synthesis decreased laterin the dayandwas0.06mgm-2 s-• sured in this study. Variation in water table was used as a qualitativeindicatorof changesin mossmoisturecontent. late in the afternoon. The midmorning peakin P anda subsequent declinelaterin Lightresponse.The dependence of P onRp is shownin the day is typicalat this studysite for dayswith moderate Figure4a. Thesemeasurements werefrom periodswhentem- conditions andis similarto the patternof atmospheric CO2 peraturewasmoderate(20ø-28øC),the watertablewascloseto the surface(0.01-0.08m belowthe surface),andvaporpressure deficitwas low (1.2-1.5 kPa). Althoughthere is some scatterin the data,a (rectangular) hyperbolic relationship be- fluxreportedin previousstudies(e.g.,in an openbogat Lake Kinosheo in northern Ontario, Canada [Neumannet al., 1994]).The declinein P after the midmorninghourscouldbe due to a combinationof severalfactors,suchas the vegetatweenP andRp canbeseen.Thesedatawerefitted(nonlinear tion's light responsecharacteristics coupledwith increasing regression)with a relationshipof the form [e.g.,Landsberg, ambienttemperatureandvaporpressuredeficit.Mossspecies 1977] are knownto attainlightsaturationat lowintensities[Harl.ey et al., 1989]. Data in Figure 4a indicate a tendency toward light P = a•a2(Rp- a3)/[a• + a2(Rp- a3)] (5) saturationabove1200 /xmolm-2 s-• which occursaround wherea • is the maximumP, a 2 is the slopeof the fittedcurve midmorning hourson cleardays.Also,temperatureandvapor higher(by 5ø-6øCand 0.2atP - 0, anda3 isthevalueof Rp at thelightcompensationpressuredeficitwere significantly point(R2 = 0.70). Thevalues ofa• anda2were0.28mgm-2 0.3 kPa,respectively) in the afternoonhours(Figure5c).With s-• and0.0004 mgCO2per/xmol ofRp,respectively. Thelight compensation point (a3) wasestimatedto be 33 txmolm-2 higher evaporativedemandin the afternoonhours,the moss surfacebecamedrier.Vascularplantgasexchange mayalsobe affectedbyincreases in temperatureandvaporpressuredeficit. s-•. SkreandOechel [1981]reported values ranging from10to 140txmolm-2 s-• (at 20øC)forthelightcompensation point Under theseconditions,the middaydepression of photosynfor Sphagnumspecies. thesishasbeenobservedin otherspecies[e.g.,Tenhunen et al., Data from periodswith hightemperature(30ø-34øC), low 1980;Raschkeand Resemann,1986]. watertable (0.21 m belowthe surface,implyingreducedmoss The peakP rate mentionedabove(0.23mg m-2 s-j) is moisturecontent),and highvaporpressuredeficit(1.9-2.4 similarto valuesobservedin other wetlandstudies(atmo- SHURPALI ET AL: CO2 EXCHANGE IN A PEATLAND ECOSYSTEM sphericCO2 flux•0.18 mgm-2 s-1 in a wetmeadowtundra • nearBarrow, Alaska [Coyne andKelley, 1975]; 0.15mgm-2 s-• '• 14,323 0.3 1991 A in an openbog at Lake Kinosheoin northern Ontario, Canada + [Neumann etal.,1994];0.1-0.2mgm-2 s-1 in fenandbogsites g atLakeKinosheo, Ontario, •Canada [Whiting, 1994]). Consid- • 0.2- + + erably higher values ofmidday atmospheric CO2flux,however, •, • [Anderson andVerma, 1986; Baldocchi etal.,1983; Monteith et • • o.1- al.,1964]), ingrasslands(0.6-1.3 mg m-2s-1[Kim and Verma, • 1990]), andindeciduous andcon!fern)us forests (0.4-1.0 mg c) m-2s-• [Jarvis et51., 1976; Verma ethl.,1986]). These differ- • • + + + have been observed inagricultu?al"•rops (1.0-2.0 mgm-2 s-• ences inCO2fluxes c•nl•e'attribut½•l todifferences inplant + ++ ++ ++ + + + + ++++ ++ ++ + + + + 0 photosynthesis and respirationratesand s0ilbiologicalactivi- ties(e.g.,rootresp!ration; decomposition of organic matter). 