Dec 26, 1997 - J. Harry McCaughey, David W. Joiner, and Paul A. Bartlett ... near Thompson, Manitoba, Canada, as part of the Boreal Ecosystem-Atmosphere ...
JOURNAL OF GEOPHYSICAL
RESEARCH, VOL. 102, NO. D24, PAGES 29,009-29,020, DECEMBER 26, 1997
Seasonal trends in energy, water, and carbon dioxide fluxes at a northern Peter
boreal
wetland
M. Lafleur
Department of Geography,Trent University,Peterborough,Ontario, Canada
J. Harry McCaughey, David W. Joiner, and Paul A. Bartlett Department of Geography,Queen's University,Kingston,Ontario, Canada
Dennis
E. Jelinski •
Department of Forestry,Fisheriesand Wildlife, University of Nebraska, Lincoln, Nebraska
Abstract. Micrometeorologicalmeasurementswere made over a northern boreal fen near Thompson,Manitoba, Canada, as part of the Boreal Ecosystem-Atmosphere Study. The measurementperiod extendedfrom the start of snowmeltuntil the early fall, at which time senescence was widespreadthroughoutthe fen. Data analysisconcentratedon identifyingseasonaltrends in energy,water, and carbon dioxide fluxesand linking them to observedsurfacecover changes.Albedos (solar and photosynthetically activeradiation (PAR)) showedlarge decreasesover the melt period, reachingseasonallowsat the end of melt. Solar albedo increasedin the summerin responseto vegetationgrowth on the fen, while PAR albedo remained constant.Incoming and outgoinglongwaveflux seasonal trendswere similar, so seasonalchangesin net radiation were driven by the net solar flux. During the springthaw, the melting of snowand ground ice was equal to about 28% of the daily total net radiation,while the soil heat flux accountedfor about 5%. Bowen ratios at this time were aboveunity. Mean Bowen ratio decreasedto 0.70 during the period betweenspringthaw and leaf-out. As the vascularvegetationcover developed,Bowen ratios decreased
to seasonal lows of 0.10-0.20
near midsummer
and then increased
to
aboveunity duringsenescence. The daily evaporativefraction (EF) was highest(->0.80) during midsummerwhen the vascularvegetationwas in full leaf and actively photosynthesizing, and EF decreasedto a mean of 0.55 during senescence. Eddy correlation measurementsof carbon dioxide flux showedthe fen acting as a net sink for CO2 onlywhile the vascularvegetationwas activelyphotosynthesizing with a dailymean flux
of -0.81 g CO2-Cm-2 d-1 (standard error= 0.16).Beforeleatingandduringsenescence the fen was a net sourceof CO2. Integrated over the studyperiod of 124 days,the fen 2 experienceda net lossof 30.4 g CO2 m- to the atmosphere. changesare influencedby trends in climatic change.Quantifying the magnitude and behavior of these exchangesis a Wetlands occupy approximately20% of the boreal forest fundamental component of determining the role of boreal [National WetlandsWorkingGroup (NWWG), 1988]. The vast wetlands and, indeed, of the boreal forest in global climate majority of these are peatlands and, as such, represent an change. immense(200-400 Pg C) store of carbon [Postet al., 1982; In the present study we quantify seasonaland daily exGorham, 1991].Although their participationin the globalcar- changesof radiation, heat, water, and carbon dioxide in a bon cycle is still the subjectof intensivestudy, it is generally northern boreal wetland. The wetland was classified a mineralbelievedthat northern peatlandsare a strongglobal sourceof poor fen supportinga diversecoverof vascularvegetationand methane to the atmosphereand a sink for atmosphericcarbon mosses. The site was one of four tower flux sites located west dioxide(presumingpeatlandsare accumulating).In the short of Thompson,Manitoba, Canada, in the Northern StudyArea term, the balancebetweenthesetwo opposingeffectsis depen- (NSA) of the Boreal Ecosystem-Atmosphere Study dent upon soil temperatureand water table status[Crill et al., (BOREAS). PreliminaryBOREAS results[see Sellerset al., Introduction
1988; Roulet et al., 1992b; Moore and Roulet, 1993; Funk et al.,
1995] indicate that in terms of its interactionwith the atmo1994] and thus is closelylinked to seasonaland daily energy sphere the fen is distinct from the surroundingforest ecosysexchangewith the atmosphere.In the longer term, these extems. Before plant growth in the spring,it is a considerable source of CO2 to the atmosphere,yet during the growing •Nowat Departmentof Geography, Queen'sUniversity, Kingston, period, midday uptake of CO2 at the fen rivals that of the Ontario, Canada. forests. Midday evaporation rates from the fen are almost Copyright1997by the American GeophysicalUnion. alwayslarger than from the forest sites. The present study exploits a time seriesof data extendingfrom spring thaw to Paper number 96JD03326. 0148-0227/97/96JD-03326509.00 earlyfall in 1994to investigatesurface-atmosphere exchanges 29,009
29,010
LAFLEUR
0.5
ET AL.: NORTHERN
BOREAL WETLAND
FLUXES
usinga relev6methodand classifiedusingacom,binationof
Km
two-way indicator species analysis (TWINSPAN) and detrendedcorrespondence analysis(DCA). These resultswill be reported in detail elsewhere.Briefly, microreliefwith respect to water table accountsfor much of the variability in vegetation. The vegetation communitieswere classifiedalong the moisturegradientinto four broad communitytypes.Extremely wet, medium speciesrich meadowsare treelessand dominated by sedges(Carex spp.) and buckbean(Menyanthestrifoliata). The brown mossDrepanocladusexannulatusis the dominant understoryspecies.The sedgefen communitytype is alsowet with little hummock development. It is dominated by sedges (Carexspp.)andbuckbean.The moss-shrub communitytypeis drier and more hummocky.Speciestypical of this community include bog birch and bog rosemary (Andromedapolifolia). Small stunted tamaracks may also be present. Characteristic bryophytesincludeMeesiatriquetra,Scorpidiurnscorpiodes, and Carnpyliurnstellaturn.Much of the area adjacentto the tower falls into this community.The driest communitiesare characterized by an overstoryof dense tamarack with hummocks dominatedby mosses(Sphagnurnwarnstorfii,S. angustifoliurn and Tornenthypnurn nitens). The total area of the fen is approximately50 ha and is kidney shaped(Figure 1). The peat surfaceis slightlymoundedin the central portion of the fen, but overall, the entire fen has little relief. The fen is underlainby 1-6 m of peat, with peat depths generallygreater than 5 m in the central portions. Below the peat lies a silt/claylens of unknownthickness.The surfaceis characterizedby classichummock-hollowtopographywith the meanpositionof the water table usuallyabovethe peat surface
I
!
