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Comparison of sap flow and eddy fluxes of water vapor from a boreal deciduous forest den Hartog, Gerry; Black, T. Andrew; Hogg, Edward H.; Yang, Paul C.; Nesic, Zoran; Zimmermann, Reiner; Neumann, Harold H.; Blanken, Peter D.; Hurdle, Patrick A.; Oren, Ram; McDonald, Kyle C.; Staebler, Ralf M. 1997

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JOURNAL  OF GEOPHYSICAL  RESEARCH,  VOL. 102, NO. D24, PAGES 28,929-28,937, DECEMBER  26, 1997  A comparison of sap flow and eddy fluxes of water vapor from a boreal  deciduous  forest  Edward H. Hogg,• T. Andrew Black,2 Gerry den Hartog,3 Harold H. Neumann,3 Reiner Zimmermann, 4 Patrick A. Hurdle, • Peter D. Blanken,2 Zoran Nesic,2  Paul C. Yang,2 Ralf M. Staebler,3 Kyle C. McDonald,4 and Ram Orens Abstract. Water flux to the atmospherewas measuredfrom a mature standof aspen (PopulustremuloidesMichx.) in Saskatchewan, Canada,as part of the Boreal EcosystemAtmosphereStudy(BOREAS). Diurnal and seasonalchangesin transpirationwere monitored usingtwo sap flow techniquesand were comparedagainstthe difference between eddy correlationmeasurementsof water vapor flux made above and below the aspencanopy.The three methodsshowedsimilar diurnal and seasonaltrendsin water flux, althoughsapflow laggedthe eddy correlationmeasurementsby about 1 hour diurnally due to changesin water storagewithin the trees. During the growing season,all methodsshoweda linear increasein midday transpirationwith above-canopyvapor pressuredeficit (VPD) up to -1 kPa, beyondwhich transpirationwas relativelyconstant (VPD 1-2.5 kPa). A similarrelationshipwas obtainedwhen total daily transpirationwas plotted againstmean daytimeVPD. The resultsare consistentwith other observationsthat stomatalconductanceof the aspencanopydecreasesat high VPD. The complementary benefitsof simultaneousmonitoringof canopytranspirationby both eddy correlationand sap flow measurementsare discussed. 1.  Introduction  1996]. This was feasible becauseof a large and well-defined verticalseparationbetweenthe tree canopyand the understory shrublayer,whichwere eachrelativelyuniform and dominated by a single species(trembling aspen and beaked hazelnut, CoryluscornutaMarsh., respectively).The measurements were made usingthe eddy correlationtechnique,by severalof us in the tower flux (TF-1 and TF-2) scienceteam, as part of the 1994 BOREAS field campaign[Sellerset al., 1995a]. In the present paper, we compare the 1994 tower-based measurementsof water vapor flux from the aspen canopy at thissitewith concurrentmeasurements of sap(i.e., water) flow within individual stemsof aspen in the same stand. Two different methods of determining sap flow were used: the heat pulsemethod(terrestrialecology(TE) team,groupTE-7) and the constantpower method (remote sensingscience(RSS) team, groupRSS-17).The objectivesof this paperwere (1) to compare measurementsof diurnal and seasonalchangein aspen water flux and (2) to use the three methodsto examine relative responsesof aspentranspirationto above-canopyvapor pressuredeficit. We also identify issuesthat may cause difficulty in making interscale comparisonsof transpiration  The Boreal Ecosystem-Atmosphere Study (BOREAS) is a large-scale,internationalfield experimentthat aimsto improve understandingof the interactionsbetween the boreal forest biome and the atmospherein order to clarify their roles in global change[Sellerset al., 1995a]. Most of the researchis beingfocusedwithin two large studyareasin westernCanada: the Southern Study Area near Prince Albert, Saskatchewan, and the Northern StudyArea near Thompson,Manitoba. One of the major processesbeing studied in BOREAS is evapotranspiration (watervaporflux) from differentvegetation types and its control by environmentaland ecophysiological factors.In 1994,measurements of water vapor flux (alongwith CO2 in most cases)were made at 10 tower sites,located in tremblingaspen(Populustremuloides Michx.),jack pine (Pinus banksianaLamb.), black spruce(Piceamariana (Mill.) BSP.), and fen ecosystems. One of the difficultiesin comparing tree-based, ecophysiologicalresponseswith tower-basedflux measurementsmade abovethe forest canopyis that the latter may includea signifmeasurements from individual icant contributionby the soil and understoryvegetation.Howtrembling aspen forests. ever, at the mature aspen site in the Southern Study Area, water vapor and CO2 fluxeswere partitionedbetweenthe tree and the understorycomponentsthrough tower-based,eddy 2. Methods correlation measurementsat different heights [Black et al., 2.1. Study Site Description  trees versus the stand level in  The site (53ø38'N, 106ø12'W,elevation600 m) is located  •CanadianForestService,Edmonton,Alberta, Canada.  within Prince Albert National Park, Saskatchewan, in the 2University of BritishColumbia,Vancouver,Canada. 3Atmospheric Environment Service, Downsview, Ontario,Canada. southern boreal forest of western Canada. It is in an extensive, 4jet Propulsion Laboratory, Pasadena, California. mostlypure stand of trembling aspenabout 70 years old, with -SDuke University, Durham,NorthCarolina. Copyright 1997 by the American GeophysicalUnion. Paper number 96JD03881. 