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BOREAS in 1997: Experiment overview, scientific results, and future directions Crill, Patrick M.; Ranson, K. Jon; Baldocchi, Dennis; Black, T. Andrew; Ryan, Michael; Berry, Joe; Kelly, Robert D.; Hall, Forrest G.; Sellers, Piers J.; Goodison, Barry; Gower, Stith T.; Jarvis, Paul G.; Williams, Darrel; Halliwell, David; Cihlar, Josef; Margolis, Hank; Fitzjarrald, David; Newcomer, Jeffrey; Lettenmaier, Dennis P.; Guertin, Florian E.; Wickland, Diane E. 1997-12-31

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JOURNAL  OF GEOPHYSICAL  RESEARCH,  VOL. 102, NO. D24, PAGES 28,731-28,769, DECEMBER  26, 1997  BOREAS in 1997: Experiment overview, scientific results, and future  directions  Piers J. Sellers,• Forrest G. Hall, • Robert D. Kelly,2 Andrew Black,3 Dennis Baldocchi,4 Joe Berry,s Michael Ryan,6 K. Jon Ranson,• Patrick M. Crill, 7 Dennis P. Lettenmaier,s Hank Margolis,9 Josef Cihlar,•ø Jeffrey Newcomer,• David Fitzjarrald,TMPaul G. Jarvis,•2 Stith T. Gower,•3 David Halliwell,TMDarrel Williams,• Barry Goodison,•s Diane E. Wickland,•6 and Florian E. Guertin •ø Abstract. The goal of the Boreal Ecosystem-Atmosphere Study(BOREAS) is to improve our understandingof the interactionsbetween the boreal forest biome and the atmospherein order to clarify their roles in global change.This overviewpaper describes the sciencebackgroundand motivationsfor BOREAS and the experimentaldesignand operationsof the BOREAS 1994 and BOREAS 1996 field years.The findingsof the 83 papersin thisjournal specialissueare reviewed.In section7, important scientificresults of the project to date are summarizedand future researchdirectionsare identified. The magnitudesand dynamicsof these exchangesand many of their controllingprocessesare poorly understoodfor most of Persuasivearguments indicate that there will be global the Earth's ecosystems.The boreal forest is a good example: warmingresultingfrom the continuingincreasein atmospheric this ecosystemencircles the Earth above 48øN, is second in CO2 concentration[Houghtonet al., 1995;Hasselmann,1997]. areal extent only to the world's tropical forests,occupiesabout However, there are uncertainties about the magnitude and 21% of the forested land surface[Whittakerand Likins, 1975], regionalpatternsof projectedglobalchangebecauseof short- and contains 13% of the carbon stored in biomass and 43% of comings in the atmospheric general circulation models the carbon stored in soil [Schlesinger,1991]. Several climate (AGCMs) usedfor climatesimulation.There is a real need to model projectionsindicatethat the greatestwarmingwill occur improve (1) our understandingof basicclimatic physicaland at these high latitudes (43ø-65øN) with the most marked efdynamic processesso that we can enhance the realism and fects within the continental interiors [Houghtonet al., 1995; accuracyof AGCMs and (2) our abilityto quantifyglobal-scale Mitchell, 1983; Schlesingerand Mitchell, 1987; Sellers et al., climate variablesand parametersto better initialize and vali- 1996a]. Large perturbationsin northern continental climates date models. Success in these two research areas should result are expectedto lead to changesin the carbon cycle and the in improvedclimate modelsand data sets,which in turn should ecologicalfunctioningof the boreal forest, which could feedprovide more credible and useful climate projections[Tren- back onto the globalclimate.Thus the sizeof the boreal forest, berth, 1992;Sellerset al., 1997]. its sensitivityto relatively small climatic variations, and its The exchangesof energy, water, and carbon between the importanceto the climate of the northern hemisphereand the atmosphereand the continentsrepresentthe lower boundary globalcarboncyclecombineto make it an important biome to condition for the atmosphericphysicalclimate systemand the understandand to representcorrectlyin global models. climatic forcing to terrestrial biota and biogeochemicalcycles. The Boreal Ecosystem-AtmosphereStudy (BOREAS) was initiated as a large-scaleinternationalinvestigationfocusedon •NASA GoddardSpaceFlightCenter,Greenbelt,Maryland. improving our understandingof the exchangesof radiative 2University of Wyoming,Laramie. energy,sensibleheat, water, CO2, and other radiatively active 3University of BritishColumbia,Vancouver, Canada. 4NationalOceanographic and AtmosphericAdministration,Oak trace gasesbetween the boreal forest and the lower atmosphere.A primary objectiveof BOREAS was to collect the Ridge, Tennessee. SCarnegie Institution,Stanford,California. data needed to improve computer simulation models of the "UnitedStatesDepartmentof Agriculture,Fort Collins,Colorado. important processescontrolling these exchangesso that the 7University of New Hampshire,Durham. effects of global change on the biome can be anticipated, in "Universityof Washington, Seattle. '•Centrede Recherchcen BiologieForestigre,Sainte-Foy,Quebec, particularthe effectsof altered temperatureand precipitation, Canada. as well as providingAGCMs with better land surfaceprocess •øCanada Centerfor RemoteSensing, Ottawa. submodelsand data setsfor the boreal zone. The field phaseof • •Atmospheric Sciences Resource Center,Albany,New York. the experimentextendedfrom 1993 to 1997 and included two •2University of Edinburgh, Edinburgh, Scotland. seriesof intensivefield campaignsin 1994 and 1996. Monitor•3University of Wisconsin, Madison. •4Forestry Canada,Edmonton,Alberta,Canada. ing of the BOREAS studyareawill continueon a reducedbasis •5Atmospheric Environment Service,Downsview, Ontario,Canada. through at least 2001 and someinfrastructuremay be retained •'NASA Headquarters, Washington, D.C. for possiblefollow-up studies.This journal specialissuebrings Copyright 1997 by the American GeophysicalUnion. together resultsfrom field work conductedin 1994, and to a lesserextentin 1996,and somepreliminaryfindingsfrom modPaper number 97JD03300. 0148-0227/97/97J D-03300509.00 eling studies.A 2-year program of follow-on analysesof the 1.  Introduction  ..  28,731  28,732  SELLERS ET AL.: BOREAS IN 1997  PHYSICAL CLIMATE  SYSTE•M BIOPHYS•  B•OGEO••  Hy•mlogy I X •  /  I  •bpn  Temperature,  ECOSYSTEM STRUCTURE  AND FUNCTION  Species Composition,Structure Pedology  Figure 1. Important interactionsbetween the boreal forest and the atmospherewith respectto global change.(a) Influence of changesin the physicalclimate systemon biophysical processes.These may feedback to the atmospherethrough changesin energy,heat,water, and CO2 exchange;(b) changes in nutrient cyclingrates;releaseof CO2 and CH 4 from the soil carbonpool back to the atmosphere;(c) changesin biogeochemicalprocessesand water and nutrient availabilityinfluencecommunitycompositionand structure;(d) changein species composition results in changes in surface biophysical characteristicsand biogeochemicalprocessrates.  BOREAS  data set will be initiated  in late 1997. Other  work  a lengtheningof the approximately150-daygrowingseasonby about 6 daysat higherlatitudes.Keelinget al.'s [1996] resulthas recently been supportedby analysisof a global satellite data record which suggeststhat the growing seasonand photosynthetic activity increasedover large areas of Europe, northern Eurasia, Alaska, and Canada for the same period [Myneni et al., 1997]. The land surfaceparameterizations(LSPs) and surfaceparameter setsusedin AGCMs have improvedconsiderablyover the last decade [Sellerset al., 1997]. Climate modelsuse many of the same formulations,submodels,and parametersas numericalweatherprediction(NWP) models;the latter alsobenefit from a continuousprocessof operationalverification.Recent experiencehas shownthat the resultsof large-scalefield experimentsare usuallytransferredfirst to NWP models[Betts et al., 1993, 1996, 1997, 1998] and later to climate models [Sellerset al., 1995a, 1997]. In any case,if a processsubmodel is responsiblefor systematicerrors in an NWP model, it will certainlylead to even larger biasesin a climate model which is not reinitialized at short time intervals.Analyseshave demonstrated that until very recently, even the best NWP models consistentlyoverpredictedthe evaporationrates and specified unrealistic winter albedo fields over the boreal region with seriousconsequences for forecastingskill [Sellera' et al., 1995b; Bettsand Ball, this issue].The reasonsfor these errors were directlyconnectedto misrepresentationsof important biophysical processesin LSPs,for example,controlson evapotranspiration, and inaccuraciesin specifyingmodel parameters,such asthe extent,type, and densityof forestbiomes.The main aims of the physicalclimate systemcomponentof BOREAS were therefore to provide data which could be used (1) to improve and validate LSPs and (2) to enhancemethodsfor deriving parameterfieldsfrom satelliteobservations, which are the only feasible,consistentmeansof providingglobal surfacedata sets. NWP modelshave alreadybenefitedfrom this work, and climate modelsare expectedto benefit in due course.  from BOREAS has been published elsewhere,notably in a specialissueof TreePhysiology [Margolisand Ryan, 1997].This 2.2. Carbon and Biogeochemistry overviewpaper describesthe BOREAS experiment,reviews The physicalclimatesystemis stronglycoupledto the global the constituentpapers of the special issue,summarizesthe important resultsof work done to date, and identifiesfuture carbon cycle. Temperature and precipitation anomalieshave research directions. been comparedwith seasonalvariations in atmosphericCO2 concentrationand isotopic analysesto show that warm years over the northern  2.  Science Background  continents  are associated  with a net terres-  trial carbon sink, while cold and/or dry years are associated The interactions between the boreal forest and the atmowith a net sourceof terrestrialcarbon[Keelinget al., 1995;Ciais spherecan be groupedinto physicalclimate systemprocesses, et al., 1995; Denninget al., 1995; Tans et al., 1990]. Together, carbonand biogeochemistry, and ecology(seeFigure 1). These theserecent studiessuggestthat the northern continentsacted are discussedin turn below, with particular emphasison the as a large sink for atmosphericcarbon,an averageof 1-2 Gt C yr-• overthe 1980s,or about15 to 30% of the anthropogenic issuesaddressedby BOREAS. CO2 flux from fossilfuel burning.The exactbiophysicalmech2.1. Physical Climate System anismsresponsiblefor this sink are unclear, although the hyAt the present rate of increase, atmosphericCO2 concen- pothesizedlengtheningof the growingseasoncouldbe a factor tration will double before the end of the next century [Hough- [Keelinget al., 1996;Myneni et al., 1997]. The boreal ecosystem ton et al., 1995]. Climate simulationsmade with AGCMs point is vast,between12 millionkm2 [Whittaker andLikins,1975] to large temperature increasesin the northern high-latitude and 20 millionkm2;the latter figurewe computefrom the continentalinteriors [Schlesinger and Mitchell, 1987;Houghton classification of DeFriesand Townshend[1994],aspublishedby et al., 1995; Sellers et al., 1996a], partly due to projected Meesonet al. [1995] and describedby Sellerset al. [1996b]. changesin the polar seaice climatologyand snow-albedofeed- Simple arithmetic implies then that on average,only 50-80 g backs.A significantwarming trend was observedin the boreal C m-2 yr-• needbe sequestered to accountfor a 1 Gt C yr- ' zone during the 1980s and early 1990s, reaching 1.25øCper global sink. As will be seen later in this paper, sucha number decade within the Canadian interior [Chapman and Walsh, is well within the range of annual carbon uptake values esti1993].Keelinget al. [1996] analyzedtime seriesof atmospheric mated from eddy correlation data acquired at the BOREAS CO2 concentrationsto showthat this warming may have led to tower sites. However, to extrapolate these measurementsto  SELLERS ET AL.: BOREAS IN 1997  the entire boreal zone, or into the future from a coupleof years worth of data, necessitatesa deeper understandingof the climatologicaland physiologicalprocesses controllingcarbonuptake, respiration,and fire frequency,how suchprocessesdepend on soil and land cover type, and how they might be affectedby change.From the outsetan important objectiveof BOREAS was to acquire the data needed to improve terrestrial carbonmodelsfor the boreal region. In particular, data are needed to improve our understandingof the dependence of carbonfluxeson physicalclimatevariationsand to develop methodsfor extractinguseful parametersfrom satellite data. 2.3.  