access
trail
FSA
375rn
flux tow,
boardwalk
FEN hut
FOREST
UPLAND
orthern Study Area Thompson Study Sites
,0
,
5,00km
A•rport
0
5
10 15 20Km
Figure 1. Map of the fen study site and its regional setting. FSA
is flux
source
area.
Boreas
tower
flux sites shown
in
in the
hollows.
Dead
and
decadent
tamarack
are common
within approximately200 m of the tower. The poor condition bottom right inset are YJP, young jack pine site; OBS, old of these trees is likely a function of changing water levels black spruce;and OJP, old jack pine. brought on by alteration of the fen hydrologyby beavers. The main hydrologicinflow is from a watershedto the north which drainsthrougha culvertunder Highway391 (Figure 1). at the fen in relation to changesin surfacecover,particularly Water also drains into the fen from the surroundinguplands, particularly the elevated land to the southwest.Piezometric vegetationphenologyand the hydrologicregime. measurementsindicated that there was little groundwaterinput to the fen. Much of the water is deflectedlaterally to the Study Site fen margin where a moat exists,though in early spring,lateral The fen is located approximately50 km west of Thompson, flows were observed over the entire fen surface. The main Manitoba, alongHighway391 at latitude 55.9øNand longitude outflow was through a degradedbeaver dam in the southeast 98.4øW(BOREAS grid 781.14,618.16).The regionis near the corner of the fen, where water is routed through a seriesof northern limit of the closed-crown boreal forest. The landwetlands and streams and ultimately drains north into the scapeis dominatedby standsof black spruce(Picearnariana) SapochiRiver. There is also a small amount of seepagefrom and, to a lesserextent, balsampoplar (Populusbalsarnifera), the west end of the fen. with jack pine (Pinusbanksiana)occupyingdrier sites.Wetlands exist in the lowland regionsand along hydrologicpathInstruments and Methods ways. The climate is dry continental,with short, warm summers, long, cold winters, and a warm-seasonprecipitation In 1993 a 160 m plywoodboardwalkwasconstructedinto the maximum.Mean annualtemperatureof the Thompsonregion fen, with an instrument hut located 120 m from the shore and is -3.4øC, and mean daily temperature is -25.0øC in January a tower platform at the terminus(Figure 1). The instrument and 15.7øCin July. Average annual precipitation is 536 mm, tower was a 12 m communication-typesteel tower guyed by with 43% of this total falling in the monthsof June, July, and anchorssunk into the clay sedimentsat the base of the peat. August. Power(120V AC) wassuppliedto the hut andtowerby a diesel The fen is speciesrich as a resultof its minerotrophicstatus. generator located on the shore of the wetland. In general, the fen is characterizedby an undercover of a Radiation instrumentswere situated on support arms near variety of Sphagnurnspeciesand an overstoryof sparselyscat- the top of the tower (10.35 m) oriented to the southeast.Net tered tamarack(Larix laricina),bog birch (Betulaglandulosa), all-waveradiation, Q *, wasmeasuredwith a net pyrradiometcr and a variety of sedges.The mean heights of the overstory (model CN-1, Middleton Inst., Melbourne, Australia) purged layerswere 3.5 m for tamarack,0.8 m for bog birch, and 0.5 m with nitrogen gas. Incoming and reflected global solar radiafor the sedge.The vegetationon the fen surfacewas described tion were measuredwith pyranometers(model 8-48, Eppley
LAFLEUR
ET AL.: NORTHERN
Labs., Newport, Rhode Island) and incoming and reflected photosynthetically activeradiation (PAR) were measuredwith quantumsensors(model LII90-SZ, LICOR, Lincoln, Nebraska). Outgoing longwaveradiation was measuredwith a pyrgeometer (model PIR, Eppley Labs.,Newport, Rhode Island). Wet- and dry-bulbtemperaturesand wind speedwere measured at heightsof 2.5, 3.25, 4.0, 5.0, and 6.0 m. Wind speed was measured with DC-generating anemometers (model 12102, R. M. Young Co., TraverseCity, Michigan). Wet- and dry-bulbtemperatureswere measuredwith singlejunctioncopper-constantanthermocouplesensorscasedin 5 mm stainless steel tubing and mounted in ventilated shields.The wet-bulb sensors were water.