0148- 0227/97/96JD- 03881 $09.00  a stem densityof about 800/ha.Aspen clones(patchesof geneticallyidenticaltrees) are readilydistinguishable becauseof differencesin bark characteristicsand phenology.The aspen trees are mostly 18-22 m tall, and stem diameters at 1.3 m  28,929  28,930  HOGG ET AL.: WATER  FLUXES  FROM A BOREAL  height are 0.15-0.30 m. The height to the base of the aspen canopyis about 15 m. The understoryshrublayeris dominated by beaked hazelnut about 2 m tall. Leaf area index reacheda seasonalmaximum(July 1994) of 2.3 for the tree canopyand 3.3 for the understoryshrub layer [Blankenet al., this issue]. The soil has a silty-clay texture with 0.08-0.10 m layer of organicmaterial at the surface,and aspenroots penetrateto a depth of about 0.6 m. Mean temperaturesof the warmestand coldestmonthsare 16øC(July) and -21øC (January),respectively (at WaskesuiLake, 30 km north of the site at 530 m elevation).Mean annualprecipitationis 463 mm, of which300 mm falls as rain between May and September[Environment Canada, 1982].  DECIDUOUS  FOREST  temperaturedifferenceover each24 h period. Typically,A T,, is recordedat night,just prior to dawn,when it is assumedthat S -- 0 [Granier, 1987]. Probeswere installedon October 26, 1993 (DOY 299), and continuousmeasurementswere made from February16, to October5, 1994(DOY 47-278). 2.4.  Sap Flow by Heat Pulse Method (TE-7)  The heat pulse velocity (HPV) method [Marshall, 1958; Swanson,1983,1994]wasusedto monitor sapflow of sixaspen at a site located about 1 km east of the main flux tower, in the same extensiveaspen stand and having similar characteristics  (0.17-0.27 m diameterand 17-22 m height).Three treesfrom each of two cloneswere selectedfor monitoring.Probeswere 2.2. Water Vapor Fluxes by Eddy Correlation (TF-1 and installedon May 26, 1994(DOY 146) duringthe springperiod TF-2) of rapid leaf expansionin the aspen canopy. The two clones Fluxeswere measuredat two heightsusingthe eddy corre- were named EL (early leating)and LL (late leating)basedon lation technique, as describedby Black et al. [1996]. Water the phenologicaldifferencesnoted on this date. Our implementation of the method used differential, vapor flux abovethe canopywasmeasuredat the 39.5 m height from a mast mounted on a 37 m tower, while fluxes above the chromel-constantanthermocouplessharing a common refershrub layer were measuredat the 4 m height on a second6 m ence junction and placed symmetrically7.5 mm above and below the heater. The thermocouplereferencejunction was tower located 40 m away from the first tower. Threedimensional sonic anemometer-thermometers were used at located25 mm laterallyfrom the heater,and all thermocouple both levels (Kaijo-Denki DAT-310 and Gill Instruments junctionswere located at a sapwooddepth of 15 mm. The 1012R2 Solent,respectively).Fluctuationsin water vapor con- heater had a power output of 0.2 W/mm along its length and centrationswere measured(alongwith CO2) usingtempera- was insertedto a depth of 50 mm. All probe elementshad a ture-controlledinfraredgasanalyzers(LI-COR 6262) at both diameter of 1.6 mm, and holes of the same diameter were heights. Additional measurementswere made with an open drilled usinga steeltemplate.Probeswere locatedat a height path H20 analyzer(Campbell-Scientific KH20 hygrometer)at of --•1.3 m on the north side of the tree and were thermally the 4 m height. Data were processedaccordingto Black et al. insulatedby wrappingwith white polyethylenepackingmate[1996], and mean fluxeswere calculatedfor each 30 min pe- rial. The six treeswere monitoredby two CampbellScientific riod. Continuousmeasurementswere made from early Octo- 21X data loggers,powered by batteries and a solar panel. ber to mid-November1993(not reportedhere) and from early Measurementsof the temperatureincreaseat the upper (T,,) were taken 60 s followinga 4 s Februaryto late September1994.Evapotranspirationfrom the and lower(T•) thermocouples aspencanopywas determinedas the differenceof water vapor heat pulse (total power 40 J), every 3 hoursfrom May 26 to fluxesmeasuredat the two heights.During the summermonths October20, 1994(DOY 146-293). On eachtree, probeswere the water vapor flux measuredabovethe shrublayer accounted removedand replacedwith new probesat a differentlocation for 23% of the total for the forestasa whole [Blacket al., 1996; on the sametree, onceduringthe summer(four treeson DOY Blankenet al., this issue]. 192 and two treeson DOY 209). Sapflux density(S, m/s)was calculatedusingthe followingequationderivedfrom Marshall 2.3. Sap Flow by Constant Power Method (RSS-17) [1958]' The constant power method [Granier, 1987] was used to k,w In ( monitor sap flow in nine aspentrees in a clone locatedwithin s: 50 m of the main flux tower. Tree characteristicswere repreXCpw•)w sentative of the stand as a whole, with stem diameters at 1.3 m  height of 0.16-0.29 m. The apparatusfor each tree consisted of two cylindricalprobes, each 2 mm in diameter, that were inserted into the sapwoodto a depth of 20 mm below the vascularcambium.The upper probewasinstalledat a heightof about 1.6 m and was continuouslyheated at a constantpower of 0.2 W. The lower, unheated probe was installedabout 100 mm directly below the heated probe. All probeswere located on the north side of trees and coveredwith a plasticshieldto minimize assymmetricaltemperature fluctuationswithin the xylem. The temperature difference between the heated and unheated probeswas monitored by a data logger every 60 s, and averageswere reported every half hour. Sap flux density (S, mm/h, i.e., volume flow rate per unit sapwoodcrosssectionalarea) was determinedusingthe followingempirical relationshipfrom Granier [1987]:  S: 428[(ATm- AT)/ATm]•23•  (1)  where AT is the temperature difference between the two probesat the time of measurement,and A T,• is the maximum  where x is the spacingbetween the heater and each thermo-  couple(0.0075m) andwherec•,wandp,. arethespecific heat and densityof sap (i.e., valuesfor water of 4180 J/kg/øCand  1000kg/m-•). Thethermal conductivity ofactivesapwood (k,,,) was estimated at 0.38 J/m/s/øC based on  k,.