Ecology  Carbon sequestrationin the boreal ecosystemamountsto the relatively small difference between gains from photosynthesisand lossesdue to respirationin the plants, roots, and soils.For roughlythe past 8000 years after the last glaciation, the boreal ecosystemhasbeen accumulatingcarbonin its soils, particularlyin deep layersof organicpeat where soil organic matter  accumulates  under water-saturated  conditions.  Harden  et al. [1992]placehistoricalcarbonaccumulationratesin these  peatsoilsin the rangeof 10-50 g C m2 yr-• throughsurface mossproduction,fine root turnover,and litter fall. On shorter timescales,primary carbonstoragemechanismsappearto be in abovegroundstandingbiomass.The progressivewarming that occurredduring the 1980sand early 1990scould have altered rates of photosynthesis, soil respiration,and fire frequencyin the region.In additionto drivingchangesin the ecophysiology of the biome, continuedwarming could eventually alter the spatial structure of the boreal ecosystem.There have been several attempts to map the future extent of the northern biomesbasedon the projectedwarmingand dryingregimedue to a "doubled-CO2" climate. Some of these suggestthat the North American boreal forestwould move north and perhaps split into two halves:one in Alaska and the CanadianNorthwest and the other in the Canadian Northeast [Rizzo and Wiken, 1992]. Such changesmay themselveshave significant feedbackson the climate systemthrough changesin winter albedosand energyfluxesover the altered land surfaces. The earliestdiscussions about BOREAS recognizedthe importance of the issuesreviewed above; they also recognized how these issuesare mutually dependent and that an integrated experimentalstrategythat addressedall three disciplinesofferedthe mostprofitablescientificapproach(seeFigure 1). BOREAS, however, has served to elaborate and sharpenthe detailed questionsrelated to these issuesand has provided partial resolutionto many of them.  3.  Experimental Objectives  BOREAS was designedto bridge a wide range of spatial scalesbecausesomeof the important governingprocessescan only be studied at very small spatial scales(e.g., the links amongleaf biochemistry,spectralproperties,and photosynthesis),but ultimately,the scientificgainsmustbe appliedwithin the contextof AGCMs, carboncyclemodels,or similar largescalestudies.A multi-scale-nested designwasdevelopedwhich permitsknowledgeat one scaleto be translatedand compared to that obtainedor inferred at different scales(see Plate 1). The governingobjectivesof BOREAS canbe statedasfollows: 1. Improve the processmodelsthat describethe exchanges of radiativeenergy,water, heat, carbon,and trace constituents between the boreal forest and the atmosphere.The approach  28,733  herewasto measurethe fluxesof energy(radiation,heat) and mass(water, CO2, and radiativelyimportanttrace gases)at severalscalesalong with observationsof the ecological,biogeochemical,and atmosphericconditions controlling them. Data collectionschemeswere designedto support development and testingof processmodelsthat will be applied to the global change issuesdescribedabove. The field observations that support processmodel development include measurementsof water, CO2, and trace gasfluxesat the small plot or leaf scale (chambers,porometers),the stand scale (towermountededdy correlationand profile instrumentation),and the mesoscaleand regionalscales(airborne eddy correlation; meteorologicalmeasurementsand analyses).At the smaller length scales,these measurementswere coordinated with a seriesof ecological,meteorological,and edaphicobservations to link thesefluxesto appropriatestatevariables(seePlate 1). 2. Develop methodsfor applyingthe processmodelsover large spatialscalesusingremote sensingand other integrative modelingtechniques.The processstudiesdescribedin objective 1 abovewere coordinatedwith remote sensinginvestigations using satellite, airborne, and surface-basedinstruments that focus on methodsfor quantifyingcritical state variables. These remote sensingstudies,combinedwith mesoscalemeteorologicalstudies,will allow us to scaleup and applyprocess modelsto regional and ultimately global scales.Some largescalevalidation techniqueswere incorporatedin the experiment designto test scale-integrationmethods directly; these techniquesincluded airborne flux and profile measurements, meteorologicalobservations,and modeling.  4.  Experiment Design and Resources  The objectivesof BOREAS relate to two spatial scalesthat had to be reconciledwithin the experimentdesign.The primary focusof objective1 is mosteasilyaddressedby local-scale (a few centimetersto a few kilometers)processstudiesthat involve detailed, coordinated in situ observations.These local-  scalestudieshad to be connectedto the larger-scalemeasurement and analysistools associatedwith objective2, which was directedtoward definingregional-scale(10-1000 km) fluxes and states.In BOREAS, as in previousfield experimentssuch as the First ISLSCP (International SatelliteLand SurfaceClimatology Project) Field Experiment (FIFE) [Sellerset al., 1992a; Hall and Sellers,1995] and the Hydrological AtmosphericPilot Experiment-Sahel,HAPEX-Sahel [Goutorbeet al., 1994], the scienceteam adopted a nestedmultiscalemeasurementstrategyto integrate observationsand processmodels over the scalerange, as shownin Plate 1. The resolutionof global AGCMs, of the order of hundreds of kilometers,defined the largestspatialdomain in BOREAS, the 1000 x 1000 km BOREAS region shownin Plate 1. This was the domain for meteorologicaland satellite data acquisition and large-scalemodeling.Sinceit is impractical,evenwith aircraft, to measure surface-atmospherefluxes over areas larger than a few tens of kilometerson a side,another scaleof investigationwasset at about50 x 50 km whichcorrespondsto the two BOREAS studyareas (Plate 1 and Figure 2). These two study areas were placed near the northern and southern ecotonesof the biome in order to study important processes associatedwith the controlling factors (temperature in the north, moisturein the south)whichare mostlikely to undergo significantchangewithin the biome as a whole. The northern study area (NSA) and the southernstudy area (SSA) were  28,734  SELLERS ET AL.' BOREAS IN 1997  Region 1000 x 1000km Satellites  Remote Sensing Aircraft Large-Scalemodels  Study Area ~ 100 x 50km  Modeling Subarea 40 x 40kin  Flux Aircraft Local Area Models  Tower Flux Measurements  Hydrology  Tower  Flux Site  ~1 xlkm  PhysiologicalStudies Radiometric  Measurements Trace Gas Measurements Soil Processes  Process Study Site ~ 1 m2 ~ 100m2  Plate 1. Multiscale measurementstrategyused in BOREAS; see text.  located near Thompson, Manitoba, and Prince Albert, Saskatchewan,respectively.The studyareasare about 500 km apart, sufficientlyfar apart to resolvethe ecologicalgradient while permitting the routine ferry of research aircraft and people between them. The studyareas themselveswere small enoughto be characterizedby usingsurfaceand aircraft measurements,yet large enough to test scalinghypothesesusing models and remote sensingimagery. Within each study area, water vapor, heat, and CO2 fluxeswere measuredat the local  scale(about 1 km) usingeddycorrelationequipmentmounted on "flux" towers(five in the NSA, six in the SSA) whichwere located in patches of relatively homogenousvegetation and soils. Three boreal forest dominants were characterized by thesetower flux (TF) measurements: black spruce,jack pine, and aspen.Two wetland fens, one in the NSA and one in the SSA, and beaver ponds in the NSA and SSA were equipped with smaller towers (Figure 2). The tower-basedmeasurementshavebeen comparedwith aircraftflux data (up one scale level) and with the biophysicalcharacteristics of the constituent vegetationin the patch as definedby small-scalemeasurements made at small plots (down one scalelevel). Because thesefew TF sitescould not capture the range of variability in the region,an additional70 auxiliarysites(marked ascategory 3 sitesin Figure 2) were selected,stratified by speciestype, productivity,and age. In these small (approximately100 x 100 m) sites,biometry and optical techniqueswere used to measurebiomassdensity,leaf area index, net primary productivity (NPP), litter fall, etc.While processmodelswill be tuned  by usingprimarily the tower flux data, the auxiliarysitescan be used as independenttest sitesfor processmodel and remote sensingalgorithm validation. Some embeddedplots (marked as category2 auxiliarysites in Figure 2) were the focus of in situ measurement  work  which measured  the variables  needed  to drive stand-levelecophysiological models,for example,sitelevel biomass, leaf photosyntheticrate, litter decomposition and quality, soil moisture,and temperature.Trace gas studies were conducted in both study areas with some specialized investigationsin the NSA where somebeaverpond sitesand a collapsedpalsa were instrumentedto measuremethane flux. (Palsasare commonfeaturesin the boreal landscape;they are shallowdepressionsin the land surfacecausedby melting of subsurfacepermafrostand subsequentcollapseof the surface. Usually, palsashave high water tables which support typical bog vegetation cover such as sphagnummoss, sedges,bog birch, etc.) A remote sensingscienceprogram was implemented (see Table 4 and Plate 2) to characterizesurfacecomponentoptical propertiesfrom the leaf level(usinglaboratoryspectroradiometers),to the canopyand standlevel (usingtowerand helicopter-mounted spectrometers),to the study area and regional level (using aircraft and satellite platforms). Microwave scattering properties were determined at the canopy level using airborne radiometersand at the studyarea and regionallevels using aircraft and satellite-basedmeasurements. A large multidisciplinaryteam of scientistswas needed to cover the measurement and modeling requirements of the  SELLERS ET AL.: BOREAS IN 199'7  (a)  97 ø 30'  98 ø 00'  98ø430 '  28,'735  56 ø 15'  • MODELING • TUDY AREA • HARD SURF. SUB-AREA  U6W5S  U5W5S  NSA-Fen  NSA-OJP •T7R9S  56 ø 00'  ROADS  [• LOOSE SURF.  p NSA-BP  (ALL WEATHER)  "•1 LOOSE SURF • TRAIL OR r• TOWER SITES -• AUX. SITES C• AUX SITES • PROFILER, RADAR, (DRY WEATHER) CART TRACK  •11TOWlS T3U9S  (CATEGORY 1)  (TE TOWER)  T5Q7SNSA-OBS  ,  (CATEGORY 2)  •.-.•.  (CATEGORY 3)  R8V8A  OR LIDAR  55• 45'  LAKES 20  KILOMETERS  570.000  Additional sites exist  alongthe transectand may not appear on this map  N  (b)  • 104ø30'  1  106 ø  54 ø 15'  !  + % ß  I  ROADS  I H3DIM  (ALL WEATHER)  -OBS H2DlS  54ø  • MODELING • STUDY AREA • HARD SURF [• LOOSE SURF. /•1 LOOSE SURF. r• TRAIL OR •--• TOWER SITES SUB-AREA  I  +  (TETOWER)  (DRY WEATHER) CART TRACK  SSA-YJP  (CATEGORY 1)  "•r• ß  Aux SITES (CATEGORY 2) AUX.  SITES  (CATEGORY 3)  53 ø 45'  •  'PROFILER, RADAR, OR LIDAR  +  ½•71• LAKES  Additional sites exist  alongthe transectand to the southof this study area.  