Wind
covered direction
with
cotton
wicks wetted
was measured
with
distilled
at the 6.0 m level with a
vane (model 12302,R. M. Young, TraverseCity). The soil thermal regime was monitored with copperconstantanthermocouplesimbedded in wooden dowel. Sensorswere placed at depthsof 0.01, 0.05, 0.10, 0.25, 0.50, 0.75, 1.00, 1.50, and 2.00 m. Two such rods were installed, one in a
peat hummock with the zero mark at the moss surface and another in a hollow area with the zero mark at the position of the water table at the time of installation.Water table changes from an arbitrary datum were measureddirectlywith a recording well locatedat the tower platform. This deviceconsistedof a float and counterweightattachedto a flywheel mounted on a potentiometer. The millivolt readingsfrom the potentiometer were calibrated againstdaily manual measurementsof water level taken on the outside of the well.
Soil heat flux wasmeasuredwith two heat flux plates(model HFT-3, REBS, Seattle, Washington),one installed in a hummock 10 cm from the mosssurfaceand the other placed in the peat soil 10 cm below the bottom of a hollow. The sensor installed in the moss hummock was found to be unreliable, and
only the hollow sensorwasusedin the followinganalysis.Since heat flux plateshavebeen shownto typicallyunderestimatethe true soil heat flux in organic soils [Rouse,1984; Halliwell and Rouse, 1987], we used a calorimetric method to correct the heat flux plate readings.The soilwas dividedinto layersbased on the spacingof the soil temperaturesensors,and heat storage for each layer was computedfrom time differencesin soil temperature and heat capacityof the layer. Soil heat capacity C, was computed as C, = xoCo + x,•C,• + x,C,, where
x....... are the volumetricportionsof organicsoil,water, and air; and C...... are the heat capacities of the organicmatter, water, and air. Porosityof the peat was assumedto be 80% in all layers,andx, waszero below the water table. There were no measurements of moisture content above the water table, and
therefore the water content in the peat hummocks was assumedto be constantat 25% (basedon unpublishedmeasurements at other wetland sitesby PML). Heat flux out of the basal layer was computedfrom the temperature gradient between the two deepest sensorsand literature values of the
BOREAL WETLAND
FLUXES
29,011
plates were installed, and the daily total ground heat flux was calculatedby the calorimetric method. We have also computedsensible,Qa, and latent, Q,,, heat storage in the air layer between the fen surface and the net radiometer. For these calculationsthe approximateequations given by Thom [1975] were used,whereby
Qw : O.5zrbe,
(2)
where zr is the referenceheight (10.35 m) and 6T and be are the half hourly rates of changeof air temperature and vapor pressure, respectively;6T and be were computed from the profiles of temperature and humidity by dividingzr in layers representativeof each sensorin the profile and summingthe results.
Energy used in snowmelt,Q .... and melting of ground ice, Q•cowas computed from daily measurements.Q snowas computed from daily estimatesof melt recorded from snow-wire measurementslocated near the tower and depth and density measurementsmade along two transects.Snow density was
unchanging duringthemeltat 225kgm-3. At thestartof melt, there was 540 mm of snowon the fen. Total daily melt energy was determined from changesin depth measuredat the snow
wiresasfollows:Qsno= Ad*0.225*Lr, whereAd is the daily changein snowdepthandL r is the latentheatof fusion(334 kJ kg-•). We assumed that thawingof the soildid not occur until all the snow was melted. Q•cewas computed from daily measurementsof thaw depth made near the ground temperature rods by probing the ground with a graduatedrod. It was assumedthat the frozen portion of the soil containedno water and that ground ice melting occurredonly when Q* was positive.The portionof the soilcontainingicewasconstantat 80% for all depths.We also assumeda uniform horizontal distribution of ground ice in the fen. Although field measurements suggestedthat this was not the case,lack of accuratespatial representationprecluded a more detailed analysis.Thus we caution that the estimatesof Q,no and Qicemay contain considerable error and are given only as rough estimates. All data from the meteorologicaland soil sensorswere recorded on a data logger (Campbell Sci., model CR7, Logan, Utah). Signalswere scannedand recordedevery 5 s, and half hourly averageswere saved in the logger memory and later transferred to computer. Eddy fluxes of sensibleQ•, and latent Q•- heat were measured by eddy correlation techniques.Sensibleheat was measuredwith a single-axissonicanemometerand fine (12.5/.•m) wire thermocouple (Campbell Sci., model CA20, Logan, Utah), and vapor densityfluctuationswere measuredwith a krypton hygrometer(model KH20, Campbell Sci.). These instrumentswere mounted at a height of 4.3 m from the fen surfaceon a swivel arm which was oriented manually into the wind. The sensor signalswere monitored on a data logger
thermalconductivity of saturated peat(0.5W m • K-•). Total (model2Ix, CampbellSci.)at a rate of 10 Hz with an averaging heatstorage(Qr;,•) wasthenequalto the sumof the storage subinterval of 15 min. Covariance software within the logger in eachof the layersplus the basalheat flux. These calculations was used to combine the wind and scalar signalsto give halfwere done over 1-, 3-, 6-, and 12-day periods, and the total hour fluxes. heat storagewas comparedto the sum of half hourly measureThe net carbon dioxide flux F•. was also measuredby eddy mentsfromthe heatfluxplates(QG,platc). The meanratioof correlation. The equipment for measuring carbon dioxide Q(;....I/Q(/,plate for thefouraveraging timeperiods variedfrom fluxes was designedfrom literature recommendations[e.g., 1.74 to 3.57, with an overall mean of 3.04. This value was then
applied as the correction factor for all half hourly heat flux plate readings.For the first 20 daysof the study,no heat flux
Suykerand l/erma, 1993;Leuningand King, 1992;Leclercand Thurtell, 1990] and field tested during the August 1993 intensive field campaignof BOREAS. Carbon dioxide concentra-