• = k•Mp•,/p. + k,•.(1 - Mp/,/p,)  (2b)  wherek, is the thermalconductivity of water (0.60 J/m/s/øC),  9histhetypicallivebulkdensity of aspensapwood (370kg/m-•) [Peterson and Peterson,1992], and M is the typicalsapwood moisturecontent of 1.0 kg moistureper kilogram dry mass. The reportedvariation in aspenmoisturecontentfrom 0.75 to 1.25 kg/kg,i.e., 100 + 25% [Gibbs,1939],would lead to +_9% error in k,w and resultant estimatesof S. The longitudinal thermalconductivityof dry wood (k,½•) wastaken to be 0.248 J m/s/øCbasedon 9/, and the resultantvoid volume [Siau, 1971]. For purposesof presentation,valuesof S from (2b) were expressed in millimeterper hour (i.e., multipliedby 3.6 x  10f').  HOGG ET AL.: WATER FLUXES FROM A BOREAL DECIDUOUS  Heat transfer theory indicatesthat the interruption of the sap streamby probesresultsin an underestimationof the true  FOREST  28,931  50  (C])  ....... HPV (EL)-- HPV (LL)-- CP ]  sapfluxdensity. For ourimplementation, woundwidthwas • 40 estimated to be 2.0-2.2 mm, including a 0.2-0.3 mm zone of  g  disturbedsapwoodon eachsideof the holesfollowingdrilling  .• 30  [Swanson andWhitfield, 1982;Barrett etal., 1995].Simulations •  =x2o  using atwo-dimensional numerical model similar toSwanson •  and Whitfield[1981] indicatedthat with our implementationa correctionfactor (a) of about 2.0 (_+15%) is warranted to obtain the actualsapflux density(S*), where  o•  40 0  S* = aS  (3)  160  180  200  220  240  260  280  300  6  A similarunderestimationof 45% (i.e., a = 1.8) wasobserved by Cohen et al. [1981] for their implementation of the heat pulse method. For each of the two clones, measurements of S* were mul-  !  140  >,  4  (b)  tiplied by the standsapwoodarea to groundarea ratio (SA) to give estimatesof canopytranspiration.SA was determined as the product of BA and FSA, where BA is the basal area to groundarea ratio of aspenstems(at 1.3 m height) in one 10 x 10 m plot centeredon each clone, and FSA is the fraction of BA occupiedby sapwood.FSA wascalculatedusingtwo radial incrementcoresfrom each of four treesper clone (total of 16 radii). The zone of activesapwoodwithin incrementcoreswas recognizedfrom staining by a methylene blue in methyl hydrate dye.  -•.  3.  this date.  I--Eddy cørrelatiøn ...... Sap fløw (HPV) I  x= 3  • 1  :•  '  0 140  : ....  160  180  200  -•.."x..%,.,.... 220  240  260  280  300  Day of year 1994  Figure 1. Daily water flux for trembling aspen during the 1994 growingseason.The aspenwas almostfully leafed out by the end of May (DOY 151), while senescenceand leaf fall 2.5. Supporting Measurements occurredin September(DOY -250-270). (a) Uncorrected A wide variety of other measurementswere made at the site flux densitywithin sapwoodof individualtrees (S), as meaduring the BOREAS experiment (for details, see Black et al. suredby the heat pulsevelocity(HPV) method (EL, and LL [1996]). In the presentanalysiswe calculatedvapor pressure clones,mean of three treesper clone) and the constantpower deficits[Jones,1992, p. 110] from half-hourly averagesof air (CP) method(mean of nine treesin one clone). (b) Water flux temperature and vapor pressureat the 39 m height. These density at the stand level, measuredby two eddy correlation measurementswere taken using an aspirated platinum resis- systemsabove and below the aspen canopy, and estimated from the HPV method(mean of EL andLL cloneswith scaling tance thermometerand a dew point hygrometer.Precipitation basedon stand sapwoodarea and sap flux density(S*) corwas measured using a weighing Belfort rain guage and a tip- rectedfor flow interruptionby probeswith a = 2.0). ping bucketrain guage.Soil water potentialwasmeasuredhalf hourlyusinggypsumblocks(depthsof 0.06, (t.16,and 0.46 m). Soil water content was measured every 1-3 days using timewas generallygreater in the EL clone during the summerbut domainreflectometry(TDR) with a Tektronix, Inc. 1602cable declined sharplyrelative to the LL clone in early September. tester and G. S. Gabel Corp. segmentedTDR probes(depth This is consistent with our observation that the EL clone had (1-1.2 m) and gravimetrically(depth 0-.0.10 m). lost most of its leavesby DOY 253 (September10), whereas the LL clone continued to have a full, green canopy beyond  3.1.  