Figure2. Mapsof (a) thenorthern(NSA) and(b) southern (SSA)studyareasof BOREAS.Towerflux(TF) sites,auxiliarysites,andmodelingsubareas are marked.TF sitesare identifiedby a studyareaprefix(NSA or SSA)anda vegetation-type identifier(OA, old aspen;YA, youngaspen;OBS,oldblackspruce; OJP,old jackpine;YJP,youngjackpine;FEN, fen;BP,beaverpond).Auxiliarysites(category 2) wereusedfor carbon modelingstudies;category3 siteswereprimarilyusedfor remotesensing validationwork.  28,736  SELLERS  ET AL.: BOREAS  IN 1997  project. In 1992, 85 science teams were selected from 229 proposalsand other solicitationsto take part in BOREAS. Most U.S. universityinvestigatorswere funded by either the National Aeronauticsand SpaceAdministration(NASA), the National Oceanographic and Atmospheric Administration  facilitate accessto the forest canopyfor Icaffbranchchamber  (NOAA), or the National ScienceFoundation(NSF). Canadian universityinvestigatorswere funded through a collaborative specialproject grant from the Natural Sciencesand Engineering Research Council (NSERC) and Canadian governmentinvestigatorsby their respectiveagencies.There was also significantparticipation by scientistsand research organizationsfrom the United Kingdom, France, and Japan. The individual projects were organized into six disciplinary groupsto facilitate coordinationduring the field phase. The objectivesof these six sciencegroupsare summarizedbelow and the namesof individualprincipal investigatorsare listed in  techniquesto characterizethe flux of trace gasesbetweenthe soil and the atmosphere, including CO2, CH 4 and nonmethanehydrocarbons(NMHCs). The TGB group alsostudied the long-term accumulationof carbon in boreal soils.  Table  4.1.  1.  Airborne Fluxes and Meteorology (AFM)  Four aircraft were used to measure turbulent fluxes; sound-  ing lidars and radars were also deployed (see Figure 3 and Table 2). One other aircraftwasusedfor operationssupportin 1994 and 1996 and for CO2 concentration measurement work in 1996. Ten meteorological stations and a dense array of upper air radiosoundingstationsoperatedover the regionduring 1994,with their data being transmittedto operationalmeteorological centers for assimilationvia the Global TelecommunicationsSystem.Severalinvestigators,includingsomewith close links to these centers,are using these data to improve mesoscaleand global-scaleatmosphericmodels.The surface network, a subsetof the aircraft, and a few of the upper air stationsalso operated during 1996. 4.2.  Tower Fluxes (TF)  The TF group'sprimary objectivewas to quantify the turbulent exchangesof energyand massbetweenthe atmosphere and a variety of boreal forest surfacecoversand to investigate the processes controllingthesefluxes.This work was designcd to be complementaryto chamberobservations and other process-oriented studieson smallerscales(TE, TGB) and aircraft studies (AFM, RSS) covering larger scales.The TF towers operated almost continuouslyduring the growing seasonof 1994,measuringradiation, heat, water, CO2, and in somecases CH 4 and other trace gasfluxes;see Figure 2 for their location. Two of the sites,one in the NSA and one in the SSA, operated more-or-less continuously from the fall of 1993 onward to characterizethe annual cycleof the energyfluxcs.Six TF sites operated during the growingseasonin 1996;see Figure 3 for the operating schedules. 4.3.  Terrestrial Ecology (TE)  Over 20 teamsexaminedthe biophysicalcontrolson carbon, nutrient, water, and energyfluxesfor the major ecosystems in the boreal landscapeand are developingmodels and algorithms to scale chamber measurementsto stand, landscape, and regionalscales.Much of the work wascarriedout at plots within the footprints of the TF sites, i.e., within the surface zonesthat contributefluxesmeasuredby the TF instrumentation, typically within a 200 m radius of the TF tower. An important focus for the TE group was the measurementof carbon cycle components.A number of free-standingtowers (TE towersin Figure 2) were installedin the studyareas to  measurements  4.4.  in situ work.  Trace Gas Biogeochemistry (TGB)  Ten  4.5.  and other  TGB  teams  used chamber  measurements  and other  Hydrology (HYD)  The HYD group consistedof eight teams. Five focusedon the measurementof snowhydrologycomponentsto support remote sensingalgorithm development;two worked on catchment hydrologicalprocessesin the SSA and NSA usingprecipitationgage networks,streamgages,and a rain radar; and one team operated a program of almost continuoussoil moisture measurementsat the TF sites during the 1994 and 1996 growingseasons. 4.6.  Remote Sensing Science (RSS)  The RSS group developcd linkagesbetween optical and microwaveremotesensingsignaturesand borealzonebiophysicalparametersat scalesthat includeleaf,canopy,andregional levelsusingficld, aircraft and satellite-bornescnsors,and radiativetransfermodels.The TE and RSS groupscollaborated in gatheringa wide range of biomctric and radiometric data at the auxiliarysites(Figure 2). Severalremote sensingaircraft were deployed in BOREAS 1994 and BOREAS 1996; see Table 4 and Figure 3. 4.7.  Staff  The scienceteamswere supportedby a staff of scientistsand supportcontractorsfrom the National Aeronauticsand Space Administration (NASA); the Atmospheric Environment Services(AES), Canada;the CanadaCenter for RcmotcScnsing (CCRS); ParksCanada;the Schoolof Forestry,Laval University; the Schoolof Forestry, University of Wisconsin;and the Canadian Forestry Service. The BOREAS staff ovcrsawthe componentsof the project which required significantlogistical effort, extendedand/or routine monitoringwork, or work that required the particular expertiseand resourcesof onc of thc participatingagencies.In the early stagesof the project, the staff carried  out the detailed  site reconnaissance  work  and  constructionplanning. During the field seasons,the staff dealt with the organizationof the tield logisticsand the day-to-day managementof field operations.The staffmonitoringprogram includedautomaticmeteorologicalstationnetwork;upper air network; hydrology,snow, and soil moisture; auxiliary site work; biometry and allomctry; radiometric calibration: stan-  dardgasesand gascalibration;thermalradianceintercomparison;global positioningsystem(¸PS) facilities.  The NASA staffwcrc alsoresponsible fi•r implementingthe BOREAS InformationSystem(BORIS) whichservesasa data organization,distribution,and short-termarchivingcenter for the project.All in all. some3(1(1 peoplewcrc workingwithin or abovethe studyareasin BOREAS 1994and around 15()people participated in BOREAS 1996.  5.  Experiment Execution  Many of the BOREAS measurementswcrc taken continuously;for examplc,the 1{)metc()rologicalstationstook 15-min  SELLERS ET AL.: BOREAS IN 1997  28,737  Table 1. PrincipalInvestigatorsand TasksAssociatedWith Each BOREAS ScienceGroup Aircraft  Sounding/Networks  Models  AirborneFluxesand Meteorology(AFM)," 14 Teams AFM-  1 Crawford  AFM-2 Kelly AFM-3  Lenschow  AFM-5  Atkinson  AFM-6 AFM-7  Martner Shewchuk  AFM-4 MacPherson/Desjardins  AFM-8 AFM-9 AFM-11 AFM-12  Betts  Dickinson Mahrt Pielke  AFM-13 Schuepp AFM-14  Sellers  AFM-15 Verseghy Vegetation Type  SSA  NSA  TowerFluxes(TF),• 1! Teams Old aspen(OA) Old black spruce(OBS)  TF-1 Black.... TF-2 den HaGtog TF-7 Desjardins TF-9  Old jack pine (OJP) Young jack pine (YJP) Fen (FEN) Soils, Forest Floor, Wetlands TE- 1 Anderson  .-. TF-3 Wofsy*  Jarvis  TF-5 Baldocchi TF-4 Anderson TF-11 Verma  TF-8 Fitzjarrald TF-10 Jelinsky/McCaughey TF-11 Jelinsky/McCaughey  EGophysiologyand Ground Carbon TerrestrialEcosystems(TE), c 22 Teams TE-2 Ryan TE-4 Berry TE-5 Ehleringer/Flanagan TE-6  Gower  Models  TE-13 Apps TE-14 Bonan TE-15 Bukata TE-16  Cihlar  TE-7 Hogg  TE-17 Goward  TE-8  TE-18  Kharuk  Hall  TE-9 Margolis  TE-19 HaGriss  TE-10  TE-20  Middleton  TE-11 Saugier TE-12 Walter-Shea  Knox  TE-21 Running TE-22 Shugart TE-23  Methane  Rich  Isotopes,Pesticides  NMHCs  TraceGasBiogeochemistry (TGB),J 10 Teams TGB-1 TGB-3 TGB-4  Grill Moore Roulet  TGB-6 TGB-7  Wahlen Waite  HYD-1  Cucnca  TGB-9  Niki  TGB-10 Westburg TGB-12  TGB-5 Zepp  Soil Moisture  TGB-8Monson  SnowProcesses, Remote Sensing Snowand Hydrology(HYD)," 8 Teams HYD-2 Chang HYD-3  Davis  HYD-4  Goodison  Trumbore  Hydrological Modeling HYD-8 HYD-9  Band Soulis  HYD-5 Harding HYD-6  Optical  Peck  Microwave  Algorithms/Modeling  RemoteSensing Science (RSS),t 20 Teams RSS-1 Deering  RSS-13 Gogenini  RSS-4 Curran  RSS-2 Irons RSS-3 Walthall  RSS-15 RSS-16  RSS-5 Goel RSS-6 Williams  RSS-10 Holben RSS-11 Markham RSS-12 Wrigley  RSS- 17 Way  RSS-14  Ranson Saatchi  Smith  RSS-7 Chen RSS-8 Running RSS-9 Strome RSS-18 RSS-19  Green Miller  RSS-20  Vanderbilt  Staff sciencesupport:infrastructureinstallationand maintenance,calibration of radiometric instruments,aircraft operations,satellite data acquisitionprogram,BOREAS Information System(BORIS), operationsmanagement. 'lAirborneeddycorrelationprofiling(5 aircraft), surfacemeteorologicalnetwork(10 stations),upper atmospheresoundings(radiosondes), lower atmosphereprofiling,modelingand analysis.  bTwolong-termtowers:*heat,H20, CO2,sometracegases; eightgrowingseason towers:heat,H20, CO2,sometracegases. CPorometryand chamberphotosynthesis, canopyand soil respiration,biometry,nutrient cycling,modeling.  dTracegaschambermeasurements including methaneandnonmethane hydrocarbons (NMHCs);studies of fire scars, beaverponds,soiland moisture gradients;isotopes. CSnowprocessesand snowremote sensing;catchmenthydrology;soil moisture, canopyinterception;hydrologicalmodeling.  •Opticalremotesensing (four aircraft),microwave/radar/gamma remotesensing (four aircraft),surface-based remotesensing studies,algorithm development.  28,738  [•[ [[ [•[  SELLERS  [ ,[ [  ,-[  , ,  [ ,  [ [  ET AL.' BOREAS  IN 1997  SELLERS ET AL.: BOREAS IN 1997 Table  2.  28,739  Remote Sensing(R-Prefix) and Flux Measurement(F-Prefix) Aircraft Deployed in BOREAS  Aircraft  Type ER-2  BOREAS Identifier RE  Science Team RSS-18 HYD-2  Home  Primary Target/Role  Equipment  RemoteSensingAircraft (R-Prefixon BOREAS Identifier) airborne visible infrared imaging vegetationproperties spectrometer(AVIRIS) MODIS airborne simulator (MAS) snow  Institution  NASA  Ames  Research  Center  in FFC-W C-130  RC  RSS-2 staff  advancedsolid-statearray spectroradiometer(ASAS) thematic mapper simulator(TMS), MODIS  RSS-20  Piper Chieftain  RP  RSS-19  airborne  vegetation properties, SART  NASA  Ames  Research  Center  simulator  (MAS), airborne tracking Sun photometer (ATSP) Polarization and Directionality of Earth's Radiation (POLDER) compactairborne spectrographic imager (CASI)  vegetationproperties,SART  Ontario  Remote  SensingOffice, Canada  Helicopter (UH- 1)  RH  vegetation and surface properties  NASA Wallops Flight Facility  airborne syntheticaperture radar (AIRSAR); P, L, C-band; fully polarimetric airborne syntheticaperture radar (CCRS-SAR); X and C-band, polarimetric  vegetationand soil properties soil moisture  NASA Ames Center  vegetationand soil properties  Canada  microwave radiometers; 18, 37, and 92 GHz; H and V  snow  gamma ray equipment  snow and soil moisture  RSS-3, 13, 20  SpectronElectronics-590,Barnes  RSS-15, 16, 17  multiband modular radiometer, C-band scatterometcr, ATSP, POLDER  DC-8  CV-580  Twin Otter (DH-6)  RD  RV  RT  TE-16  HYD-2  soil moisture  RA  HYD-6  Electra  FE  Flux MeasurementAircraft (F-prefixon BOREAS Identifier) AFM-3 flux, H, LE, CO2, limited local but mainly regional flux atmosphericchetnistrylidar measurements,chemistry  FK  AFM-2  flux; H, LE, CO2  Twin Otter (DH-6)  FT  AFM-4  flux; H, LE, CO2, O•  Centre  for  Remote Sensing (CCRS) National  Research  Council, Canada  Aerocommander  Kingair  Research  local and regional scale (5600 km) flux measurements local and regional scale flux measurements  LongEZ  FL  AFM-1  flux; H, LE, CO2  local scale(5-60 kin) flux  PA-34  FB  AFM-14  CO2 conccntrations(1996 only)  profiles (0-10000'), regional gradients  NOAA National Weather Service  National  Center  for  Atmospheric Research, Colorado University of Wyoming National  Research  Council, Canada NOAA, Oak Ridge  measurements  NASA  GSFC  Scc also Figurc 3.  