29,012
LAFLEUR
ET AL.: NORTHERN
BOREAL
WETLAND
FLUXES
tion and flux densitywere measuredusing air obtained at the sonic anemometer
arm. This
air was drawn
down the tower
through 6.35 mm ID Bev-a-line IV tubing to a fast response infrared gas analyzer (IRGA) (model LI-6252, LICOR, Lincoln,Nebraska)at the baseof the tower.Airflowwasmeasured with a massflow sensor(model 5860E, Brooks Instruments, Hatfield, Pennsylvania),and cross-correlationanalysiswas
S(Q* - Q•)
œ:"
+ y)
(3)
where S is the slopeof saturationvapor pressureversustemperature(a functionof air temperature),and 3/is the psychrometric constant.Q* and air temperature were obtained by extrapolationfrom a nearbyBOREAS flux tower, in this case the youngjack pine (YJP) site 8.5 km to the east.The extrapused to determine the travel time of the air from the tube inlet olation was done using linear regressionof coincident data to the IRGA. The direct output from the IRGA, whichwas run betweenthe two sites.Q* from the YJP wascloselycorrelated in absolutemode, was calibrated daily using two span gases with Q* at the fen; the linear relationshiphas a slopeof 0.94, ranging between 330 and 400 ppmv and flowing through the a coefficient of determination (r 2) of 0.944,and a standard IRGA at the same pressureas that of air down the tower. errorof 46.1W m-2. Air temperature waslesswellcorrelated, These field calibrations,linear over the range of ambient carwith a slopeof 1.12,r2 = 0.83, ands.e. = 2.67øC.Q, was bon dioxideconcentrationsand unchangingwith temperature, determined as a percent of net radiation based on the mean were constantlycorrectedfor pressurechangesrecordedwith a percentagefor 10 days preceding and following the missing gaugepressuretransducer(model PX142, Omega, Stamford, data. The coefficientin (3), a, is a measureof the departure Connecticut). The output from these instrumentswas also from equilibriumconditions.The mean dailyvalue for a at the monitored on the 21X data logger. The logger covariance fen determinedfrom availableE C fluxesfor the periodssursoftware,with the same10 Hz measurementfrequencyand 15 roundingthe missingdata recordwas 0.97 with SE (standard min averagingsubintervalusedfor the sensibleand latent heat error) = 0.026. fluxes,was used to computethe half hourly averagefluxes. Postseasonprocessingof the eddy correlation fluxes included
the so-called
"Webb"
corrections
for fluctuations
in
temperatureand water vapor density[Webbetal., 1980;Leuning and Moncrieff, 1990]. Since emissionsfrom the krypton lamp in the hygrometerare absorbedby both water vapor and oxygen,the water vapor flux is corrected to accountfor this absorption [Tanner et al., 1993]. As well, the flux associated with a changein carbondioxide storage[Hollingeret al., 1994] was added to the eddy correlation flux to obtain the final net flux of carbon
dioxide.
Evapotranspiration Estimates
For the present studythere was a need to constructa complete seasonalrecordof evapotranspiration (E). As the direct eddy correlation (EC) measurementsof E were discontinuous, two alternate means were used to estimate E for the
missing time periods. Two methods were used. The first method employedthe Bowen ratio energybalanceapproach. Followingthe suggestion of Thom [1975],we plottedprofilesof adiabaticallycorrectedtemperature (T) againstvapor pressure (ea) and computedthe best fit line. The productof the slope of this line and the psychrometricconstantgives the Bowen ratio /3. E was then determinedfrom the energybalance equation as E = (Q* - Qs)/[;•(1 + /3)], where the quantity (Q* - Qs) is available energy from net radiation minus heat storage,and ;• is the latent heat of vaporization. Bowenratioswere computedusinga customcomputerroutine
that plotshalf-houraverageprofilesof temperatureandvapor pressureand allowsvisualinspectionfor spuriousdata points causedby dry wet-bulbwicksor poor ventilation.Thesepoints were removedfrom the profile, and the slopeof ea versusT was recomputed(see Halliwell and Rouse [1989] for full details). Using this technique,95% of the missingdata were retrieved.
Data Collection and Analysis Periods
Data were collectedfor the period April 9 to September19 (DOY 99-162), 1994. Becauseof instrumentcalibration,instrument malfunction, or generator maintenancesome instruments were not operational for the entire field season.The sonicanemometertransducersand fine wire thermocoupleare extremelysensitiveto adverseweather,particularlyrain, sothe eddyfluxesof sensibleand latent heat and CO2 were occasionally interrupted.This problemwasmore prevalentin the early part of the record. An extensiveperiod of generator malfunction occurredfrom July 1 (DOY 182) to 5 (DOY 186), inclusive,at which time no data were recorded.Eddy flux measurementsof CO2 did not begin until early June (DOY 153). The BOREAS experimentwas designedaroundperiodsof concentratedeffort referred to as intensivefield campaigns (IFCs). Rather than focus on these periods,we divided the data set into distinctperiods based upon changesin surface coverat the fen. There are four main periods:spring,preleaf, green, and senescence."Spring" includesDOY 99-138 and is subdividedinto snowmelt(DOY 99-118) and soilthaw (DOY 119-138) periods.The remainingdata periodswere basedon field observationsof phenologyof the vascularspecieson the fen and, as such, are somewhatarbitrary divisions."Preleaf" extendsfrom the end of thaw to the time when leating of the vascularvegetationspecieshad began (DOY 139-158). The "green" period extended from leating until senescencewas observedthroughoutmost of the fen (DOY 159-243). The "senescence"period included the later stagesof senescence and leaf fall of the vascularplants (DOY 244-262). We note that our preleaf period correspondscloselyto the BOREAS IFC-1 (DOY 144-167), IFC-2 (DOY 200-220) fall near the middle of our green period, and our senescence period correspondsto IFC-3 (242-262). The presentationand discussion of data belowfocuson the springand phenologicalperiods;however, we also present data for the three BOREAS IFC for comparisonwith other works.