Results  and Discussion  Seasonal Changes in Daily Water Flux  In the aspenclonemonitoredby the constantpower method, S was generally only about half of that measuredby the heat pulsemethodin June and July (Figure la) but remainednear midsummerlevelsuntil mid-September.In contrast,the clones monitoredby the heat pulsemethod showeda gradual decline in S startingin late July. This resultedin a greater similarity in the magnitudesof S for the two sap flow methodsduring the latter part of the growingseason.In particular, the magnitude of S in the LL clone was very similar to that measuredby the constantpower method in August and September. For each of the two clones monitored by the heat pulse method, daily transpirationwas estimatedfrom S* (as describedin section2.4). The basalarea (BA) ratio was 0.0038 and 0.0063 for the EL and LL clones, respectively,and the fractionof sapwood(FSA) was0.51 for both clones,basedon measurementsmade in July 1994.Thus the SA ratio was0.0020  Both the sap flow and the tower-based,eddy correlation measurements (Figure 1) showeda rapid increasein water flux from the aspencanopyin late May 1994(DOY 142-151) during the period of rapid leaf expansionwhen the leaf area index increasedfrom 0.5 to 1.6 [Blacket al., 1996;Blankenet al., this issue].Water flux was generally greatest from mid-June to early August, when the aspen leaf area index was near its seasonalmaximum of 2.3 and declined in September during the period of leaf senescenceand abscission. The pattern of short-termvariation in mean daily sap flux density(S) wasgenerallysimilaramongthe aspenclonesmonitored, but differenceswere noted in the magnitudeand broad seasonalpattern(Figure 1a). On DOY 147,S by the heat pulse methodwasabouttwiceas high in the early-leafing(EL) clone comparedto the late-leafing(LL) clone (EL mean, 21 mm/h, for EL and 0.0032 for LL. On the basis of the above, with a = range 19-28 mm/h; LL mean, 11 mm/h, range 8-14 mm/h). S 2.0, total estimated transpiration over the growing season  28,932  HOGG ET AL.' WATER  FLUXES FROM A BOREAL DECIDUOUS  FOREST  0.4 i  measured but expected if transpiration rates were actually lower in this clone).  0.3  3.2.  We selectedthe period from June 9 to August 17 (DOY 160-229) asthe "peakgrowingseason"for an analysisof mean diurnal changesin water flux. This correspondsto the period following full expansionof the aspencanopyin all clonesbut prior to the onsetof criticallylow soil moisturein late August. Also during this period, the leaf area index of the understory  0.2  0.1  0  '  , Lii. ,  Mean Diurnal Trends and Changes in Water Storage  hazelnut  ,  was near its seasonal maximum  value  and was rela-  tively stable[Blacket al., 1996]. Mean diurnal changesin water flux during the peak growing Day of year 1994 seasonare shownin Figure 3, expressedas a ratio of the hourly Figure 2. Soil water content (0-0.3 m depth, time-domain flux to the mean hourly flux over this 70 day period. The eddy reflectrometry(TDR) measurements)and daily precipitation correlation measurements show that mean water flux from the during the 1994 growing season. aspencanopyreacheda maximumnear or slightlyafter solar noon(1305centralstandardtime (CST) at thislocation).Low values were recorded from 2100 to 0600, when global solar (DOY 147-261) was 222 and 297 mm for EL and LL, respec- radiationwas <50 W/m2. An analysisof the diurnaleddy tively. The averageestimatedtranspirationof 254 mm for the correlationmeasurements,in relation to vapor pressuredeficit, two clones combined was similar to the total water flux of 266 solarradiation,and canopyconductance,is givenby Blankenet mm based on the tower flux eddy correlation systems.Varia- al. [this issue]. tion in standwater flux densityamongdaysand throughoutthe Mean diurnal changesin sap flow laggedchangesin towerseasonalso showedgood agreementbetween the heat pulse and eddy correlationmethods(Figure lb). It shouldbe noted that the measurementsmade by eddy correlation included both transpirationand evaporationof interceptedrainfallfrom -- Eddy correlation 1 120  140  160  180  200  220  240  260  (el)  theaspen canopy. Wedidnotmeasure canopy evaporation  ....  "•"Sap flow (HPV)/  directly,but it is probablyabout 8% of total rainfall, basedon  the resultsof Clements [1971]in an aspenforestin Ontario, Canada, with similar stand characteristicsand precipitation patterns.  The constant powermethodwouldgivemuchloweresti-  mates(notshown) if theresults werescaled to givetranspiration on a standbasis.This is partly becausethe magnitudeof  S measuredby the constantpower methodwas generally smallerthanfor the heat pulsemethod(Figure la). Also,when the resultsfrom heat pulse method were scaledto give estimatesof standtranspiration,a correctionfactor (a - 2.0) was  ß  •  0  3  6  9  ,  12  15  i  appliedto S to account for sapflowinterruption byprobes, but such a correctionis not applicableto the constantpower method. Muchclosercorrespondence hasbeenobtained elsewhere,involvingcomparisons betweenthe constantpower  18  21  2z  I -- Eddy correlation I  (b)  methodand the other measurements of transpiration[Granier  J  et al., 1990;Diawaraet al., 1991].Early installation of the  probespriorto springthawmayhaveresulted in reduced thermal contact between probes and xylem following freezethaw cycles.However, there was no evidence of a further decline in S during the growing season,and lower thermal contactshouldnot have affectedrelative responses. The late summerdeclinein standwater flux density(Figure lb) was accompaniedby a gradualdecreasein volumetricsoil moisturecontent(at depth of 0-0.3 m) from -0.28 to 0.14, duringa periodof scantrainfall (Figure 2). Soilwaterpotential at the 0.46 m depthwasnear -0.03 MPa in Juneand Julybut by mid-August(DOY 230) had fallen to -0.4 MPa (gypsum block sensor).Drought stressmay have been a factor in causing the overalllate-seasondeclinein transpiration,particularly in the EL clone,where foliage color had begunto changein mid-August. In contrast, the aspen clone monitored by the constantpowermethod showedno evidenceof sucha decline, possiblydue to deeper rooting or locally moister soils (not  'll•.