0.3 1992 Seasonal 'vari.ation. Midday(averaged between 1000and B 1400hours)P duringthe 1991season is plottedin Figure6a. In the beginning•6f [he growingseason(middleto late May) + the vascularcanopywasjust developingand the value of P was + + + + July 23, 1991 0.25 + ß 0.20- ß ß ß ß ß +++ ß ß ß ß ß •++1 I MAY 0.15- 120 JUN 150 180 JUL I AUG 210 •- SEP 240 I+ OCT 270 300 Day of Year 0.10- Figure 6. Seasonaldistributionsof canopyphotosynthesis in (a) 1991 and (b) 1992. 0.050- -0.05 - small(0.04-0.07mgm-2 S-•). The valueof P increased to 0.12mgm-2 s-• duringthefirstweekof Juneandrangedfrom 0.09to 0.24mg m-2 s-• duringthe periodfrom the second 2100 + 1800+ +++ 1500- week of June to the third week of July. During this period, temperature (22ø-28øC)and vapor pressuredeficit (0.8-1.2 kPa) were moderate, and the water table was close to the surface(0.00-0.07 m). + + + + + + 1200 _ + + + 600_ Following a peak(0.24mgm-2 s-•) duringthethirdweekof July,P declinedto a low of 0.06mg m-2 s-• in the second + + + + week of August(day 227). The water table droppedcontinuously(from 0.08 to 0.23 m below the surface)from late July until the end of August. The reduced values of P during the secondweek of Augustwere associatedwith high temperature (30ø-34øC)and moisturestress(water table was0.21 m below + + 300 + 0 + + 30 +Ts ß 24- + + + ++ + o ß kPa).Thisdropin P wasfollowedby20 mm of precipitationon oo • August 16(day 229) and moderate temperature (18ø-22øC) and low vapor pressuredeficit (0.7-1.0 kPa) during the third + ß 00 + + • ++ + + • • + s-• in the secondweek of October. DuringlateMayin 1992(Figure 6b),P waslow(0.01-0.04 • mg m-2s-•).These values are similar tothose observed dur- + , 600 weekof August.In thisperiod,P reached0.23mgm-2 s-•. 1.2 i• August The values ofP declined 0.17 mg m -2sand -•by the end of to 0.05 mg m-2 s-•from in late September 0.03 mg m-2 ++ + + 6 thesurface, andvapor pressure deficit wasbetween 1.9-2.4 OO ß ß +++ +++ ++ 1.6 c oo ßO ß VPD 960 ' 15•00 1200 ' 1800 0.8 2100 ingthisperiodin 1991.Therewasa dryspellbeginning in the lastweek of May until the secondweek of June.Photosynthesis Figure 5. Daytimepatternof (a) canopyphotosynthesis, (b) declined from0.12mgm-2 s- • duringthefirstweekof Juneto 0.03mgm-2 s-• duringthe second weekof June.Thesesmall perature(degreesCelsius)andvaporpressuredeficit(kPa) on July 23, 1991. periodin 1991)werelikelydueto high-temperature(29ø-30øC) and moisturestress(water table was about 0.18 m below the Time photosynthetically activeradiation(Rp), and(c) surface tem- values of P (as comparedwith those in the corresponding 14,324 SHURPALI ET AL: CO2 EXCHANGE IN A PEATLAND ECOSYSTEM • 7.5 '• 5.0- • 2.5- cY ing this period in 1992 the water table was lower, and peat temperaturewas higher,which causedgreater releaseof soil CO2, thus resultingin more negativevaluesof F. 1991 A In thesecond weekof Junethevalueof F was0.9g m-2 d- • in 1991and-7.9 g m-2 d-• in 1992.Thisdifference in F was associatedwith the contrastingconditionsthat prevailed during this time in 1991 and 1992. Moderate temperature(22ø27øC), vapor pressuredeficit (0.7-1.5 kPa), and high moss moisturecontent(water table 0.04-0.06 m belowthe surface) conditionsfavoredhigh canopyphotosynthesis and small soil CO2 release during this period in 1991. On the other hand, highertemperatureandvaporpressuredeficitand lower moisture content(the water table was 0.16-0.18 m below the surface) in 1992 reducedP. In addition, the decreasein water table and high temperature resulted in a release of large I 0 ß ß ß • -2.5- •o -5.0Z 7.5- .,• m -10.0 7.5 1992 B • •zl 2.5- o o o }• o o" rO • -2.5- • -5.0- {D 0 ø8 o 0 • o water table elevations, however, were different. The water ta- o [.r.1 z amounts of soilCO2(about10g m-2 d-1) to the atmosphere during this time in 1992. Midseason. During the first week of July to the first week of August,temperature(18ø-28øC)and vapor pressuredeficit (0.8-1.2 kPa) were generally moderate in both years. The o 7.5- o '• ß -10.0MAY I JUN I JUn I AUO I SaP I OCT 120 150 180 210 240 270 300 Day of Year Figure 7. Seasonaldistributionsof daily net ecosystemCO2 exchangein (a) 1991 and (b) 1992. ble dropped(0.00-0.14 m) belowthe surfaceduringthis time in 1991 (implyingcontinueddepletionof mossmoisturecontent and hence decreasingphotosyntheticrates and increasing soil CO2 flux). In 1992 the water table was abovethe surface (resultingin low rates of soil CO2 release).Accordingly,the valuesof F indicatedoppositetrendsduring this period in the 2 years:F declinedfrom4.4 to 1.4 g m-2 d-• in 1991and increased from2.8 to 4.2 g m-2 d-• in 1992. In 1991 the drop in the water table continueduntil the end of August.During the middleof Augustin 1991the watertable was low (0.21 m below the surface).The increasedaerated depth below the surfaceand high temperaturecauseda sub- surfaceand vapor pressuredeficit was between 2.5 and 3.0 kPa). stantially greatersoilCO2 flux(about13 g m-2 d-1) to the Ample rainfall (about 145 mm) raisedthe water table above atmosphere.Also, high-temperature(30ø-34øC)and moisture the surfaceduring the third week of June. Canopyphotosyn- stressconditionsresultedin a significantreductionin photothesisincreased to 0.20mg m-2 s-1 by late June.Subsequent synthesis.Hence the peatlandwas a sourceof CO2 releasing rainfall during the remainder of the seasonkept the water 9.2 g m-2 d-• duringthistime. Duringthe sameperiodin table at or abovethe surface.Canopy photosynthesis ranged 1992, however, conditionswere favorable for high photosynfrom 0.13 to 0.23 mg m-2 s-1 duringthe periodfrom the thesisand smallsoil CO2 flux. Accordingly,the peatlandwas a secondweek of Julyto the third week of August.In thisperiod, sink for CO2 during the middle of August with a peak CO2 temperature was moderate, ranging from 18ø to 24øC, and uptakerate of 4.9 g m-2 d-• in the thirdweekof August. vapor pressuredeficit was 0.5-1.4 kPa. Thesevaluesof P are Late season. The photosyntheticrate declinedtoward the comparablewith those observedduring the secondweek of end of the season.Soil CO2 releasealsodecreased.In 1991 the June to the secondweek of July in 1991, when temperature, water table was below the surfaceduring the period from the vaporpressuredeficit,and water table conditionswere similar. end of Augustto mid-October,and the soil CO2 flux exceeded The valuesof P rangedfrom 0.13mg m-2 s-1 in earlySep- •he uptakeby canopy photosynthesis. Thusthe ecosystem retemberto 0.03mgm-2 s-• in mid-October. mained a source of atmosphericCO2. The magnitude of F decreasedtowardthe end of the season(F rangedfrom -8.1 Daily Net EcosystemCO2 Exchange to -2.5 g m-2 d-1 duringthe monthof September andfrom The net daily exchange(F) of CO2 betweenthis ecosystem -2.5 to -0.7 g m-2 d-• duringthefirst2 weeksof October). and the atmospherewascalculatedby integratingF c data over 24-hour periods: F- Fc(day) + Fc(night) (6) In 1992, however, moisture conditionswere nonlimiting, and theecosystem wasa sinkuntilmid-September (F • 1.5 g m-2 d-i). Towardthe endof the season (mid-September to mid- October),due to rapid reductionin canopyphotosynthesis, the A positivevalue of F impliesthat the