The finalmissingdatarelatedto periodswhenno instrument signalswere recordedbecausethe generatorwas inoperable (e.g., day of year (DOY) 181-186). For theseoccasions we usedthe Priestley-Taylor(P-T) approachbecauseof its robust- Meteorology nessand limited data requirements.The P-T approachcalcuMean daily air temperature and precipitation during the lates evaporationas studyperiod are shownin Figure 2, and a comparisonwith
LAFLEUR ET AL.' NORTHERN
BOREAL WETLAND FLUXES 0.80
AprilI MayI JuneI JulyI Aug.I Sept. 70 I
I
I
I
I
s•pn ng thaw Pr•e;f green senescence
6O
20-
0.60 • o
5O
0.40 J
104O
-- solar
o.•o I•
3O -10
2O
0.00 -20
29,013
'
,
95
10
110
,
125
,
,
140
155
,
Day
-3O
90
110
130
150
DaY
170
90
of
210
230
250
270
,
170
185
of
,
,
,
,
,
200
215
230
245
260
Year
Figure 3. Trend in solar and photosyntheticallyactive radiation (PAR) albedosduring the study.
Year
Figure 2. Trend in mean daily air temperature Tair (solid line) and daily total precipitationP (bars).
tion growth and was virtually constantat 0.055, SE = 0.012. The vascularplant specieson the fen beganto leaf near DOY 151 at whichtime the solaralbedobeganto increasegradually. Albedo reacheda seasonalhigh of 0.18, SE = 0.0127,when the fen was in full leaf (DOY 204-220). Senescence on the fen beganin earlyAugust(DOY 218-225) and appearedto cause a slightdecreasein solaralbedo.By the end of the record,most of the vascularplants were senescent,appearingyellow or brown, and leaf fall was widespread.Albedo averaged0.155,
climatenormalsis givenin Table 1. In general,the studyperiod was warmer and considerablydrier than the long-term normals.Mean daily temperaturesfor the monthsof May to September averaged1.9øCabovenormal, with June and September averagingmore than 3øCabovenormal. The trend for the studyperiod showsthe temperatureclimbingabovefreezingin May, peaking in June, and stayingrelativelywarm through to September.Mean monthlyprecipitationfor May to September SE = 0.011, at this time. Seasonaltrendsin the longwavefluxes,shownin Figure 4a, was 38.7 mm, averaging25.1 mm below normal. July was the wettest month with 69.5 mm of rain. indicate that although both incoming and outgoing componentsfollowed similar seasonalpatterns,their daily behaviors were less synchronized.Both longwave fluxes were smallest Results during snowmeltand increasedsteadilyuntil near the summer Radiation Balance solstice(DOY 171) after which there was no trend. The inThe seasonaltrends in mean daily global solar albedo and cominglongwaveflux showsa greater daily variation than the PAR albedo are comparedin Figure 3. The greatestrange in outgoingflux. Moore et al. [1994] found similar seasonalpatalbedosoccurredbetweenthe presnowmeltand postsnowmelt terns in longwavefluxes for a fen in northern Quebec. The conditions.The solaralbedorecordbegantwo daysbefore the mean daily net longwaveflux had virtually no seasonaltrend start of melt with values near 0.70. Solar
albedo
decreased
throughoutthe melt reachingthe seasonalminimum (0.090.10) during the days following final melt. This also correspondedto the time of highestwater table conditionsin the fen. The PAR albedo record began five daysinto the melt at which time it was about 0.10 larger than the solar albedo, and it appearedto decreaseat a faster rate than the solar albedo during snowmelt.At the end of the melt, PAR albedo was around 0.05. Both solar and PAR albedosincreasedsuddenly in responseto a snowfallat the end of the melt (DOY 123). After
snowmelt
the solar albedo showed a distinct
seasonal
trend which couldbe linked to phenologyof the vegetationon the fen, while the PAR albedo showedno responseto vegeta-
450
E
•
350-
250-
X3 150' •
-50-150
300
1.O
29O
Table 1. Mean Daily Temperature T and PrecipitationP Comparisonfor the Study Period, April to September1994, and Climate Normals Derived From the Atmospheric Environment ServiceMeteorologicalStation at Thompson,Manitoba
T1994 , øC T ..... !, øC P 1994,mm P ..... i, mm
April
May
June
July
August
September
-2.0 -2.4 18.9 28.0
7.7 6.3 27.3 45.8
15.4 12.3 45.8 71.5
16.5 15.7 69.5 84.3
14.4 13.8 31.8 77.7
11.4 7.2 19.0 63.4
280
270
260
i
95
115
135
155
Day
,
175
of
195
,
215
I
235
i
0.0
255
Year
Figure 4. Trendsin (a) daily incoming,(El) , outgoing(L o), and net (L,) longwavefluxes;and (b) surfaceradiative temperature (rrad) and normalized extraterrestrial radiation (rex).