•. ß  0  3  6  9  12  15  18  21  24  Hour (CST)  Figure 3. Mean diurnalwater flux for tremblingaspenfrom June 9 to August 17, 1994 (DOY 160-229), expressedas a ratio of the hourlyflux to the overallmean hourlyflux recorded duringthisperiod.Comparisonsof diurnalresponses obtained by the eddy correlationmethod againstsap flow by the heat pulse (HPV) method every 3 hours (a) and the CP method every 0.5 hours (b) are shown.For sap flow measurements, vertical  bars indicate  standard  errors  in relative  diurnal  re-  sponses,basedon N = 6 trees for HPV and N = 9 trees for CP. Shadedareasindicatewhere evapotranspirationfrom the aspen canopyexceededthe rate of upward water flux in the sapwood(at 1.3 m height). Mean solarnoon occursat the site at 1305 CST.  HOGG ET AL.: WATER FLUXES FROM A BOREAL DECIDUOUS  basedwater flux, both usingthe heat pulse (Figure 3a) and constantpower (Figure 3b) methods. With the heat pulse method the diurnal pattern was similar to that recorded by eddy correlation, but nighttime fluxes were relatively higher. This could be partly a result of slighterrors in probe spacing with the heat pulsemethod. However,significantnighttimesap flow can occurin aspen,basedon our observationthat sapflow was greateston nightswith high vapor pressuredeficits(see Figure 4). With the constantpowermethod,nighttimesapflow was low and tended to increasemore slowlyin the early morning. Both sap flow methodsshoweda more gradualdeclinein diurnal water flux between 1800 and 2100, comparedwith the eddy correlation measurements. The mean diurnal changein water storagewithin the aspen canopyand stem (above -1.5 m height) can be derived from Figure 3 and estimatesof mean daily transpiration, assuming that the mean net changein water storagefrom one day to the next is small (i.e., mean daily transpiration= mean daily sap flow). First, diurnalchangesin water storagecan be expressed as a fraction of mean daily transpirationby summingthe continuouspositivedeviationsof water flux (shadedareas) from the tower versussap flow. On the basisof this, mean diurnal changesin water storageusingheat pulse (Figure 3a) and the constantpower method (Figure 3b) were 13.5 and 11.6%, respectively,of mean daily aspentranspirationfor DOY 160229. From the eddy correlation measurements,mean transpiration over this period was 2.43 mm/d basedon a total water flux from the aspencanopyof 186 mm and assumingthat 8% of the rainfall recordedduringthis period (16 mm) wasintercepted and evaporated. This gives estimated mean diurnal changesin water storageof 0.33 and 0.28 mm for the two sap flow methods,which is equivalentto about I hour of midday transpiration. For comparison, the aboveground pool of readily availablewater in conifer standswith comparablecharacteristicscan be as great as 0.5 mm or about one quarter of daily transpiration [Schulzeet al., 1985; Diawara et al., 1991; Ciencialaet al., 1994]. 3.3.  Water Flux in Relation to Vapor Pressure Deficit  Another usefulmeansof comparingthe three methodsis to examine the ability of each to detect relative transpiration responsesto changesin environmentalconditions.In the fol-  0.9  ß  FOREST  .  28,933 -90  ,  -- Eddy correlation ..,a,... Sap flow (HPV) -- Sap flow (CP) ] •  '80  60 E  0.6  • 0.5  50 •  __=x 0.4 '•  o.3  •  0.2  40  =x  30  •-  20  0.1  10 0  202  203  204  205  206  2.0  1000  • VPD- RsI •  800•-  1.5  _  600  ._  -2  -e  400 • 3  0.5  -200 • 0  202  203  204  2O5  20{  Day of year 1994 (CST)  Figure 4. (a) Water flux for July 21-24, 1994 (DOY 202205), as measuredby eddy correlation(tower) and by two sap flow methods(S). For sapflow, meanvaluesare shownfor the CP method(ninetrees)andthe heatpulsemethod(EL andLL clones,sixtrees).(b) Environmentalconditionsrecordedat the 39-m height during this period.  day sap flux density(S) up to --•1 kPa, but for VPD greater than this, S was relativelyconstant.For the heat pulse method this relationshipwasparticularlystrikingwhen the resultsfrom both clones (N = 6 trees) were used to estimate canopy transpiration(Figure 5b). The magnitudeof the transpiration estimatesbasedon heat pulse was similar to that from eddy correlation,and the latter method also showeda tendencyfor water flux to remain  constant for VPD  > I kPa. For all three  methodsthe slope of the relationshipbetween flux and VPD was not significantlydifferent from zero, if only the dayswith  lowinganalysis wepresenta comparison'of responses to above- VPD > 1 kPaareconsidered (N = 28 days,rangeof r 2 from canopyvapor pressuredeficit (VPD) duringthe peak growing season,when soil moisture was not limiting. The VPD was expectedto be the most important environmentalfactor governingtranspirationof thisforest,especiallyduringthe midday period when solar radiation is not limiting on most days [see Blanken et al., this issue]. For sapflow by the heat