ecosystemis a sink for valuesof F rangedfrom0.4 to -2.9 g m-2 d-1. Seasonallyintegrated values. Integration of the daily net atmosphericCO2. The seasonaldistributionsof F in 1991and 1992 (Figure 7) indicatesignificantday-to-dayand interannual ecosystemCO2 exchangeindicated that this ecosystemrevariations. leasedabout71 g C m-2 overa periodof 145days(mid-May Earlyseason.ThevalueofF was-1.4 g m-2 d-• in late to mid-October) in 1991. As was discussedabove, moisture Mfi•f1991,andit rangedfrom -5.5 to -3.6 g m-2 d-1 during stressconditionsprevailedduringthe later half of the growing the sametime in 1992. Peat decompositionand plant respira- seasonin this year, which reducedthe uptake of CO2 by the tion exceededcanopyphotosynthesis early in the season.Dur- vegetation.The decreasein water table led to enhancedsoil SHURPALI ET AL: CO2 EXCHANGE IN A PEATLAND ECOSYSTEM aeration and increasedrelease of soil CO2. Wetlands have beenreportedto be possiblesourcesof atmosphericCO2 under moisturestressconditions.Billingset al. [1982] measured CO2 exchangefrom intact coresfrom the wet coastalarctic tundra at Barrow,Alaska.They found that loweringthe water table from the surfaceto 0.05 m belowhad a pronouncedeffect in reducingnet carbonstorageand concludedthat greenhouse warmingcouldchangethis ecosystem from a sinkto a source. Oechelet al. [1993]madewhole-ecosystem CO2 flux measurementsover five seasonsat Toolik Lake, Alaska. They reported a net carbonlossto the atmosphereat all sitesmeasured.They 14,325 was a sourceof atmosphericCO2. These resultsindicatethat the predictedhighertemperatureand lowerwater table elevations,due to greenhousewarming[e.g.,Gorham, 1991],could significantly affectthe net ecosystem CO2 exchangein northern wetlandsand,consequently, changethem from sinksto sources of atmosphericCO2. Acknowledgments. This studywas supportedby the Atmospheric ChemistryProgramof the National ScienceFoundationunder grant ATM-9006327 and the University of Nebraska Center for LaserAnalytical Studiesof Trace Gas Dynamics.Specialthanksto Robert attributed the carbon loss at their sites to a decrease in the Clement and Andy Suykerfor their help in data collection.H. D. Earl water table,enhanceddrainage,and soilaeration.The average and SheldonSharp providedvalued assistancein maintenanceof inrate of annualCO2 lossat their sitesrangedfrom 34 to 156 g struments.JamesHines assistedin data computation.We thank Sandy Verry and Art Eliing for help with the infrastructureat the study-site, C m-2 yr-•. Whiting[1994]foundthat the open bog and SharonKelly for the stenographicwork, SheilaSmithfor preparingthe interior fen sites near Lake Kinosheo, Ontario, released car- figures,and Joe Berry and Max Cleggfor their reviewof this paper. bonto the atmosphere at ratesbetween 9 and21 g C m-2, Journal Series, Agricultural Research Division, University of Nerespectively, over a 153-daygrowingseason.Measurementsof braska,Lincoln, paper 10863. CO2 exchangeby Grulkeet al. [1990] alsoindicateda net loss of carbon(53.4g C m-2 yr-•) fromanuplandtussock tundra site in Alaska. The 1992 season was wetter and had conditions favorable for highphotosynthesis andlow ratesof soilCO2 release.Accordingly,thisecosystem wasa sinkfor atmosphericCO2with a net accumulation of 32 g C m-2 overa 145-dayperiod(mid-May to mid-October).Previousinvestigations have reportedwetland sitesto be sinksfor CO2 duringwetter periods.Coyneand --2 Kelley[1975] measureda net seasonaluptake of 40 g C m References Anderson, D. 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