Responses of net ecosystem exchanges of carbon dioxide to changes ...

JOURNAL

OF GEOPHYSICAL

RESEARCH,

VOL. 104, NO. D24, PAGES 31,421-31,434, DECEMBER

27, 1999

Responsesof net ecosystemexchangesof carbon dioxide to changesin cloudiness: Results

from two North

American

deciduous

forests

LianhongGu, JoseD. Fuentes,and Herman H. Shugart Global EnvironmentalChangeProgram,Departmentof EnvironmentalSciences,Universityof Virginia, Charlottesville

Ralf M. Staebler AtmosphericEnvironmentService,EnvironmentCanada,Downsview,Ontario T. A. Black

Departmentof Soil Sciences, Universityof BritishColumbia,Vancouver,Canada

Abstract. We analyzedhalf-hourlytower-basedflux measurementsof carbondioxide (CO2) from a boreal aspenforestand a temperatemixeddeciduousforestin Canadato examinethe influencesof cloudson forest carbon uptake. We showedthat the presenceof cloudsconsistently and significantly increasedthe net ecosystem exchanges (NEE) of CO2 of both forests from the level under clear skies. The enhancement varied with cloudiness,

solar elevationangles,and differedbetweenthe two forests.For the aspenforestthe enhancementat the peak rangedfrom about 30% for the 200-25ø interval of solar elevationanglesto about 55% for the 550-60ø interval.For the mixed forestthe enhancementat the peak rangedfrom more than 60% for the 300-35ø intervalof solar elevationanglesto about 30% for the 650-70ø interval.Averagedover solar elevation angles>20 ø, the aspenand mixedforestshad the maximalNEE at the irradiance equivalentto 78 and 71% of the clear-skyradiation,respectively. The generalpatternsof currentskyconditionsat both sitespermit further increasesin cloudinessto enhancetheir carbonuptake.We found that both forestscan tolerate exceedingly large reductionsof solarradiation(53% for the aspenforestand 46% for the mixedforest)causedby increasesin cloudinesswithout loweringtheir capacitiesof carbonuptake. We suggestthat the enhancementof carbonuptake under cloudyconditionsresultsfrom the interactions of multiple environmentalfactorsassociated with the presenceof clouds. 1.

Introduction

Clouds, as a natural weather element at a given location, stronglyinfluenceenvironmentalconditionson the groundsurface via radiative transfer, latent heating, and precipitation [Bennerand Curry,1998].Thereforeit is expectedthat clouds can have important ramificationson CO2 exchangesbetween terrestrialecosystems and the overlyingatmosphere.Field observationshave shownthat the highestrate of forest net ecosystemexchanges (NEE) of CO2 (i.e., the mostnegativevalue, followingthe NEE sign convention)often occurson cloudy rather than on sunnydays[Priceand Black, 1990;Hollingeret al., 1994;Fitzjarraldet al., 1995;Sakaiet al., 1996;Freedmanet al., 1998].Other studieshavefoundthat for a givenirradiance level,overcastdaysgenerallyhavea higherNEE rate (in terms of the absolutevalue) than cleardays[Fan et al., 1995;Baldocchi, 1997;Gouldenet al., 1997].To explainsuchobservations, severalmechanismshave been postulated.They include increasesin diffuseradiation[Priceand Black, 1990;Hollingeret al., 1994;Fan et al., 1995;Gouldenet al., 1997],decreasesin the respirationof sunlitleaves[Baldocchi,1997],reductionin va-

por pressuredeficit(VPD) [Freedmanet al., 1998],and stomatal dynamicsassociated with light fluctuations[Fitzjarraldet al., 1995; Sakai et al., 1996]. Although observedat stand levels throughtower-basedflux measurements,the enhancementof carbonuptakeundercloudyconditionshasregionalandglobal implicationsbecausecloudinesshasbeen increasingovermany regionsof the world [McGuffieand Henderson-Sellers, 1988; Henderson-Sellers,1989; Karl and Steurer, 1990; Russak, 1990;

Angell, 1990; Kaiser and Razuvaev,1995;Abakumovaet al., 1996]. It is of particularinterestin North America where increasedvegetationactivitieshavebeenobserved[Keelinget al., 1996;Myneniet al., 1997]and a terrestrialcarbonsinkhasbeen suggestedby eddy covariancemeasurements[Wofsyet al., 1993],isotopicanalyses of atmospheric CO2 [Ciaiset al., 1995], and modeling studies [Fan et al., 1998], while increasesin cloudinesshave been observed [McGuffie and Henderson-

Paper number 1999JD901068.