data from early in 1994 to the end of 1996, and satellitedata werecollectedregularlyoverthe sameperiod(Figure 3). Some flux towersran continuouslyand a few scienceteamswere in the field during the entire growingseasonsof 1994 and 1996, but for most investigationsinvolvingcomplexequipmentand moderate-sizeteams, it was not practical or necessaryto sustain a continuouspresencein the field, so special periods, intensivefield campaigns(IFCs), were definedfor coordinated  runningoperationsin 1994 [Sellerset al., 1994]. During IFC-93 and the five BOREAS 1994 field campaigns,operationswere coordinated out of two centers, one in the NSA and one in the  SSA. Each center was equipped with FM ground-to-ground and VHF ground-to-airradio links, telephones,fax machines, etc., to maintain real-time management of the airborne operations and related surfacework. Nightly meetings of the participating scientists,air crew, and managersanalyzedthe revisits. The IFCs, each of which lasted about 20 days, were sults of the day's completed operations and set up the next spacedto catch the major phenologicalevents,includinga day's activities. Weekly sciencesymposiaprovided a useful snow hydrology/RSScampaign in the winter, spring thaw, forum for sharing scientific results and reviewing recent green-up,peak greenness,and the beginningof senescence progress.This continuousexchangeof information and ideas betweenBOREAS staff and scienceteam memberspermitted (Figure 3). In August 1993, many of the BOREAS investigatorsand continuousrefinement of the experiment design and operastaff were in place in the studyareas as part of the "shake- tions and better integration of the various scientific compodown" intensivefield campaign,lFC-93. This 21-day IFC was nents in the project. The BOREAS 1994 fall field campaign, IFC-3, ended in used to test the experimentinfrastructureand investigator's instruments as well as to refine coordination and communicaSeptember 1994, and the first scienceworkshopwas held in tion procedures. All of theseexperiences were pooledto refine December of that year. On the basisof the resultsfrom early the four-volumeexperimentplan which servedas the basisfor analysespresentedat the workshop,it became apparent that  28,740 Table  SELLERS ET AL.: BOREAS IN 1997 3.  Organization of Papersin This SpecialIssue No. of  Overviewof Papers  Papers  Section6.1: Carbon-Water-Energy Fluxes small-scalefluxesand physiological measurements (chambersand enclosures) standand plot-levelfluxesand dynamics(flux towers) landscape-scale fluxesand surface-boundary layerinteractions(aircraft,sondes,profilers)  6.1.1 6.1.2 6.1.3  10 17 15  Section 6.2.' Trace Gas Fluxes 6.2.1  small-scalefluxes(chambersand enclosures) standand plot-levelfluxesand dynamics(flux towers)  6.2.2  Section6.3: Soil and SnowMoistureand Runoff point measurementsand modelingof soil moisture dynamics stand-levelsoil moistureand snowdynamics(modelsand measurements) landscape-scale precipitationand soil moisture(models,radar estimates  6.3.1 6.3.2 6.3.3  Section6.4: Remote SensingScience groundand aircraft measurements of biophysicaland opticalcharacteristics, and understoryand canopyreflectance radiative transfermodelsand algorithmdevelopment landscape-scale land coverand biophysicalcharacteristics algorithms radiation and atmosphericeffects  6.4.1  6.4.2 6.4.3 6.4.4  Each sectionis devotedto a specificsciencearea and is further subdividedby spatialscale.  there were a number of gapsin the 1993-1994 data set, and in addition the meteorologicalrecord showed1994 to be an unusualyear: a new recordhad been set for the longestfrost-free period at Prince Albert National Park, while the NSA had experiencedthe driest year on record. A full accountof the BOREAS 1994 field campaignsmay be found in the work of Sellerset al. [1995b]. During 1995 the decisionwas made to take additional field measurementsduring 1996 to fill the 1994 data gaps[Sellerset al., 1996c].The BOREAS 1996 campaignsinvolvedfewer aircraft and scientistson the groundthan BOREAS 1994 as the objectives were strictly defined by shortcomings in the BOREAS 1994 data sets and important findingsfrom early analyses.Particularlyimportant was the decisionto start TF measurements earlier (in March) andend later (in October)in 1996, comparedwith 1994, to better coverthe growingseason (Figure 3). This was done becausethe BOREAS 1994 model-  Table 4.  ing and measurementresultsindicatedthat variationsin spring thaw and fall temperatureswere critical in determininginterannualvariability in ecosystemproductivity.  6.  Overview of Papers in This Special Issue  The 83 papersin this specialissuewere contributedby the sciencegroupslisted in Table 1. The papershave been organized into four major sections(see Table 3). The first three sectionscoversurfaceenergyand massexchangeprocesses and the fourth final sectionis devotedto remote sensingscience. The findingsof thesepapersare summarizedbelow. 6.1.  Carbon-Water-Energy Fluxes  Carbon, water, and energy fluxes were measured in BOREAS usinga comprehensive rangeof techniquesranging from leaf-scaleobservations made with chambersand porom-  BOREAS RSS Ground, Aircraft, and Satellite Instruments With Measurements and Related Products Measurements  Products Ground-Based  PARABOLA  Sensors  Laboratory and field  3-bandbidirectionalradiance BRDF, spectralalbedo spectralradiancespectroradiometers understoryand tree componentreflectances  NASA  radar backscatter  DC-8/AIRSAR  NASA ER-2/AVIRIS,  spectralradiance  RemoteSensingAircraft/Sensor forest type, biomassdensity,canopymoisture forest type, canopymoisture,atmosphericproperties  MAS  Piper Chieftain/CASI  spectral radiance  BRDF  NASA  NASA helicopter  TMS, ASAS, MAS, POLDER MMR, SE-590, POLDER  Radar  C-band  spectralradiance,bidirectionalradianceBRDF, albedo,foresttype,FPAR spectralradiance BRDF, FPAR canopyscatteringprofiles  C-130  backscatter  AVHRR  Landsat SPOT  TM  ERS-1, 2 SAR JERS- 1 SAR Radarsat SIR-C/XSAR GOES  scatterometer  spectralreflectance,emittance spectralreflectance,emittance spectralreflectance  Remote SensingSatellites land cover, FPAR land cover,FPAR, LAI, biomassdensity land cover  radar backscatter radar backscatter radar backscatter  freeze/thaw  radar backscatter  land cover,biomassdensity PAR, albedo,downwellingirradiance  spectral radiance  land cover land cover  SELLERS ET AL.: BOREAS IN 1997  28,741  Leaf Chambers/Bags M•ddleton  et al.  Dang et al. (th•s •ssue; 1997) Flanagan et al. Flyan et al. Ryan et al. (1995) Sullivan et al. (1997) Saug•er et al. (1997)  Leaf spectra Middleton  et al.  Woody Respiration Ryan et al. Lavlgne and Ryan (1997)  Moss/Soil  Chambers  Normanet al.  Long Term Carbon  Rayment andJarvis  Turnover  Nakane et al.  Winston et al.  Harden et al.  Roots  Gouldenand Cnll(1997)  Ryanet al.  Steeleetal.(1997)  Trumbore and Harden  :: :::  Hardenet al.  Figure4. Schematic summarizing papers in section 6.1.1'small-scale fluxesandphysiology withchambers and enclosures.  the driersubplot,whichsuggests that near-surface andsurface decomposition ratescouldbe significantly enhancedundera warmer,drier climate.Winstonet al. [this issue],who worked mainlyin the NSA, extendedmanyof their CO2 fluxmeasure6.1.1. Small-scale fluxes and physiologywith chambers mentsthroughthe wintersof 1993-1994and 1994-1995.The of observed wintertimeCO2fluxesrangedfrom0.5 and enclosures. Six papersdescribesoil fluxesand four pa- magnitude to 1.0 g CO• m 2 d •. The lowestfluxeswerecorrelated with persdescribe lea[-andbranch-scale measurements (Figure4). the midwinterminimumtemperatures.Accumulatedover the A BOREAS specialissueof 77c½Physiology [MargeIls Ryan,1997]has1()papersthatrcporton a varietyof BOREAS longwinterperiod,thesefluxescontributean importantfrac-  ctcrs (section6.1.1), throughintermediate-scale studiesconductedwith fluxtowersandsupporting measurements (section 6.1.2),up to regional-scale studieswhichusedaircraft,meteorologicalarrays,andsatellites(sections 6.1.3and6.4.4).  ccophysiological studics; the resultsof someof thesearc dis- tion of the annual carbonbudget.The winter fluxesseem to agreewell with the eddycorrelationmeasurements takenat cussedhcrc for complctcncss. Norma•tel al. [this issue]comparedsoil CO2 fluxesmca- the northernblacksprucesite(NSA-OBS).Gouldenand Crill chambers at NSA-OBSto measure the surcdby sixdift'crcntchambermethodsas fieldedby partici- [19971usedautomated pantsin BOREAS(Figure5a). It wasdemonstrated thatsys- CO, fluxesfrom the feathermossand sphagnummosscovers tematic differences exist between the measurement methods. and deepersoilsbeneaththe blacksprucecanopy.Maximum occurredbetween5øCand 8øC,andmoss Adjustment factorsof bctwcen (1.8and1.4areneededto bring net photosynthesis wasestimatedto accountfor all the data into agreement.These differencesare actually and surfacenet photosynthesis uptakeand50 to 90% smallerthanwcrcanticipatedprior to the BOREAS intercom- between10and40% of wholeecosystem of the whole ecosystem respiration from above-canopy eddy parisonbasedon thedisparate techniques used.Biasesamong  the techniques wereconstantacrossthe rangeof fluxesmea- correlation measurements. Winston etal. [thisissue] reportthatthe •4Ccontentof the sured(1-8/•mol m 2s •).Rayment andJarvis[thisissue] used releases showed a progressive shiftto lighter,thatisolder, an "open"chamberthat maintainedan interiorpressure very CO,__ closeto externalatmospheric pressure, therebyreducingmass carbon as the winter wore on, which suggeststhat the source from recentlyfixed,modern flow effects on the soil GO, flux measurements. CO• fluxes materialfor the CO2fluxchanged locateddeeperin were measuredmore or lesscontinuouslyfrom a subploton carbonto olderorganicmaterial,presumably the floorof a blackspruceforest(SSA-OBS)overa periodof the soil.Hardenet al. [thisissue]alsousedisotopesto invesof carbonby surfacepeatsandmosses about 3 months in 1994; the fluxeswere found to be strongly tigatethe sequestering correlatedwith soiltemperatureat 5 cm depth.Nakaneet al. near the NSA-OBS site' these two surfacetypeshave sequesof 40-60 g C m-2 yr-• overthelast90years, [thisissue]measured soilCO,__ fluxesat wetanddrysubplots at teredanaverage --2 in deepersoilsreleased20-50 g C m the sameblacksprucesiteasRaymerit andJarvis[thisissue]. andnet decomposition Theyalsomeasured carbonstorage components (litterfall,soil yr • overthe sameperiodleadingto an averagenet exchange --2 carbon,etc.).Thedrierplothaddecomposition ratesthatwere of soilcarbonwithin the rangeof + 10 (efflux) to -50 g C m roughly doublethosefromthewetplot,presumably because of d • (uptake)over the last century.Hardenet al. [thisissue] differences aredueto (1) therateof the muchwarmersoiltemperatures and aerobicconditionsat pointoutthatsite-to-site _  28,742  SELLERS ET AL.: BOREAS IN 1997  .o  o•  o  o  decompositionof surfacepeats,(2) ratesand methodof lateral spreadof moss,and (3) the historyand rate of decomposition in the deeper soil layers.Sitescan therefore vacillatebetween sourceand sink statusbased on climate variability. The primary control on carbon accumulationin peats and moss is drainage,while deep carbonstorageseemsto be a functionof respiration,decomposition,drainage,and fire history. The isotopicrecord found in soil organicmatter has been usefulin determiningdecadal-scalecarbonaccumulationrates in the NSA soils. Analyses of soil carbon and its isotopic compositionrevealeddifferencesin carbondynamicsbetween sitesin the NSA [Trumboreand Harden, this issue].The jack pine siteshad the lowestsoil carbon concentrations,and the fastest turnover occurred in the surface organic and mineral layers.Black sprucesiteswith mosssurfacelayerswere found mainly on clay soilswith degradingpermafrost.Thesesiteshad net carbonuptake rates that were similar to thoseat the jack  pinesites(50-100 g C m-2 yr-•), but theyalsohad slower ([.