29,014
LAFLEUR
1.00
spring thawPrt•-af
ET AL.' NORTHERN
green ence
0.80
0.60'
BOREAL WETLAND
FLUXES
6). Total storagewasequalto 41% of net radiationduringthe soil thaw period. Although not shown in Figure 6, latent and sensibleheat storage in the air layer below the net radiometer were also computedfor the springperiod. Daily averagesfor Q, and
werenearzero,rangingfrom -1.3 to 1.0W m-2. Thesevalues
0.40'
comprisedabout 1% of total storageand lessthan 1% of net 0.20'
radiation.
Heat storageduringthe summerperiodswas almostentirely comprisedof the soil heat flux. On a daily basis,Qr;/Q* averaged 6%; however,QG/Q* was typicallynear 25% during Day of Year the midday.Again, Q, and Q,• comprisedlessthan 1% of Q* Figure 5. Trend in daily radiation efficiency,ratio of net to on a daily basis,with Q, almostan order of magnitudelarger incomingsolar radiation (Q*/Ki) for the study. than Q,•. Near sunriseand sunset,Q,/Q* varied from 50 to 100%, and Qw/Q* rangedfrom 30 to 70%. Both are related to rapid changesin air temperatureand smallvaluesof Q* that typicallyoccur at these times. (Figure 4a) and showeda net lossof energyvaryingbetween Diurnal average energy storagefluxes show different pat-2 and -98 W m -2 with a seasonal mean of -52 W m -2. ternswhen separatedinto vegetationcoverperiods(Figure 7). Surfaceradiativetemperature(Trad) was derivedfrom the During the preleaf period, Q• was almost symmetricalwith outgoing longwave fluxasTrad = (Lo/•O')ø'25with• = 0.96. net radiation (Figure 7a). Daytime energygains dominated The trend in mean daily Trad is comparedto the extraterres- over the nighttime losses,and the mean daily soil heat flux is trial solarinput in Figure 4b. Temperaturesincreaseafter melt 11.6W m 2.For thegreenperiod,Qc,,isoutof phasewithnet until the solstice.Despite a 40% decreasein potential solar radiation, peaking sharply 2-3 hours before solar noon and inputs to the system,radiative temperatureswere relatively fallingoff graduallythroughoutthe daylighthours(Figure 7b). constantthroughoutthe remainder of the growingperiod and The meandailyflux (5.4 W m-2) wasonlyabouthalf of the into the early fall (Figure 4b). The phaseshift betweenpeak value of the preleaf period. During senescenceand leaf fall, solar inputs and extendedmaximum radiative temperatureis mean daily Q• wasnear zero, with an almostequal numberof causedby the large thermal inertia of thesewetland systems hourswith energylossesand gains(Figure 7c). This pattern and is likely to be greater than that of the surroundingforest wasa responseof the largethermalcapacityof the wet fen soils environments. to the shorter days and cooler nighttime air temperaturesin The combined effectsof changesin albedo and longwave the fall, causinglarge and sustainedenergylossesat night. exchangeon net radiation are examinedthroughthe radiation efficiencyterm which is defined as the ratio of net radiation to Eddy Fluxes of Heat and Water incomingsolar radiation Q*/K/ (Figure 5). As net longwave Eddy correlation measurementsof heat and water vapor exchangeshowedno distinctseasonaltrend (Figure 4a), radi- were made on 126 daysthroughoutthe study, althoughsome ation efficiencyis almostentirely controlledby albedo.Q*/K i dayshad only a few hoursof measurement.We have examined increasedsharplyasthe snowcoveron the fen wasablatedand closure of the energy balance for all dayswith more than 6 peaked at approximately0.80 in the period before green-up, hoursof measurementsduring the daylighthours(Figure 8). when albedowasat its seasonallowest(Figure 3). Throughout Closure appearedto be lower during the springperiod than the rest of the summerand early fall, Q*/K, decreasedgrad- later in the season.This may have been causedby underestiually in responseto increasingalbedo and leveled off during mation of the energy used for snowmeltand for thawing the the early fall. Mean radiation efficiencyfor the green period soil. It was alsopossiblethat limited fetch may haveresultedin was0.55, SE = 0.006,which is comparableto valuesfrom other poor energy balance closure. Figure 1 showsthat fetch was wetland studies[Lafieuret al., 1987;den Hattog et al., 1994]. most limited directly toward the north and south shoresof the 0.00
i
95
115
i
135
i
155
i
175
i
195
i
215
I
235
I
255
Heat Storage 25O Figures6 showsthe daily averageheat storagecomponents and net radiationfor the springperiod,and Figure7 showsthe 200 diurnal trendsin thesecomponentsfor the three summerpe150 riods. Only daily estimatesof most heat storagecomponents were availableduring the springperiod, thus daily valuesfor 100 the melt and thaw periodsare presentedtogether(Figure 6). 50 Soil heat flux was only a minor componentof total storage (16%) duringthe period of snowmelt,and the energyusedin snowmeltaccountedfor most (89%) of the total storageflux. -50 As a proportion of net radiation, total storagewas 31%, and 0") 0 0 '•-'•-04 04 O0 O0 snowmeltwas 28%. Similar resultswere found for the period Day of Year of soil thaw. The energyconsumedin melting groundice accountedfor 82% of total storage.On average,this amountof Figure 6. Daily heat storagecomponentsduring the spring energyaccounted for about1.8mmd-• of thawovera 19-day periods(DOY 103-138). Q• is soil heat flux, Q .... is latent period.Soilheatflux (Q G) accountedfor 18% of total storage, heat used in melting snow, and Q ice is latent heat used in with its importanceincreasingthroughoutthe period (Figure melting ground ice.