Sellers,1988; Henderson-Sellers,1989; Karl and Steurer, 1990; Angell, 1990]. At present,we lack systematicapproachesto examinethe relationshipbetweencarbonuptake by terrestrialecosystems and cloudiness.Previousstudiesfocusedon the comparison betweentwo categoriesof sky conditions:cloudy (overcast) andcleardays.The comparison waseither on an individualday

0148-0227/99/1999JD 901068509.00

basis[PriceandBlack,1990;Hollingeret al., 1994;Fitzjarraldet

Copyright1999by the AmericanGeophysicalUnion.

31,421

31,422

GU ET AL.: FOREST CARBON UPTAKE AND CLOUDS

al., 1995;Sakaiet al., 1996] or on a multiple-day/seasonal basis [Fan et al., 1995;Baldocchi,1997; Gouldenet al., 1997;Freedman et al., 1998]. In the latter case,the differencesbetween overcastand clear dayswere often shownthrough the NEEphotosynthetically activeradiation(PAR) relationship.These comparisonsshowstraightforwardlydifferencesin the carbon uptake between sunny and overcastdays.However, two important methodologicalissueshaveyet to be solved.First, the classification of skyconditionsinto merelytwo categoriesis an oversimplificationof the real situation.It is critical to know how gradual changesin sky conditionsaffect the carbon exchange.Second,the effect of solarelevationangleson canopy photosynthesisshould be teased out from the influence of cloudson the carbonexchanges.As the solar elevationangle increases,the proportionof the light reachingdeepercanopies also increasesbecauseof the reducedlight extinction[Ross, 1981; Campbelland Norman, 1989]. Consequently,the radiation use efficiencyincreaseswith solarelevationangles[Gu et al., 1999].In order for an overcastday to have the samesolar radiation level as a clear day the Sun must be in a higher position.Therefore the higherNEE rate found under the overcastconditionat the sameradiation level with the sunnycondition can be partly attributed to the increasein the radiation use efficiencyof direct beam with solar elevationangles.To solvethese issues,new approachesare needed. The three major objectivesin thisstudyare asfollows:(1) to develop a methodologyfor the studyof the relationshipbetween cloudinessand the carbonuptake by terrestrial ecosystems, (2) to quantifythe influencesof cloudson carbonuptakes by two North American deciduousforests,and (3) to identify potential factors contributingto and controllingthe responseof forest carbonuptake to changesin cloudiness.In particular, we want to answerthe following questions.Is the enhancementin forest CO2 uptake on cloudydaysconsistent over time and statisticallysignificant?What is the optimal sky conditionfor forest carbonuptake? Obviously,forest carbon uptake is hindered if the cloudsare too thick and the PAR is reduced too much. Then it is logical to ask at what threshold cloudsbecomelimitingfactorsfor forestcarbonuptake?Given the observedtrendsof increasingcloudiness overmanyregions of the world [McGuffieand Henderson-Sellers, 1988;Henderson-Sellers, 1989;Karl and Steurer,1990;Russak,1990;Angell, 1990;Kaiserand Razuvaev,1995;Abakumovaet al., 1996],we also want to know if the current levels of cloudiness, as observed at these two sites, allow for further increases in cloud-

sistsprimarily (>90%) of tremblingaspen(Populustremuloides)and is locatedin the southernboreal forestof Canada (53ø63'N,106ø20'W)away(>300 km) from anthropogenic activities.Randomlylocatedthroughoutthe landscape, the forest also includesa small percentage(, • -• E