%.wIOWri) eoueIonpuoo leIewoI$  decompositionrates, due to high soil moisture contentsand lower temperatures,which resulted in the accumulationof more carbon in their soils.The fen sites had the largest soil  carbonstores(1.2 x 105g C m-2) andthehighest productivities(200-400g C m-2 yr-•). Slowdeepsoilrespiration offset '  '  '  I  '  about 15% of the carbonuptake rates in wetlandsand about 45% in upland sites.Deep soil respiration/decomposition rates were observedto decreasewith depth. Deep soil carbon de-  composi, ratesasdetermined from •4C andsoilinventory  --  '  i  i  {  I  work agreewith the findingsof Winstonet al. [this issue]. Middletonet al. [thisissue]acquiredleaf-levelmeasurements of gas exchange,chemistry,morphology,and spectraloptical propertiesduring the BOREAS 1994 IFCs at all of the SSA forestcoverTF sites(Figure 5b). Leaf net photosynthetic rates were found to be comparablebetweenthe old (SSA-OA) and the young(SSA-YA) aspensites,but leaf transpirationrates were significantlydifferent; the SSA-YA leaves transpired about 30% more than the SSA-OA leaves.The black spruce shootsat SSA-OBS exhibited the lowest photosyntheticrates amongall the foresttypes.The conifers(SSA-OBS, SSA-OJP, SSA-YJP) were observedto have peak photosyntheticratesin late summerto early fall, while the deciduousspeciespeaked in midsummer(see alsoSullivanet al. [1997] and Saugieret al. [1997]).Dang et al. [1997,this issue]examinedthe profilesof photosynthetically activeradiation (PAR), leaf nitrogen,and photosyntheticcapacityat two jack pine sites,an aspensite and two black spruce sites in the NSA. Beer's law descriptions (exponentialextinction)fit the PAR profilesdownthroughthe canopyfairly well (r = 0.73-0.92) under both cloudy and clear-skyconditions.Leaf nitrogendecreasedwith the fraction of absorbedPAR in each of the forest stands(relationships were similaramongthe speciesexceptfor an alderunderstory),  Figure 5. (opposite)(a) Comparisonof soil CO2 fluxesmeasuredby different investigatorsand instrumentsat a common test site in the SSA in BOREAS 1994,from Norman et al. [this issue];(b) leaf assimilationratesasmeasuredby porometryfor severalspeciesat the SSA TF sites,from Middletonet al. [this issue].Note that understory(hazel, bog birch) as well as the dominant species(bs, black spruce;jp, jack pine, ws, white spruce)were sampledat each TF site (OA, old aspen;YA, youngaspen;OJP, old jack pine; YJP, youngjack pine; OBS, old blackspruce);(c) plot of leaf stomatalconductance against photosyntheticcapacityfor several speciesin the NSA, from Dang et al. [this issue].  SELLERS ET AL.' BOREAS IN 1997  28,743  Net Radiation, Albedo  Tower Flux (Eddy Correlation)  Hodges and Smith  Blanken et al. Baldocchiet al. Jarviset al.  (SSA-OA) (SSA-OJP) (SSA-OBS)  Patteyet al.  (SSA-OBS)  Betts and Ball Shewchuk  Local-Scale Kimball  Flux Models  et al.  Frolking et al. Bonan and Davis  ...  Humidity Sensors  Goulden et al. (NSA-OBS) McCaugheyet al. (NSA-YJP) LaFleur et al. (NSA-Fen)  Suykeret al.  Betts et al.  (SSA-Fen)  Productivity Gower  et al.  Respiration Lavigneet al.  Sapflow Hogg et al.  Hogg and Hurdle(1997)  Figure6. Schematic summarizing papers in section 6.1.2;stand andplot-level carbon-water-energy dynamics. and photosynthetic capacitydecreased significantly with leaf thesis,averaged0.44, 0.29, and 0.43 for aspen,blackspruce, Differencesin CUE between nitrogenexceptfor thesinglecaseofjackpineduringearlyleaf and old jack pine, respectively. flush.Aspenleaveshadhigherphotosynthetic capacities than the NSA and the SSA sites were small for the conifers but of higherrootrespiration in theNSA. conifersfor the same leaf nitrogen. Good correlationswere largerfor aspenbecause 6.1.2. Stand and plot-levelcarbon-water-energy dynamics. found betweencanopyphotosynthetic capacityand remote sensing spectral vegetation indices(NDVI andsimpleratio), The firsttwo papersin thissectiondealwith radiationfluxes but the relationships varieddependingon the canopyscaling and albedo measurements measured over forested sites in procedure. In all cases it wasfoundthatphotosynthetic capac- BOREAS; most of the remaining 15 papers describeeddy from the flux towers(TF sites)and ity correlated wellwithstomatal conductance (Figure5c) but correlationmeasurements of patch-scale carbondynamics inferredfrom these decreased lesssteeplythandid PAR downthroughthe canopy. analyses observations takenwithinfootprintsof the TF Flanaganet al. [thisissue]correlatedleaf isotopiccontents, andsupporting specifically 13Cto I:C ratios,to inferthe degreeof stomatal sites(Figure 6). Hodges andSmith[thisissue]analyzedsurfacenet radiation restrictionon photosynthesis. Despitethe very low stomatal acrossthe BOREAS region.Three caliconductances observedamongthe forestspeciesin BOREAS (Rn) distributions bratednet radiometerswere comparedwith 22 Rn instruments of photosynthesis, in agreement withgasexchange andmete- at 21 sitesin BOREAS; R,fields generatedfrom GOES oborologicaldatapublished byDanget al. [1997].Measurements servationswere alsocomparedwith the in situ measurements.  their results indicated that there was little stomatal limitation  of the carbonisotopeconcentration of cellulosefrom individual tree growthringsvariedwith summerprecipitation and temperature, suggesting thattreeringcarbonisotopes maybe usedto infer the effectsof past environmentalconditionson  The results show that one make of radiometer, used at 15 out  photosynthesis.  to create time series of albedo estimates over the different  of the 21 sites,underestimated R,by about5% in the daytime  andby about45% at night.BettsandBall [thisissue]analyzed theBOREASmesonetdata(seenotebyShewchuk [thisissue])  Ryanet al. [thisissue]and Lavigneand Ryan [1997]esti- covertypessampledin BOREAS throughout1994and 1995. dailyaveragealbedosin summerrangedfrom matedannualautotrophicrespirationfrom chambermeasure- Representative mentsof foliage,woodytissue,andfine rootsof aspen,black 0.083 for coniferous sites, 0.15 for aspen, to 0.20 at grass  were0.13for conifers, 0.21for spruce,andjackpineforestsin the SSAandthe NSA during (airport)sites;winteralbedos the BOREAS 1994growingseason.Mean foliagerespiration aspen,and 0.75 for the grasssite (Figure7a). The winter per unit leaf areawasfoundto be similaramongexpanded albedosof the forestsitesvariedwith canopyclosureand with leavesfor all species at 10øC.Woodrespiration wasstrongly solar diffuse/direct flux ratios but never exceeded 0.3. These differentfrom thoseusedin the seasonal, with high ratesin midsummerthat coincidedwith numberswere significantly underestimawoodgrowth.Finerootrespiration declinedby abouta factor EuropeanNWP modelwhichled to a systematic of 3 throughoutthe season,but rates were similar among tion of the near-surfaceair temperature(Figure7b). The note species. Annualcostsof autotrophic respiration for thewhole of Bettset al. [thisissue]comparedsurfacehumiditymeasuresystem wereestimated to be 310-610g C m-2. Carbon use mentsfrom the operationalAES sensorat Meadow Lake, efficiency (CUE), the ratioof net production to net photosyn- Saskatchewan,with a colocated BOREAS mesonet sensor,  28,744  SELLERS ET AL.: BOREAS IN 1997  (a)  i  I  1994  /  -Grass#1  •1.  .  - Grass#2  , /  ,I 11 d  ........ Aspen  0.8  .....  Conifer  0.6  I  0.4 i  showingthat the operational instrument has significantbiases that could be important to surfaceclimate studies. Figure 8 summarizesthe flux data reported by the TF sites during BOREAS 1994; it can be seen that the coniferous vegetationis characterizedby low evaporativefractions(ratio of latent heat to the sumof the latent and sensibleheat fluxes) and low CO: uptake rates during the growingseason.Boreal deciduousland cover,while showinglarger transpirationand CO,_tiptake rates than its coniferouscounterpart,is still much lessactive than deciduousforestsin temperate North America [e.g., Baldocchiand l/ogel, 1996; Baldocchiand Harley, 1995]. The papers summarizedbelow provide more details on these results.  Blankene! al. [thisissue]presentedresultsfrom eddycovariance  measurements  of latent  and  sensible  heat  flux  made  aboveand below the aspenoverstoryat the SSA-OA site during 1994. Before leaf emergence,most of the availableenergy was converted to sensible heat flux, but latent heat flux dom0  60  120  (b)  180  240  300  360  DayofYear,1994  20[  1996  were330and113mmolm : s •, respectively (Figure9a).The  ••  canopyconductancesof both speciesincreasedwith increasing photosyntheticphoton flux densityand decreasingsaturation deficit at the leaf surfaceand were not limited by soil moisture. Forest canopyconductancewas directly proportionalto forest leaf area index. The daytime average ratio of the latent heat flux to the equilibrium latent heat flux (the Priestley-Taylor alpha) suggests that the understoryhazelnuttranspirationwas mainly energy controlled, while the overstory aspen canopy transpirationrate was more limited by stomatal conductance. [togg et al. [this issue]measuredsap flows through aspen bolesat the SSA-OA site in BOREAS 1994 usingtwo different techniques(seealsoHoggand Hurdle [ 19971).Both techniques produced similar results and compared well with the eddy  o o  '•  •  inated after leaf emergence.During the full leaf period, daytime mean dry leaf aspen and hazelnut canopyconductances  10  lO  o  correlation -20  1  3  5  7  9  11  13  15  17  19  21  Day of Year, 1996  Figure 7. (a) Annual courseof surfacealbedoover different BOREAS cover types during 1994, from Bettsand Ball [this issue]. Note that the winter albedo over the grasslandsites varies around 0.7, while the winter albedos over the forest sites  are much lower, 0.12-0.30; see section 6.1.2: (b) effects of overestimatingsurfacealbedo on the performanceof the European numerical weather prediction (NWP) model. The [nodel  assumed  a winter  surface  albedo  of about 0.8 over the  boreal zone, while the true value was around 0.1-0.2; see  Figure 7a. Shownhere is the ECMWF (European Center for Medium-Range Weather Forecasting)model near-surfaceair temperaturefor the SSA site as predicted24 hours ahead of the start of the model run, comparedwith the mean observations from the BOREAS meteorological towers for the same time. (The NWP runs were initialized with observedtemper-  measurements  described  above in terms of diurnal  and seasonalpatternsand magnitudes,althoughthe sap flow time serieslagged the eddy correlation data by about 1 hour due to water storagechangesin the trees. All methodsshowed that as expected,transpirationincreasedwith air vapor pressure deficit  until  about  1 kPa but then  remained  almost  con-  stant for higher vapor pressuredeficits. Baldocchiet al. [this issue]report on the variationsof radiation, sensible, and latent heat fluxes above and below a boreal  jack pine canopy(SSA-OJP) during the 1994growingseason. The sum of the measuredhourly averagesensible,latent and soil heat fluxes, and canopy heat storagewas about 8% less than the net radiation, indicatinga systematicunderestimation of one or more of the measured energy fluxes. When the canopywasdry, daily evapotranspiration was lessthan 2.5 mm  d • withmostgrowingseason valuesbeingmuchlessthan1.5 mmd • (Figure9b).Thisextremely lowratewasattributed to  the low LAI (1.9-2.2) and low stomatalconductancesobserved at the site. Factors restrictingstomatal openingwere low soil atures).Note how the (24 hour) predictedsurfaceair tempermoisturesupply,limiting atmosphericsaturationdeficitsand ature is alwaysmuch lower than observationsbecausethe surface is calculatedto absorbonly a small fractionof the incident the low photosyntheticcapacityof the needles.Typically,20solar energy, exceptwhen the model predictsmelting condi- 40% of the total energy exchangedoriginated at the forest tions (marked by horizontal bar in figtire) in which case the floor underneaththe sparsejack pine canopy. Jarviset al. [this issue]summarizeeddycovarianccmeasurecalculatedalbedo is reset to a value representativeof a snowfree surface,around 0.2. which is actuallycloseto the observed ments of CO2, water vapor, and sensibleheat fluxesabove a winter albedo for the forest. ECMWF output are from Holl- black spruce (SSA-OBS) forest over a 120-day (May 23 to ingsworth(personal communication,1996) and the analysis September 21) period in 1994 (Figure 9c). Average midday andfigurearefrom Huemmrich(personalcommunication, 1996). evaporativefractions(latent heat flux dividedby the sumof the latent and sensibleheat fluxes) for the three IFCs were 0.34-  SELLERS ET AL.: BOREAS IN 1997  --  28,745  0  o -  O>-•LLO  I i.