LAFLEUR
ET AL.: NORTHERN
50O
BOREAL WETLAND
FLUXES
29,015
2.00
ill'
30O
1.5o,
0
1 .oo, llllllllllllllllllllllllllllllllllllllllllllll
1oo 75
0.50
-ll--e e
0.00
5o
110
25
155
170
185
200
215
of
Year
230
245
260
Figure 8. Seasonal trend in daily energy balance closure, sumof sensibleand latent heat fluxes(QH + Q•:) to available energy,net radiation minusheat storage(Q* - Qs).
-25
--50
.......
08
r ........
I........
12
16
] ........ 20
• ........ 24
].... 04
(•MT 5OO
100 75
5O
25
X -25
-501 08
12
16
20
24
O4
GMT 5OO
senesoenoe
400
C
300 200
100 0 -100
lllll
lllllllll
II
Ill
lll|ll
ll|ll
lllllllllllll|l
I
lOO
,,,
140
Day
x
•
125
75
---- QQ
1= 50
fen. In order to test this hypothesisthe data set was stratified on the basisof fetch to height ratios into quadrantsof good (_>1:100) and poor (_ we caution that given the assumptionsin the methodsand accumulation of errors, this value may not be statisticallydifferent
o. 1
0.0
from zero. However, it is clear that the fen was a net sink for
-0.1
carbon
-0.2 -0.3
0.3
c
0.2 o. 1
0.0 -0.1
-0.2 $8ne$cenc8 -0..3 "I ....... 08
I ....... 12
s.e.
I ....... 16
I ....... 20
I ....... 24
I"' 04
GMT
Figure 13. Average diurnal trends in carbon dioxide eddy
fluxes(Fc) for phenological periods:(a) preleaf,(b) green,and (c) senescence.Periods are defined in the text. Points are means for all available half hours, and bars represent the standarderror (SE).
dioxide
for most of the summer
of 1994 and that the
growing seasonuptake was balanced off by the strong net sourcein the shoulder seasons.Given that the study period covered only parts of the spring and fall shoulder seasons, beginning 23 days after final snowmelt and ending several weeksbefore first snowon the ground,in all likelihood the fen was a net source of CO2 on an annual basisfor 1994. There are few micrometeorologicalinvestigationsof CO2 exchangein peatlands with which to directly compare our results.Coyneand Kelley [1975] measuredmean daily F•. of
-1.96 g CO2-Cm-2 d-• overa wet meadowsitenearBarrow, Alaska,resultingin a net seasonal uptakeof 40 g CO2-Cm-2. A smallermeandailyflux (-0.46 g CO2-Cm-2 d-1) was measuredin midsummerat a water-stressedbog at Kinosheo Lake in the Hudson/JamesBay lowlands by Neumann et al. [1994]. More recently, at an open peatland in northern Minnesota,Shurpaliet al. [1995]measureda seasonallyintegrated
netlossof 71 g CO2-Cm-2 duringa dryyearanda netuptake of 32g CO2-Cm-2 duringa wetyear.Theseauthors foundthat midseasondaily fluxes varied from -1.2 to -0.38 g CO2-C
During the senescenceperiod the fen was once again a net sourceof CO2 for most of the diurnal cycle.Uptake was still occurring throughout the daylight hours with the strongest signalin the midmorning(1500-1600 UT). Althoughmuchof the broad leaf vascularvegetationwas in senescenceby this time, the tamaracktrees and, presumably,the mosseswere still photosynthesizing. Mean daily F c for the senescence period
was2.24,SE = 0.34,g CO2-Cm-2 d-1 One curiousfeature of the dailyplotsof F• is that nighttime respirationappearsto be larger in the preleaf period, averag-
m-2 d- • duringthedryyearand- 1.14to -0.76 g CO2-Cm-2 d-• duringthe wet year. It appearsthat midseason fluxes observedat the BOREAS fen are comparablewith thosefound in other peatlands.The differencesare likely due to several factors, one of which is total productivity of the system.Unfortunately,sufficientinformationon productivityis lackingto make comparisonsbetween studies.
Summary and Conclusion Seasonalchangesin surfacefluxesfrom this boreal wetland
ingnear0.2 mg CO2-Cm-2 s-1, thanin the greenor senes- were closelylinked to changesin surfacecover. The dynamic cenceperiods,averaging near0.10-0.15 mg CO2-Cm-2 s-• nature of thesechangesis clearlyrepresentedthroughanalysis The reasonfor this behavioris not clear, and we acknowledge the difficultyof measuringeddycorrelationfluxesat night.Two explanationsfor this result are possible.One is that duringthe preleaf period the error barson the nighttimefluxesare large and overlapthe rangein F• for the other periods,in whichcase the apparentdifferenceis an artifact of variabilityin the measurements.In biophysicalterms, one would not expectlarger soil respirationrates in the preleaf period becausethe soils were coolerthan in the other periods.Thuswe alsoproposea physically basedhypothesis whereCO2producedduringwinter and late fall is trapped in the soilsuntil after the soil thawsin
of the radiationbalance.Albedos(solar and PAR) undergoa sixfolddecreasefrom its yearlymaximumto minimum over the few weeks of final snowmelt.As shownelsewhere[Lafieur et al., 1993],the magnitudeof this changeis probably2-3 times larger than for the surroundingforest environmentswhere trees mask the albedo of snow.Development of the vascular
The storagecomponentwassignificantonly at certaintimesof
was similar
vegetation cover affectssolar albedo but not PAR albedo,with solar albedo increasingfrom 0.10 immediatelyafter snowmelt to 0.17 at the peak of the green period. The longwavefluxes evolvein concertover the studyperiod and are more closely related to the thermal inertia of the whole surface-atmosphere spring.It is then releasedin sporadicevents(e.g., bubbles) system.Consequently,the net longwaveflux showedno seawhichmayhavecausedthe largestandarderrorsin Figure 13a. sonal trend, and net radiation is largely dependent on solar Storage of CO2 was included in all fluxes discussedabove. inputs. For the growing season,Q*/K i averaged0.55 which to other wetland
studies.