pulsemethodthe readingsat 1500 CST were greateston the average(Figure 3a) and were thus consideredto be representativeof middayconditions.With the constantpower method, midday was also representedby a measurementperiod near the diurnal maximum,from 14.5 to 1500CST. For vaporpressuredeficit(VPD) and middaywater flux basedon eddycorrelation,we usedthe averageof the four half-hourly measurementsbetween 1300 and 1500 CST. This periodwaschosento broadlycompensatefor the 1 hour diurnal phase lag of sap flow measurementsand to smooth the apparentlyrandom nature of variabilityamong adjacenthalfhourly eddy correlationmeasurements(Figure 4). When plotted againstmidday VPD (Figure 5a), both sap flow methodsshowedan approximatelylinear increasein mid-  0.01 to 0.05). Thus fluxesof water from the aspencanopywere nearly the samefor dayswith VPD > 1.5 kPa, comparedwith dayswith VPD between1.0 and 1.5 kPa (Table 1). When total daily water flux (2400 period beginningat 0100 CST, i.e., solar midnight) was plotted againstmean daytime VPD (0700-1900 CST), there wasalsoa tendencyfor fluxesto level off for VPD > I kPa (Figure 6). For constantpower method and the eddy correlation measurementsthe slope of the relationshipfor VPD > I kPa was not significantlydiffer-  entfromzero(N -- 21 days,r2 from0.01to 0.03).In contrast, the heat pulse method indicated a slight increasein water flux  for VPD > 1 kPa (r 2 = 0.26, p < 0.05), but the slopeover thisrangewasless(0.9 mm/d/kPa)than for VPD < I kPa (3.1 mm/d/kPa). The tendencyfor canopy transpiration to level off at high VPD has been reported in studies conducted elsewhere, includingtropical rain forest [Meinzeret al., 1993; Granier et al., 1996] as well as temperate forests and woodlands[e.g., Lopushinsky,1986; Price and Black, 1989; Goulden and Field, 1995].When observedin forestswith a high aerodynamiccon-  28,934  HOGG ET AL.: WATER FLUXES FROM A BOREAL DECIDUOUS  lOO  ,-,  8o  •  60  (a) ß  ._  ß ß  m  x  []  40  []  20  o cPI 0  2.5  0.5  •  0.4  .?  •  0.3  mm  m  ß  []  "- 0.9  • o.1 0.0  !  0  !  0.5  !  1  1.5  2  2.5  FOREST  ductancein thisforest decreasesexponentiallywith VPD. Field measurementsof leaf stomatalconductancein tremblingaspen conductedelsewhere[McCaugheyand Iacobelli,1994] are also consistentwith these relationships. Sincewater flux from the aspencanopyis governedlargely by the leaf stomata,it may seem surprisingthat stomatalresponseswere tuned in such a way as to produce relatively constanttranspirationratesover a rangeof VPD. It is possible that sucha result could be explainedby leaf level modelsthat link stomatalresponsesto photosynthesis rates and environmental conditionsat the leaf surface[e.g.,Collatzet al., 1991]. An alternative explanationis that stomatalconductancewas beingconstrainedby the needfor aspento maintainleaf water potentials above the point where catastrophiccavitation of xylem would occur [Tyreeand Sperry,1988;Sperryand Pockman, 1993; Goulden and FieM, 1995]. Under these circumstancesthe hydraulic resistancefrom soil to leaf would ultimately determine the maximum transpirationrate that could be sustainedby the canopy [cf. Meinzer and Grantz, 1991]. Further studyof this mechanismcould be helpful in making refinementsto modelsof transpiration,at spatialscalesranging from singletreesto the landscape[e.g.,Jarvis,1976;McNaughton and Jarvis, 1991; Monteith, 1995; Sellerset al., 1995b; Wil-  liams et al., 1996].  VPD at 39 rn height (kPa) 13-15 CST  Figure 5. Midday water flux for trembling aspenon 70 days 3.4. Scaling Issues and Sources of Error during the 1994 growingseason,in relation to the abovecanIn this study,the three methodsof estimatingcanopywater opy(39 m height)vaporpressuredeficit(VPD). Flux measureflux gavebroadlysimilarresults,in termsof relativeresponses mentsare as describedin Figure 1. Sap flow comparisons are to above-canopy VPD. Suchagreementshouldstrengthenconbased on measurements at 1500 CST for the HPV method and 14.5-1500 CST for the CP method. For measurements of VPD fidence in the observedoverall responsesto VPD, sincethe and water vapor flux density by eddy correlation method potential sourcesof error are very different and largely inde(above-minusbelow-canopyreadings),averagesfor the period 1300-1500  CST  are shown.  ductance,sucha responseindicatesgradualclosureof stomata  .-. 40  (a)  astheVPDofambient air(e.g.,above canopy) increases. This • interpretation shouldbe applicable to the borealaspensite ._•30 examinedin the presentstudy,whichbecauseof canopyroughnessand smallleaf size(30-50 mm in lengthandwidth) hasa  •, g 20  high (--•50-200mm/s)aerodynamic conductance [Blankenet  7j  al., this issue].More specifically,our observationof a constant midday transpirationrate over a range of VPD > 1 kPa suggeststhat stomatalconductancewas inverselyproportionalto VPD over this range. It is also consistentwith the results of analysisby Blankenet al. [this issue]that aspencanopycon-  i•m•l• m --•m mß mm•mmm  •0 ß 0  0.0  •  • m  0.5  HPV (EL)  Deficit  for DOY  • cPI  m  1.0  Table 1. Comparisonof Midday Water Fluxesfor Trembling Aspen Under Different Rangesof Vapor Pressure  m HPV (LL)  ,  1.5  2.0  ß  m •m  m  160-229  Vapor PressureDeficit, kPa 0.0-0.5  Number of days  15  0.5-1.0  27  Sapflux density(S, mm/h) Constantpower method 12 33 Heat pulsemethod (EL clone) 17 49 Heat pulsemethod (LL clone) 17 42 Stand Water Flux Density (ram/h) Eddy correlation 0.13 0.25 Sap flow (heat pulse) 0.08 0.23  1.0-1.5  14  >1.5  14  •nEddy correlation ß Sap flow (HPV) 1 0  38 68 52  40 68 52  0.34  0.36  0.30  0.30  0.0  ,  !  