{-)  I o  1.1'3  0  o  '•-  m.  o.  i.q.  o  o  o  o.  0  (s/•m/fim)xnl:l•00 Aepp!l/•  UO[JO'eJ-.I OA!J'eJOd'eA:l Al•pp!l/•  r-:. o  m. o  m. o  q o  UO!J01•J-i OA!Jl•JOdl•A 3 A•PP!PI  c,q. o  m c5  I  I  I  o. o  i.q. o,  o. "'7  (s/•m/fim)xnl-t•OO A•pp!PI  28,746  SELLERS  ET AL.' BOREAS  (a) 800• • • • •' I20 (b) 4 SSA-OA i Aspen I_  400  o,  i  ø ø  ß  I  r . '•'••o__. I  I.  -  -I.  Hazelnut  200 _  •  '•'  10  ,, lO( •4 I  5  '-' 3  •'  • m 1  IN 1997  SELLERS ET AL.: BOREAS IN 1997  28,747  tissue,foliage, and soil respirationrates and temperature measurementsto estimatehalf-hourly ecosystemrespirationrates at six coniferousBOREAS sites.These were compared with nocturnal eddy correlation estimates.Soil surfacerespiration was the largest contributor (48-71%) to the total flux and  adding the downwardCO2 exchangeand the soil respiration measuredby soil chambers.Midsummer photosynthesis ap-  foliar respirationwasa significantcontributor(25-43%) at all the sites.The nocturnaleddycorrelationmeasurements(at u *  curredin earlyJuly,were13 •mol m 2 S-1.Mid-Mayto early Octobernet ecosystem exchange (NEE) was -88 g C m-2  proachedlightsaturationabove1000-1200•mol m-2 s- 1 and decreasedin responseto increasingtemperature and saturation deficit. Maximum middayphotosyntheticrates,which oc-  > 0.25 m s-l) werepoorlycorrelated to estimates obtained (sink). This substantialproductivityis consistentwith the obfrom scalingup chamber measurements;estimatesof ecosystem respirationfrom chamberswere 1.2-1.5 times greater than those from eddy correlation. Gouldenet al. [this issue]used eddy correlation equipment to measure CO2 exchangebetween the atmosphere and the NSA-OBS site for 2 years (March 16, 1994 to February 19, 1996) (Figure 9d). This is the longestcontinuouslyrunning eddy covarianceCO2 flux record of the BOREAS TF teams. Nocturnal CO2 fluxeson windy nightswere used to develop a respiration versus temperature relationship for the forest which was then applied to separate net exchangeinto gross photosynthesisand respiratory fluxes. Gross photosynthesis waslargelya functionof PAR flux and air temperaturewith no apparent effectsdue to high evaporativedemand or soil water content. Under moderate light levels, photosynthesiswas higher under cloudythan under sunnyconditions.A singleset of regressionrelationshipsdescribingthe responseof gross photosynthesis to PAR flux and temperatureand the response of respirationto temperatureaccountedfor 72% of the variation in hourly CO2 exchange.Maximum rates of photosynthesis at the NSA-OBS site were small compared to those measured at a deciduous Harvard forest site [Wofsyet al., 19931. McCaugheyet al. [this issue] report on measurementsof sensibleand latent heat and CO2 fluxestaken above the stand at the NSA-YJP site over 119 days in BOREAS 1994. The PAR and clear-skysolar albedoswere approximately0.054 and ().136, respectively,during this time, and the averageevaporative fraction during this period was about 0.3. Nighttime CO2 fluxcs were estimated by using a regressionrelationship between  fluxes  measured  in neutral  to near-neutral  conditions  servedhigh water table and is in marked contrastwith the loss of carbonover the correspondingperiod measuredat the drier NSA-fen site in the sameyear [Lafieuret al., this issue].Roulet et al. [this issue]report that their beaverpond site in the NSA  wasa strongsource of CO2(183g C m- 2) overthe120growing seasondaysof 1994(section6.2.2),whichsuggests that in some boreal landscapes,beaver pondscould make a significantcontribution to the regional carbon budget. Productivityand carbon allocationwithin an ecosystemare criticallyimportant for determiningsite carbonbudgets.Gower et al. [this issue]analyzedtheir detailed measurementsat the TF and auxiliary sitesto show that abovegroundnet primary productivity(ANPP) for the BOREAS forestsiteswas55-310  g C m-2 yr-• for aspen,blackspruce,andyoungandoldjack pine covers, low values when compared to most temperate sites. Thirty to 40% of the ANPP, as estimated at the BOREAS forest sites,fell to the surfaceas detritus;only 6070% wasretainedas biomassincrement.Black spruceecosys-  temscontainedthe highestcarbonstocks(39-48 kg m-2), aspenwasintermediate (16-18kgm-2), andjackpinewasthe lowest(5-8 kgm-2). Leafareaindices rangedfroma lowvalue of 1.25 for jack pine sitesto 5.6 for black sprucesites.Fine root  NPP estimatedwith minirhizotrons rangedfrom 30 g C m-2  yr-• at SSA-OAto 115g C m-2yr-• atNSA-OBS[Steele etal., 1997]. Root elongationwas highlycorrelatedwith soil temperature (10 cm) for aspen,black spruce,and jack pine sitesat both the NSA and the SSA [Steeleet al., 1997]. Three papers present stand-scalesimulation modeling resultswhichusedTF flux data and supportingmeasurementsfor validation. Kimball et al. [this issue]discussenergy and water balancesimulationsperformed with the BIOME-BGC model.  and soil temperature at 1()- and 75-cm depths.The regression In their model, total ET was calculated to be insensitive to estimatesfor June and July agreedquite well with the modeled ecosystemleaf area index, and canopy conductanceto water values calculated by Ryan et al. [this issue].It was estimated vapor averaged67% of its maximum(unstressed)value during that the standfixed224g C m 2 duringthisperiod. the growingseason,with low light and low temperatures(not LaJteuret al. [this issue]collectededdy covariancemeasure- low humidity)causingthe reductions.This reductionin canopy mcnts of sensibleand latent heat, and CO2 fluxes and related conductancecorrelateswith the stablecarbon isotopework of climatic variablesover a period of 124 days (April 9 to Sep- Flanagan et al. [this issue].Frolking [this issue]assessedthe tember 19) in 1994over the NSA fen site. Albedos(solar and effect of climate anomalieson carbon dynamicsat the NSAPAR) decreaseddramaticallyafter snowmelt;during full leaf, OBS site with a black spruce-mossprocessmodel. Net ecosys--2 solar and PAR hemisphericreflectanceswere 0.18 and 0.055, tem fluxesof carbon(sinks)were calculatedto be 120 g C m respectively.Mean evaporativefractionsincreasedfrom about yr • in 1994and 90 g C m-2 yr-• in 1995.The effectsof 0.5, during the snowmeltperiod, to 0.59 at leaf-out, to 0.83- introducingclimate anomalies,in the form of 2-month periods 0.91 near midsummer, and then decreased to around 0.5 at of warm, wet, cool, or dry conditions, into the model runs senescence.The fen acted as a net sink for CO2 only when varied with the timing of the anomalyand were not anticipated vascularplantswere activelyphotosynthesizing to give a daily from simple relationshipsof grossprocesseswith climate. If meanfluxof -0.81 g C m 2 d-i. Overthe 124-dayperiodthe the mosslayer remained intact to insulate the soil, large diffen lost30.4g C m 2 to the atmosphere. While the authors ferences in air temperature were simulated to have only a point out that there is considerableuncertaintyin the latter minor (5-10%) effecton decomposition. BonanandDavis[this figure, there is a marked contrastbetween this fen and the one issue]assessed the ability of their land surfaceparameterizain the SSA, which was relatively productive(i.e., acted as a tion (NCAR LSM1) to reproducethe fluxes of energy and carbonsink) duringthe June-Augustperiod; seeSuykeret al. carbonas measuredby eddy covarianceat aspenand jack pine [this issue]who report on atmosphericCO2 exchangesmea- sites.Using generic vegetation parameters,their calculations suredat the SSA-fen site from mid-May to early October 1994. generallymatchedthe observednet radiation and eddy covariCanopy photosynthesisat the SSA-fen site was calculatedby ance CO2 and sensibleheat fluxesbut overestimatedthe latent  28,748  SELLERS ET AL.: BOREAS IN 199/  Airborne Eddy Correlation Dobosyet al.  (Intercomparisons)  Soundings/ABLStudies  Kaharabata et al.  (Footprints)  Wilczaket al. (WindProfiler) Kiemleet al. (Airborne Profiles) BarrandBetts(Analysis) Barret al. (Radiosondes)  Desjardins et al. Ogunjemiyo et al. Oncleyet al. MacPherson andBetts  (Footprints) (Vegetation IndexandCO2 Flux) (Regional Transects) (GustEvents)  K. Davis et al. (AirborneProfiles)  rn t I •  Meso eteorolog'ca / Network  Shewchuk /  /  r•• •  _•-• •b> ,Ft '  /,_ ,• •"•5•F,/•/ •//  Models  Sun et al. (Lakes, MesoscaleEffects) Vidale et al. (MesoscaleCirculations) Cooperet al. (LSP Radiation)  Figure 10. Schematicsummarizingpapersinsection6.1.3; lanscape-scalecarbonwater-energydynamics and surface-atmosphere boundarylayer interactions. heat flux and did not capturereal differenceswithin vegetation couplingof the boundarylayer fluxes from surfacefeatures types,for examplebetweenthe jack pine sitesin the NSA and with increasingheight. The differencesin scale between airSSA. craft- and tower-measuredfluxeswere also examinedby Des6.1.3. Landscape-scalecarbon-water-energy dynamics and jardinset al. [this issue],but the large differencein footprint surface-atmosphereboundary layer interactions. Figure 10 sizebetweenthoseplatformsmakesit questionable to compare summarizesthe regionalscalestudiescarried out in BOREAS flux values directly. Among other things, it appearsthat the usingaircraft, soundingdevices,meteorologicalnetworks,and towerfluxfootprints(around104-105m2 in unstable condicoupled land-atmospheremodels.Shewchuk[this issue]de- tions) may have been drier than the aircraft flux footprints scribesthe mesoscalemeteorological network that was de- (around107m2 for a 10-kmpass),asevidenced bydifferences ployedover the BOREAS 1000 x 1000km regionin 1994 and in sensibleand latent heat fluxes.Desjardinset al. [this issue] ran continuouslythrough 1996. Two siteswere located in the compared fluxes measuredby the Twin Otter, from short SSA, one in the NSA with another nearby, and the remaining passesover specificTF sites(SSA-OA, SSA-OBS, SSA-OJP, six were placed at or near airfieldsin the region to allow for NSA-OBS, NSA-OJP, NSA-YJP), with the TF measurements easy deploymentand servicing.The BOREAS years of 1994 themselves(Figures11a and lib). The aircraft fluxeswere on and 1995 showedwarmer air temperaturesthan normal; 1994 was0.7øCabovethe long-termmean. Wilczake! al. [thisissue] (a) (b) present resultsobtained with a 915-MHz profiling radar lo750 © .............. ¸ YJ P NSA cated near SSA-OJP during BOREAS 1994. The device ,7 ..... rOBS J... '• ..... rOBS • 600_• OJP yielded profilesof wind velocity,virtual temperature,and atmosphericboundarylayer (ABL) depth. m, 450 E 450 ..