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Clear-Sky Clearness Index

Sine of Solar ElevationAngles

Figure 5. Relationshipsbetweenthe clear-skyNEE of CO2 and clear-skyclearnessindexfor (a) the borealaspenforestin 1994 and (b) the temperatemixedforestin 1997.

linear patternsshownin theseplotsare similarto thoseshown in Figures3 and 4 for the clearnessindex. From Figures3, 4, 7, and 8 it is clear that the maximalsolar radiation doesnot result in the maximalNEE. It is interesting to askthe followingquestion:At which irradiancelevel doesa forest ecosystemhave the maximal NEE rate.9Equally interestingis the questionat which irradiancelevel under a cloudy sky does a forest ecosystemproduce the same NEE rate as under a clear sky.9We calculatedvaluesof the optimal and critical

relative

irradiances

for each 5 ø interval

The critical

relative

mixed temperate forest (Table 3). While the aspenforest showsa generaltrend of decreasesin both the optimal and the critical

relative

irradiances

with

increases

in solar

elevation

angles,the pattern for the mixed forest is more complex.For

highsolarelevationangles(>40 ø) the aspenand mixedforests have closeoptimal relativeirradiances.For low solarelevation

for these two

forests.The optimal relative irradiancefor a 5ø interval was obtainedby solvingthe corresponding cubicregressional equation for its maximum.

Figure 6. Scatterplots and cubic regressionsbetween the clear-skyNEE of CO2 andthe sineof solarelevationanglesfor (a) the boreal aspenforest in 1994 and (b) the temperate mixed forest in 1997. The obtainedcubicregressionequations (givenin Table 2) are usedto calculateF c(/3) in Equation(6).

irradiance

for a 5 ø

interval was obtained by solvingthe correspondingcubic regressionalequationfor its root at which the NEE equalsthe clear-skyvalue (the relativeirradiance- 100). Figure9 shows the optimal and critical relative irradiancesagainstthe solar elevation angle at the center of each 5ø interval. Both the optimal and the critical relative irradiancesvary with solar elevationangles.For the boreal aspenforest in 1994 the opti-

Table 3. Optimal and Critical Relative Irradiances(%) for Every 5ø Interval of Solar Elevation Angles >20 ø at the Boreal Aspen Forest Site in 1994 and the Temperate Mixed Forest Site in 1997

Aspen Forest in 1994

Mixed

Forest

in 1997

Intervals

of/3 20o-25 25o-30 30o-35 350-40 400-45 450-50 50ø-55 550-60

ø ø ø ø ø ø ø ø

mal (critical)relativeirradiancechangesfrom 83% (65%) for the 200-25ø interval to 76% (46%) for the 550-60ø interval (Table 3). For the temperatemixedforestin 1997the optimal (critical) relativeirradiancechanges-from 75% (54%) for the 200-25ø intervalto 67% (36%) for the 650-70ø interval(Table 600-65 ø 3). Averagedoversolarelevationangles>20 ø,the optimaland 650-70 ø critical relative irradiancesare 78 and 53%, respectively,for Average the boreal aspenforest and 71 and 46%, respectively,for the

Optimal

Critical

Optimal

Critical

83 81 78 76 78 76 75 76

65 59 53 49 53 49 50 46

75 57 70 75 71 78 76 74

54 34 48 47 48 53 50 47

69 67

39 36

71

46

78

53

GU ET AL.' FOREST CARBON

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Clearness

Index

1997

0.,•

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0.8

Index

Figure 7. Relationshipbetweenthe 20-pointmeansof relativechangesin the magnitudeof NEE of CO2 relativeto clearskies(%NEE) andthe clearness indexfor differentintervalsof solarelevationangles.Thex coordinatesare the mediansof valuesof the clearnessindexof the corresponding 20 points.The error bars

represent the 95% confidence interval.For clarity,errorbarsare shownfor onlyonecurvein eachplot. (a, b) Borealaspenforestin 1994.(c, d) Temperatemixedforestin 1997.

angles(