0.5  1.0  1.5  2.0  AverageVPD at 39 m height,7-19h CST  Figure 6. Daily averagesap flux density(a) and total daily water flux densityfrom the aspenstand(b) for 70 daysduring the 1994 growing season,in relation to the averagedaytime (0700-1900 CST) vapor pressuredeficit (39 m height).Flux measurementsare as describedin Figure 1.  HOGG ET AL.: WATER FLUXES FROM A BOREAL DECIDUOUS  pendentbetweenthe tower-basedand sapflow methods.How-  FOREST  28,935  increasein sapflux densitiesfor severaldaysfollowingprobe changesin midsummer(DOY 192 and 209), which may acwater flux among the three methods,in terms of relative sea- count for someof the scatterin Figure 5. sonal and diurnal responses,degree of short-termvariation, Accurate determinationsof canopytranspirationfrom sap and absolutequantitativeestimates. flow measurements canbe achieved,but quantitativereliability It is apparent that for clonal speciessuch as aspen, differ- can only be assuredwith a carefully conducted,intensiveexencesin clone phenologyand physiologicalfunctioningcan be perimental design[e.g.,Hatton et al., 1995]. Although we oba major problemin relatingleaf- or tree-levelmeasurementsto tained comparableestimatesof canopytranspirationfrom one larger-scalemeasurements.This problemappearedto be most of the sapflow methods(heat pulse),cautionwould be advised critical duringthe periodsof springleatingand late seasonleaf in applyingthis techniqueas the sole meansof making quansenescence,when differencesin clone phenologywere most titative measurements.With the constantpower method, sap apparent. The resultsalso suggestincreasedvariation among flux measurementswere lower than expected,based on the clones during periods of moisture stress.In contrast, clonal eddycorrelationmethod,indicatinga possiblereductionin the variation should have had a smaller effect on above-canopy sensitivityof probesinstalledprior to springthaw. eddy correlation measurements,becausetower fluxes would One of the difficultieswith the eddy correlation measurenormally representspatial averagingfrom many clones. ments was the high degree of short-termvariation which did For mean diurnal responses changes in water storage not appear to be related to changesin key environmental largelyaccountedfor the 1 hour lag betweensapflow and eddy factorssuchasVPD and solarradiation(Figure4). This is also correlation measurements, as discussedabove. Differences in apparent in the relationship between midday water flux and relative diurnal responseswere evenobservedbetweenthe two VPD (Figure 5b), where both sapflow methodsgavegenerally sap flow methods(Figure 3). This could reflect differences more stableand consistentvalues.This was partly becauseof among sites and clones, but the lower early morning values evaporationfrom the aspencanopyduring and following rainrecordedby the constantpower method might be a result of fall events,which was not recorded by the sap flow methods changesin heat storage[Grime et al., 1995]. With both meth- (e.g., four outlier dayswith VPD < 0.3 kPa and tower water ods, there are also potential errors in preciselydefiningcon- flux density > 0.15 mm/h in Figure 5b). Another factor to ditionsof zero sapflow. Sincesignificantsapflow mayoccuron consideris that for eddycorrelation,the areasof forest (footnightswith high VPD [Greenet al., 1989], predawn sap flow print) being measureddiffer greatlyfor the above-and belowshould not be assumedto be zero exceptwhen VPD is very canopy measurements,and both are affected by wind speed small. and direction.Although the aspenforestwasrelativelyuniform Sapflow readingscan alsobe affectedby changesin sapwood within 500 m in all directions from the tower site, there was moisturecontent(seesection2.4), but on a diurnalbasis,such variabilityin the degreeof energyclosure(i.e., sumof sensible changesare expectedto be small. The total quantity of mois- and latent heat flux versusavailableenergy) among adjacent turein theaspenat thesiteis >4 mm(>40 m3/ha),basedon half-hourlymeasurements.These issuesare examinedin more a minimumstemvolumeof 120 m3/ha[HalliwellandApps, detail by Blankenet al. [this issue]. 1996]. Since the estimated diurnal change in water storage within the aspenstandwas about 0.3 mm (section3.2), the 4. Conclusions mean percentagechangein moisture content of sapwoodxylem shouldbe <8%, leading to a potential error of <3% in All three methods showedthat midday transpiration from estimatesof sapflux density(section2.4). Even thisestimateof the aspen canopy has a striking tendency to remain constant error is probablyhigher than necessary,sincethe calculation for above-canopyVPD ranging from 1.0 to 2.5 kPa. Since does not include diurnal changesin moisture content of the midday aerodynamicconductanceis high, such constancyin transpiration rates reflects gradual stomatal closure as VPD foliage. Forobtaining quantitative measurements ofwaterfluxatthe increasesbeyond 1 kPa, which is supportedby other studiesof canopylevel, tower-basedmicrometeorologicalmethodshave stomatalresponsesin this