-; • Severalpapersdescribeintercomparisonsbetweenairborne  750 I ©............ ¸OA SSA  • 6øø/_=OJP O  _  _  flux measurements, both aircraft to aircraft and aircraft to  tower.Dobosyet al. [this issue]presentextensivecomparisons amongthe four flux aircraft involvedin BOREAS, all basedon wing-to-wing flights spaced throughout the BOREAS 1994 IFCs. Overall, the smallestcorrelations(most scatter) were between the LongEZ and the Twin Otter, while the Twin Otter-King Air and Electra-King Air pairingshad higher correlations(lessscatter).At least someof the reducedcorrelation for the LongEZ-Twin Otter comparisonis becausethe air-motion equipment and techniquesused on the LongEZ, with its smallsizeand payloadare different in manywaysfrom the other three aircraft.  Significanteffortswere devotedto determiningthe geometry of the surfacesourcezonessampledby the airborne flux systemsand in correlating the airborne fluxes with surface covertypes.An encouragingstudyof aircraftflux footprintsby Kaharabataet al. [this issue]made use of SF6 releasesto compute diffusionprofileswithin the lower boundarylayer; their results indicate that the tower and low-level flux aircraft mea-  surementscan be expectedto yield site-specificestimatesof the surfacefluxes.The work also exploresthe progressivede-  --  3OO  3OO  +  150  150  150  300  450  600  750  0/•  H + LEm W m-2 (Tower)  t • t  150  300  450  600  750  H + LE inW m-2 (Tower)  (c)  (d)  -0.1 ß  ß  d ß ßß ß  o  -0.4  1.8  2.0  2.2  2.4  2.6  2.8  SpectralGreennessIndex  3.0  -0.6  -0 5  -0.4  -0.3  -0.2  -0.1  0  CO2 flux(mgm-2s-1)  Figure 11. Comparison of the sum of sensibleand latent  heat fluxesmeasuredby the Twin Otter (FT) aircraft over shortrunscloseto severalTF towersin (a) NSA and (b) SSA, from Desjardinset al. [this issue]. Comparisonof aircraftmeasured(FT) daytime(c) CO2 fluxeswith spectralgreeness index and (d) latent heat fluxeswith CO2 fluxesas measured over a grid within the SSA; see Ogunjemiyoet al. [this issue].  SELLERS ET AL.' BOREAS IN 1997  average higher than the towers for latent heat, less than the towersfor sensibleheat, and comparableto the towersbut with muchmore scatterfor CO2. The bestcomparisons (both in magnitude and in degree of scatter)wcre for momentum fluxes. Two-dimcnsionalpatternsof surface-ABL couplingare presentedby Ogunjemiyo½tal. [this issue]for multiple grid flights in both studyareas(Figures 11cand 1ld). Strongcorrelations were observedbetweenCO2 flux and spectralgreennessindex, a very encouragingresultfor modelerstryingto estimatelargescale surface fluxes over the boreal zone. These  results concur  with similar findingsfor grasslands[Cihlar et al., 1992], leafscalestudiesin BOREAS [Dang et al., 1997], and theory [Sellers et al., 1992b].At the same time, sensibleheat fluxeswere noticeablydecoupledfrom the atmosphere-surfacetemperature difference (see also Mahrt et al. [this issue], Fining and Blad [ 1992],and Hall et al. [1992]). Oncleyetal. [thisissue]documentedseasonaland spatialflux changesalong a regional transect,from just south of the SSA, up over the NSA, and into the tundra near Churchill, located near the southwestcorner of Hudson Bay. Traces of CO2 fluxes from three of these transects, in BOREAS 1994 IFC-1,  IFC-2, and IFC-3, showchangeswith latitude, season,and land cover. There were also large-amplitudevariations along each transect,superimposedon the other trends. Airborne flux measurementstaken over lakes highlight the complexity that must be addressedin modeling the regional surface energy budget. Oncleyet al. [this issue]estimate that aircraft-level fluxes measured in their regional transectsoriginated from surfaceareas (footprints) that were up to 40% water (lake surfaces).They also estimate that surface heat storageaccountedfor 30% (forests)to 40% (tundra) of the net radiation, with a large portion of the storagebeing assignedto the numerous lakes in the region. The length scalesof the flight-levelfluxespoint to specificforcing mechanismsand to the difficultiesfaced in mergingaircraft and tower flux data. A detailed analysisof flux fieldsover and around severallakesin the SSA by Stln et al. [this issue] indicates that the areaaveragedsensibleheat fluxes in the SSA were 12-24% lower than they would have been if no lakeswere present.A welldefined divergent lake breeze circulation was observed over three lakes in the SSA during the day by the BOREAS flux aircraft with the circulation strength dependent on lake size and wind speed.The modelingstudyof Vidaleet al. [this issue] investigatesthe contribution of mesoscalecirculationsto the large-scalesurface-atmospherefluxesof heat, water, and momentum.They concludethat thesecontributionscan be significant under low-wind conditions and are generated by the differencesbetween the energy balancesassociatedwith the different surfacetypesfound in the region,for example,lakes, dry uplands, and wet forested areas. They argue that these mesoscaleeffects should be parameterized in large-scale atmosphericmodels. Aircraft measurements, radiosondes, surface measurements  28,749  and Betts[this issue]calculatedarea-wideestimatesof entrainment rates from a budget analysis,based on data from the BOREAS radiosondenetwork. They found the entrainment parameter A•e above the boreal forest to be 0.21, which is in closeagreementwith the generallyacceptedvalue of 0.20. The surface available energy, calculated by summing the sensible and latent heat fluxes derived from the budget analysis,was within 4% of the independent estimate provided by the BOREAS meteorologicalnetwork radiation measurementsof Shewchuk[this issue].They estimatedthe mean surfaceevaporative fraction over the forest during the summer to be 0.49, which can be comparedwith the value of 0.71 observedover the grasslandFIFE site [Smithet al., 1992]. Ban' et al. [this issue]compared the surface fluxes derived from the radiosondebudgetanalysiswith the eddy fluxesmeasured by the Twin Otter aircraft for the three growingseason IFCs  of BOREAS  sensible  1994.  and latent  The  sum of the aircraft-measured  fluxes was 25%  less than  the radiation  network estimate of available energy, in contrast to the 4% underestimategiven by the budget analysis.However, the aircraft-derived estimate of the evaporative fraction was 0.52, which agreeswell with the budgetanalysisfigure of 0.49, which implies that the aircraft measurementsmay be systematically and proportionatelyunderestimatingthe turbulentfluxes.The authorspoint out that both methodshavevalue for the studyof landscape-scalefluxes, with the budget analysisproviding a large-area integral view, while the aircraft provide insightinto spatial variations and associationsbetween different surface flux regimesand land cover type. Davis et al. [this issue(a)] describe studies of the convectiveboundary layer based on aircraft  measurements  in BOREAS  1994. Convective  bound-  ary layer divergenceduring the midday period and early afternoon was often observedto dry out the atmosphericboundary layer; this drying may have had feedback effects on the vegetation which could have further reduced evapotranspiration, thus reinforcing the drying of the ABL; see also discussionby Sellera'et al. [1995b].MacPhersonand Betts [this issue]document the structureof the strongABL vorticesseen by aircraft at low levelsin 1994in conditionsof deepABLs and low winds. Cooperet al. [this issue]used the BOREAS meteorological observations,soil and vegetation parameters, and satellitebased  estimates  of  radiation  fluxes  to  drive  a surface-  atmosphereflux exchangemodel for an area coveringmost of the BOREAS domain. According to sensitivityanalysesconducted with their model, root depth, soil moisture, and soil depth are the most important quantitiesin controllingtranspiration fluxes. The radiation balance of the BOREAS region was studiedby a number of BOREAS investigatorsusingsatellite data analyses(section6.4.4). 6.2.  Trace  Gas Fluxes  Carbon exchange(CO2, CH 4, and CO) was the principal focusof the TGB group,largelybecauseof the great interestin the carbon balance of the boreal ecosystembut also because early measurementsshowedthat the fluxes of nitrogen gases were small exceptin very recentlyburned areas. Most of the BOREAS trace gas studieswere conductedwithin the NSA, which can be roughly divided into drained upland sandysoils usually dominated by jack pine and aspen; lesswell drained clay-rich soils dominated by black spruce;and wet peat soils dominated by sphagnumand sedges(Figure 12). TGB mea-  and analyseswere all used to explore exchangesamong the land surface, the boundary layer, and the free atmosphere. Inversion-level(ABL-top) entrainment,the captureof air from the free troposphere into the ABL, is an important factor which affectsmanyof theseexchanges. Kiemleet al. [thisissue] used an airborne laser to remotely senseprofiles of aerosols and water vapor that were used to build 2-D cross-sectional maps of entrainment structures and estimate entrainmentzone water vapor fluxes. They found good agreement with surements were focused at the NSA tower sites, some landdirect eddy-covariance measurementsof the samefluxes.Ban' scapetransects,a chronosequenceof sites disturbedby fire,  28,750  SELLERS ET AL.' BOREAS IN 1997 CH4 and N20 fluxes (SSA-OA) Simpson et al.  Peroxide  Fluxes  Hall and Claiborn  CH4 Chambers Amaral and Knowles (Dryer Sites) Moosaviand Crill (Transects,Water Table Effects) Savage et al. (Transects,Drainage Effects)  Fire Sites Burke et al.  Zepp et al. Beaver  Fen (CH4)  Pond Tower  Rouletet al. (CO2,CH4) Bourbonniereet al. (CO)  LaFleuret al. (CO2) Suykeret al. (CO2,CH4)  Figure 12. Schematicsummarizing papersin Section6.2; tracegasfluxes,principallyCH4 and CO. (CO2 fluxesare coveredin Section6.1).  and a small tower at the NSA beaver pond. The CO2 flux ment program and were able to model CH 4 fluxesusingcormeasurements of the TGB groupare summarized in section6.1.1; relationswith surfaceorganicmatter and the soil temperature at 20 cm in NSA upland soils. Weakly emitting sites were in this sectionwe summarizethe TGB CH 4 and CO research. 6.2.1. Small-scale fluxes with chambers and enclosures. usually dominated by black sprucewith a feather moss or Amaral and Knowles[this issue]found that drier sites,dominatedby aspen,jack pine,or birchwith a vascularplantground coverand a thin (1-5 cm) surfaceorganiclayer,often actedas (a) 250 strongCH 4 sinks(Figure 13a). These siteswere warmer and drier with faster nitrogen cyclingand shorterpath lengthsto CH 4 oxidationlayers.Consistentwith early indicationsin tem•-- 250 perate soils,Amaral and Knowles[this issue]determinedthat • 'el E the location of CH 4 oxidation activitywas near the surfaceof the mineral soil and at the baseof the organiclayer. A signifE 250 icant positiverelationshipbetween CH 4 uptake and surface C:N ratio was also observed.Their microbiologicalwork dem• 250 onstratesthe potential for a near-surfaceorganic layer of drainedsoilsto produceCH 4 underanaerobicconditions.This effect  is obvious  documented  on the macroscale  for well-drained  but has not been well  soils.  