LAFLEUR
ET AL.: NORTHERN
On a seasonalbasis,energy storageis largest in the spring period when melting of snowand ground ice occur, at which time it is equivalentto 20-30% of net radiation. During the summerperiods,Q,, is typicallyonly 6% of net radiation on a daily basisbut may equal 25-30% of Q* at midday.Nighttime lossesof energyfrom storagebecomegreater in the late summer and early fall when air temperaturesare decreasingand wetland soilsrepresenta large store of heat. Energy balance closure during the study averaged 82.2% and had a distinctseasonaltrend. Closurewas poorestduring the spring periods and improved throughout the study. This behavior may have been the results of underestimation of energystoragetermsin the snowmeltand soil-thawingperiods. Wind directionseemedto have little effect on energybalance closure.Although no direct estimatesof the eddyflux footprint were made, we interpret this to mean that the sourcearea for the eddy fluxes (measuredat 4.3 m) remained on the fen surfacefor all wind directionsand mostwind speeds. Partitioningof availableenergyinto latent and sensibleheat varied considerablyduring the study.Bowen ratios were high in the spring,decreasedto a midsummerlow, and increasedin the late summer/fall.During the green period, latent heat flux consumed,on the average,69% of availableenergydecreasing to 54% by the end of the study. Lowering of the water table and reducedtranspirationbecauseof early senescenceof the vegetation may be responsiblefor this decreasein EF. The total 200 mm drop in water table during the studywas largely the result of lower than normal inputs of precipitation. The direction and magnitude of net COg fluxeswas closely linked to phenologyof the vascularplants on the fen. The fen was found to be a net sink of COg only while the vascular specieswere activelyphotosynthesizing, a period of about 85 daysin 1994. Mean daily net COg fluxesshowedthat the fen was a net sourceof COg earlier and later in the field season. We estimatedthe total net exchangefor the studyperiod was
nearzero(30.9g COg-Cm-g). However,giventhatthe study period only included a small portion of the snow-free spring and fall periods,we expectthat overthe wholeyear, at leastfor 1994, the fen was a net source of COg. This finding is of primary importanceas we attempt to monitor and model the dynamicsof the boreal ecosystem,especiallywhen considering the northern boreal regions as possiblecarbon sinks for elevated atmosphericCOg concentrationsor asthe missingsinkin current carbon transfer models [Intergovernmental Panel on ClimateChange(IPCC), 1992].While northernforestsmay be responsiblefor balancingthe globalcarboncycleand, through aerial fertilization, slowingthe increasein atmosphericCOg levels [Idsoand Kimball, 1993;Kolchuginaand [/inson, 1995], the role of boreal peatlandsmust be considered.If fens are a large componentof the boreal forest,then in someyearsthey maybe net carbonsources,and the effectof the forestsinkwill be reducedor erased.This also suggestsan important positive feedbackwith climate change.Under a warmer climate, water tables in peatlands may be reduced, switchingthese systems from net annual sinksto sourcesof COg. The net effectsof this
changewouldhaveto be balanced againstreducedmethane emissionsfrom the same systems[Rouletet al., 1992a]. We cannotyet answerthe questionwhether borealwetlands sustain or, even decrease, the carbon sink of the forests. It is
known that soil COg emissionsare negativelylinked to water
table[Funket al., 1994]andthat on a seasonal basis,peatlands can be a net sourceor sink of COg dependinguponwater table conditions[Shurpaliet al., 1995].In termsof the presentstudy,
BOREAL WETLAND
FLUXES
29,019
1994 was not a representativeyear; compared to long-term climate normals, 1994 was slightly warmer than normal and much drier. The lower water table conditions
at the end of the
study almost certainly increasedsoil COg emissionsand may have decreasedphotosynthesis. Although it is not likely that mean fluxesduring the green period would be much different in a wetter year, it is possiblethat under wetter conditionsthe green seasonwould be extendedby 3 to 4 weeks. A longergrowing seasonmight be sufficientfor the fen to accumulate more COg and become a net annual sink of carbon. More information is needed on the distribution of wetland types within the boreal forestsand the variation in COg flux dynamics between these different types. The answersto these question are most likely to be addressed by linking surfaceatmosphere exchangesof carbon for northern wetlands to properties obtained through remote sensing [e.g., Whiting, 1994]. Such linkageswill allow researchersto monitor these transfersover a much larger scale than is possiblewith point measurements and are key objectivesof the BOREAS program.
Acknowledgments. We would like to thank Kristan Boudreau, Greg Bryant, Andrew Costello, Blair Mantha, Robert Metcalfe, and Michael Skarupa for their assistancein the field. The assistanceof G. den Hartog in designingthe CO2 systemand of Kim Burton for data processingis gratefullyacknowledged.This researchwas supportedby a specialcollaborativegrant from the Natural Sciencesand Engineering ResearchCouncilof Canada,NSTP grantsfrom the Department of Indian and Northern Affairs Canada. DJ was supportedby an EcoResearchDoctoral Fellowship.DEJ wassupportedby a grant from the National Aeronauticsand SpaceAdministration(NAG 5-2331).
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(ReceivedApril 24, 1996; revisedOctober 22, 1996; acceptedOctober22, 1996.)