species.The maximumtranspiration clear advantagesover other methods,partly becausethey are rate of the aspen canopy may be indirectly governed by hygenerallynonintrusiveand becausethey integrate fluxesover draulicresistancefrom soilto leaf, and the need to avoidxylem large areas. Sap flow measurements,in contrast, are made cavitation. within small regionsof sapwoodin individual trees, leading to The resultsof this study indicate the benefits of using difdifficultiesin "scalingup" to the canopylevel. One problem is ferent methodsto simultaneouslymeasuretranspirationfrom that the rate of sap (water) flux in the vicinityof probesmay the aspencanopy:Eddy flux correlationmeasurementsmade not be representativeof the averageflux acrossthe cross- above and below the canopyare generallysuperior in providsectionalarea of sapwoodin singletrees,becauseof radial and ing quantitative,daily, and seasonalestimatesof water flux at azimuthalvariation in flow rates [Cohenet al., 1985;Hatton et the standlevel. Another clear advantageof eddy flux correlaal., 1995; Swanson,1994]. Sap flux density may also differ tion is its versatilityin measuringfluxesother than water vapor significantlyeven among adjacenttrees for a wide variety of (e.g., CO2, sensibleheat, trace gases,etc.). Flux estimatescan reasonsand large sample sizes may be needed to obtain a be also independentlyverified by determining the degree of reliable statisticalsample.As indicatedin section2.4, results energyclosure.On dry dayswithout morning dew, all of the are also influencedby changesin sapwoodmoisture content water flux is due to transpiration,but over longerperiodswith and the interruptionof sapflow by probes.Lossesof sensitivity dew and rainfall events, the measured water flux also includes can also occur during the growing seasondue to progressive an evaporation component. woundingresponses[Swansonand Whitfield,1981; Swanson, Sap flow measurementson individualtreescan be subjectto 1983].This did not appearto be a majorproblemin the present considerablebias when "scaled up" to the stand level but study,but with the heat pulse method, we noticed a 10-15% appear to be more stable than eddy correlation over periods ever, there were often considerable differences in estimates of  28,936  HOGG  ET AL.: WATER  FLUXES  FROM  ranging from <1 hour to several days. Such stability arises largely from their insensitivityto atmosphericconditionsand becausethe locationsbeingsensedare fixed (in contrastto the variable footprint of forest being sensedby eddy correlation). Another important advantageof sap flow methodsis that they can provide a more direct measurementof tree transpiration; that is, evaporationof interceptedwater is not included.However, becauseof changesin water storage,sap flow measurements tend to be laggedrelative to eddy correlationmeasurements and environmentalconditions(by about 1 hour for mean diurnal responsesin the presentstudy,but seeHollinger et al. [1994]). The use of both techniquessimultaneouslyis mutually helpful in identifying problems and methodologicalweaknesses, becausethe potential errors in each technique are different and generallyindependent.This can strengthenthe reliability and usefulnessof functional relationshipsthat would be obtained by either method alone.  Acknowledgments.The authorsfrom UBC (TF-1) gratefullyacknowledge funding from the Natural Science and E•gineering ResearchCouncil(NSERC) of Canada,includinga CollaborativeSpecial Project grant in supportof universitiesparticipatingin BOREAS and an operatinggrant to T. A. Black. Fundingwas providedby Canada's Green Plan to the authorsfrom AES (TF-2) and CFS (TE-7). Work by the authors at JPL (RSS-17) was conducted under contract to the National Aeronauticsand SpaceAdministration,and we thank JoBea Way at JPL for stronglysupportingthis study.Field operationswere made possibleby the supportof personnelat Prince Albert National Park, especiallyMary Dalman, Murray Heap, and PaulaPacholek.We alsothank S. G. Chen, J. Deary, G. Edwards,J. Fuentes,B. Goodison, T. Herzog, E. Kanemasu,X. Lee, M.D. Novak, C. Russell,I. Simpson, G. Thurtell, and others within BOREAS for on-site assistanceand support. Useful commentson the manuscriptwere provided by D. Halliwell, D. T. Price, and I.D. Campbell (CFS).  References Barrett, D. J., T. J. Hatton, J. E. Ash, and M. C. Ball, Evaluation of the  heat pulsevelocitytechniquefor measurementof sap flow in rainforest and eucalyptforest speciesof southeasternAustralia, Plant Cell Environ., 18, 463-469, 1995. Black, T. A., et al., Annual cyclesof water vapour and carbon dioxide fluxesin and above a boreal aspenforest, Global ChangeBiol., 2, 219-229, 1996.  Blanken, P. D., T. A. Black, P. C. Yang, H. H. Neumann, Z. Nesic, R. Staebler, G. den Hartog, M.D. Novac, and X. Lee, Energy blance and canopyconductanceof a boreal aspenforest:Partitioningoverstory and understorycomponents,J. Geophys.Res., this issue. Cienciala,E., H. Eckersten,A. Lindroth, and J.-E. Hallgren,Simulated and measuredwater uptake by Picea abiesunder non-limitingsoil water conditions,Agric. For. Meteorol., 71, 147-164, 1994. Clements, J. R., Evaluating summer rainfall through a multilayered largetooth aspen community,Can. J. For. Res., 1, 20-31, 1971. Cohen, Y., M. 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