Energy balance and canopy conductance of a boreal aspen forest: partitioning overstory and understory components

摘要

The energybalancecomponentswere measuredthroughoutmost of 1994 in and abovea southernboreal aspen(PopulustremuloidesMichx.) forest (53.629øN 106.200øW)with a hazelnut(CoryluscornutaMarsh.) understoryaspart of the Boreal Ecosystem-AtmospherSetudy.The turbulentfluxeswere measuredat both levelsusingthe eddy-covariancetechnique.After rejection of suspectdata due to instationarityor inhomogeneity,occasionalerratic behaviorin turbulent fluxesand lack of energybalance closureled to a recalculationof the fluxesof sensibleand latent heat usingtheir ratio and the availableenergy.The seasonaldevelopmentin leaf areawasreflectedin a strong seasonaplatternoftheenergybalance.LeafgrowthbeganduringthethirdweekofMay withamaximumforestleafareaindexof5.6m2m-2 reachedbymid-JulyD.uringthe full-leaf period, aspenand hazelnutaccountedfor approximately40 and 60% of the forest leafarea,respectivelyS.ensibleheatwasthedominantconsumerofforestnetradiation during the preleaf period, while latent heat accountedfor the majority of forest net radiation during the leafed period. Hazelnut transpirationaccountedfor 25% of the forest transpirationduringthe summermonths.During the full-leaf period (June 1 to September 7) daytimedry-canopymeanaspenandhazelnutcanopyconductancewsere330mmol m-2s-1(8.4mms-•) (70%ofthetotalforesctonductancaen)d113mmoml -2s-• (2.9 mms-j)(24%ofthetotalforestconductancere),spectivelMya.ximumaspenand hazelnuctanopcyonductancweesre1200mmolm-• s-• (30mms-•) and910mmolm-2 s-I (23mms-l), respectivealyn,dmaximumstomataclonductancweesre490mmoml -2 s-1(12.5mms-1)and280mmolm-2s-• (7mms-•),aspenandhazelnurt,espectively. Both specieshoweda decreasein canopyconductanceasthe saturationdeficitincreased and both showedan increasein canopyconductanceas the photosyntheticactive radiation increased.There was a linear relationshipbetween forest leaf area index and forest canopyconductanceT.he timing,duration,andmaximumleaf areaof thisdeciduous borealforestwasfoundto be an importantcontrolon transpirationat both levelsof the canopy.The full-leaf hazelnutdaytimemeanPriestleyand Taylor[1972] a coefficientof1.22indicatedtranspirationwaslargelyenergycontrolledandthe quantityof energy receivedat the hazelnutsurfacewasa functionof aspenleaf area.The full-leaf aspen daytimemeanc•of0.91indicatedsomestomatalcontrolontranspirationw,ithadirectly proportionarlelationshipb6tWeenforestleaf areaandforestcanopyconductancev,arying c•duringmuchof the seasonthrougha rangevery sensitiveto regionalscaletranspiration and surface-convectivbeoundarylayer feedbacks.