Moosaviand Crill [this issue]observedCH 4 effiuxesat all sites along their experiment transect which extended from someof the driestuplandsoilsto inundatedwetlandsnear the NSA beaver pond tower (NSA-BP) and the NSA-OBS site. Fluxes along this transect varied by 4 orders of magnitude; strongCH 4 sourcesrequire a consistent,highwater table, as at the beaver pond mire (Figure 13b). Their work showsthat upland sitesmight be convertedfrom net sinksto net sources by minor changesin water table; also, low soil temperatures strongly inhibited methane flux rates even under saturated conditions.  Bubieret al. [1995] reported a large range in CH 4 fluxes(3 ordersof magnitude)from different plant communitiesin the NSA fen complex,with the largestfluxesfrom open graminoid fens and the smallestfrom frozen peat plateaus and treed peatland areas. Temperature at the average position of the water table was the best predictor of seasonalaverage CH 4 fluxes,and floatingpeat sitesmaintainedhighfluxesduringthe dry late summerperiod in 1994.The correlationsbetweenCH 4 flux and plant communitydistributionmay be usefulfor scaling fluxesfrom the chamberto the landscapeusingremotesensing. Savageet al. [thisissue]conducteda dark chambermeasure-  •  I• Spruce I 250 Aspen• 250  0  50  100  150  200  Days since first measurement  (b) 3000  ,  .-. 2500  --  Mire  ,•  Pond  •-u2000 E  E 1500 ,= 1000 5OO  0 138  158  178  198  218  238  258  Day of Year, 1994  Figure 13. (a) Methane (CH4) consumptionby vegetation type in the NSA estimatedfrom chambermeasurements, from Amaral and Knowles[this issue];(b) methane productionas measuredover a beaver pond site in the NSA, from Moosavi and Crill [this issue].  SELLERS ET AL.: BOREAS IN 1997 Airborne  28,751  Soil Moisture  ..•ecketal. (7-ray)  Hydrological Modeling Nijssen et al.  •  •.•i• •;.•. • ,•,'• ,• InSituSoilMoisture•;• •  •  Cuenca et al. (TDR)  ',A'A  .  ,•;.• •;• SnowPhysics  •;,• •  RainRadar  Schnur etal  Hardy etal.  • •  Y.••/ Levine and Knox  Figure 14. Schematicsummarizingpapersin section6.3; soil and snowmoistureand runoff.  report lower values from a beaver pond in the SSA. This indicatesthat beaverpondscouldbe significantcontributorsto the boreal carbon cycle with source strengths4 to 5 times larger than the sink strengthsobserved for boreal soils by Hardenet al. [thisissue]or asestimatedfor borealpeatlandsby Gorham [1991]. The surfacewaters of the NSA beaver pond site were also shownto evolveCO, most likely from degradationby incident radiation of dissolvedorganiccarbon [Bourbonniere et al., this issue].This illustratessome of the difficultiesof estimatinga regionalflux of CO to the atmosphere. Simpsonet al. [this issue]measuredCH 4 and N20 fluxes part byBubieret al. [thisissue](section6.3.1). Firedisturbance isanimportant fac•0rin theborealforest. above the SSA-OA stand using a tunable diode laser. Small Burkeet al. [thisissue]measuredfluxesof CO2 and CH 4 across emissionsof both gaseswere observedfor the measurement 16,1994),around1.9-2.5ngm-2 a chronosequence of burnedsiteson both clay (typicalof less period(April16-September well-drained black spruce dominated sites) and sandy soils s-1 for N20 and 21-28 mg m-2 s-1 for CH4. The authors measurements with some(below (typicalof well-drainedjack pine dominatedsites).All of the correlatedthe above-canopy siteswere net sinksof atmosphericmethanewith medianfluxes canopy)chambermeasurementsto concludethat CH 4 emisrangingfrom -0.3 to -1.4 mgC m-2 d-1. Medianfluxesof sion from scatteredanoxicpatchesin the aspenforest overcarbondioxidefrom the forestfloor to the atmosphereranged whelmed CH 4 uptake by the larger drier areas. This result between1 and2 g C m-2 d-1. Bothecosystem characteristicsshould be comparedwith the observationsof Amaral and (e.g.,soilandvegetationtype) andburninghistory(time since Knowles[this issue], describedabove, who found the drier burn and fire intensity)appearto have someeffect on atmo- areas to act as strongCH 4 sinks;however,they did not comsphericmethaneconsumption and carbondioxideemissionby ment on the signof the net CH 4 flux on intermediate(stand these forest soils.Their resultssuggestthat soil CO2 effluxes level) scales,as Simpsonet al. [thisissue]have done. Understandingthe sourcesand sinksof hydrogenperoxide from uplandblack sprucestandsmay not be immediatelyimpactedby fire. By 2 yearspostfirethere appearsto be a signif- (H202) and organicperoxides(ROOH) is importantfor unicant reduction in soil CO2 flux, while soil respiration rates derstandingatmosphericchemistryin generaland acid depositionin particular.Hall and Claiborn[thisissue]usedconcenrecoverto preburn levelsby 7 yearspostburn. from the tower of the SSA-OJP Carbon monoxide fluxes were slightly higher in more re- tration gradientmeasurements centlyburned areas[Zeppet al., this issue].However, CO flux site to infer depositionvelocitiesfor thesecompounds.They wasshownto be affectedby thermogenicand photogenicpro- report daytimeand nighttimedepositionvelocitiesfor hydrogenperoxide of 5 cms- • and1 cms- • respectively, withmuch ductionaswell as plant consumptionand production. 6.2.2. Stand and plot-level trace gas dynamicswith towers. lower ratesfor organicperoxides.The highestdepositionrates The boreal forest is in a constantstate of regenerationdue to occurred from 1100 to 1500 LT, indicating that deposition continuousdisturbanceby fire and beavers,insects,and to a rates are primarily turbulencelimited. These rates are similar lesserextent man. Roulet et al. [this issue]made continuous to values obtained over deciduous forests in the southern measurementsof fluxesover a beaver pond in the NSA using United States. a flux gradient method on a 1.25-m tower. It was discovered 6.3. Soil and Snow Moisture and Runoff that the beaverpond was a strongand continuoussourceof Soil moisture and snow measurementsin BOREAS ranged bothCO2 andCH4;678g m-2 GO2 (183g C m-2) and11.3g m-2 CH4 (8.3g C m-2) for the 120-day measurement period. from point measurementsin the vicinity of the TF sitesto Subsequent surveysof otherbeaverpondsin andnear the NSA airborne measurementsover the TF sites,alongshort transects for surface emissionsand near-surface CO2 concentrations within the studyareas,and alongan extendedtransectbetween showedsimilar results,althoughStaeblerand Edwards[1997] the studyareas(Figure 14). Traditional catchmenthydrologi-  sphagnummossground cover and a thick organiclayer of 20-50 cm; thesesitestended to be colder and wetter with long pathsto the zone of CH 4 oxidation.Unlike other studies,the effects of H20 on CH 4 uptake could not be observedin BOREAS becauseof limited variability in the soils over the seasonat mostof the uplandBOREAS sites.However,Savage et al. [this issue] observedthat a single heavy precipitation eventin early springchangedmanyof the jack pine sitesfrom CH 4 sinksto sourcesfor a short period of time. The problem of scalingthe chambermeasurements of theseborealwetlands to larger regionsusingremote sensinghas been addressedin  28,752  SELLERS ET AL.: BOREAS IN 1997  Wet Conifer  Regen RegenVeg(South)  Ground, Sparse Veg Typically BumArea  Open Water Mixed Agrb 1985-90  Fire  1980-84  Fire  1970-71, 77-79 Fire 1945-69  Fire  Road GPS  (b)  Field Ohs.  04-06 I 01-09 I  55-57 58-b0  13-15  16-18 19-21 22-24 25-7/ 28-3½!  3 1-3 3 34  -36  3 / - 39 •0-42  •3-45 -16-48  i [  i  i:1  _  _l .  i  [  .  _][[[.I  [.  I [[  (c)  ,:%..":,;'.,,½.......: • ....,.... ..  ½.' ½,.'.-,_ ß -. :•-'.  ..  '",:.:'.i.: ....  ..-:  .. ß  :•:.,,'•;..;:..,.•..:....:. -•;.-. •' ,,•..;•,.: :.:•;..,'•.:•' -.:,• . :..., . .  ..'%...( •'.,-,.., :. .  .  Plate 2. Use of regionalremotesensing to monitor(a) landcoverandfire disturbance fromSteyaert et al. [thisissue];(b) regionalFPAR fromCihlaret al. [thisissue];and(c) soilsfreeze-thaw statusfromWayet al. [thisissue](imagebrightness proportional to radarreturnrendering darkareasasfrozensitesandlightareas as thawed).  SELLERS ET AL.' BOREAS IN 1997  (a)  28,753  (b) 40  ß '" '  '  ...  20  '  !  •.  •,  •,',q' •  %•!  //  ,,,,""  •  /  Drainage  WettinFront 4I'•',•• Capillary ,'  120  t  Rise--  ,  -  :  x--Zero-Flux "o-.oPosition  ,%.\• • •  \-1•,•_  140  ,,,  160  0  /  _  -  800  = I•  I '""x;  20  1,5-  II  10  o/ "•/ - --DOY 154  ..':.' ---DOY 160  t/ o' ." ,..'" ......DOY164 il / !./ ........... BoY I&,, !....";,,, I,,, I ,,, 400  -.- I I',,__ \  '  'Water • / / /• ',.. Table , / / / :• '-._ DOY150 Position,/ :t •..__. , I / / / ./ t: /? --- DOY152 \'•/ \'#1  Snow Survey  30  -.  •'-...,.,_•\• \ \\  100  ß  .  "... •1_• •\\  , • :.,  ----Predicted  / Evapotranspiration -  ""--. re\\ ".. . • •\\  80  •  35  ,/ / • ..' .............. ................................ . I! #  :  60  1  /,--  ,/, "1-  40  '  1200  ABS [Total Head (cm)]  1600  2000  o|''''''''''' 60  70  ''''''''' 80  90  ' 100  I 110  Day of Year, 1994  Figure 15. (a) Absolutevalueof total head(or soilwaterpotential)asa functionof depthmeasuredusing time-domainreflectrometryat a tube at the NSA-OJP site,from Cuencaet al. [thisissue];(b) comparisonof modeledandobserved(snowsurvey)snowdepthsfor a plot at the SSA-OJPsite;seeHardyet al. [thisissue]. cal studieswere performedin both the SSA and the NSA; also, km x 5 km surroundingthe tower sites.The model simulated in the SSA a precipitation radar was deployed during the redistribution of soil moisture in the saturated zone due to BOREAS 1994. A number of supportingmodeling studies local topographiceffects.Model estimatesof latent, sensible, were made of soil moisturedynamics,snowphysics,and catch- and groundheat fluxes,and net radiationwere comparedwith tower observations.The model-predictedheat fluxes agreed ment hydrology. 6.3.1. Point measurements and modeling of soil moisture reasonablywell with those observedfor the NSA-OBS, SSAdynamics. Soil moisturemeasurementswere collectedvia in ORS, and the SSA-OA sitesafter the model had been modified situ time domain reflectrometry(TDR) observationsin the to incorporatemossmoisturestorageeffectsand temperature vicinity of the tower sites [Cuencaet al., this issue](Figure feedbacksto canopyresistance.A slighttime lag in the diurnal 15a).The typicalmeasurement designusedinvolvedtransects cyclesof latent and sensibleheat was attributedto deficiencies of five sitesspaced5-20 m apart. These transectsprovideda in the ground heat flux representation. 6.3.2. Stand-level soil and snow moisture dynamics. Soil unique opportunityto studythe space-timevariabilityof soil moistureprofilesover footprintsrepresentativeof the imme- moisturetransectswere monitoredvia an airborne gammaray diate area surroundingthe tower sites.In addition to the ex- sensorfor selecteddates from August 1993 to the fall of 1994 pected evolution of the depth profiles through the season, [Pecket al., this issue].Comparisonsof the airborne observathese measurementsystemsshow that strong soil moisture tions with TDR probes at the SSA-OA site showedthat the consistently underestimated(by a factor variationsin the near-surfacelevel (depthsto 20 cm) occurred aircraftmeasurements 2) the near-surfacemeasurements takenby TDR. even at spatialscalesof lessthan the transectlengths.These approaching variationswere especiallypronouncedearly in the seasonand However,the flightline wassomedistance(almost1 km) from followingprecipitationevents.Cuencaet al. [this issue]also the tower, so spatialvariabilitymay explainat leastpart of the estimatedevaporativefluxes in the vicinity of the NSA-OJP difference.In the vicinityof the SSA-OJPsite,the aircraft and and SSA-OJPtowersfrom changesin soil moistureprofiles. in situ measurementsagreed more closely.Especiallyduring Comparisonswith tower-measuredfluxes indicatedthat the the dry periods,spatialvariabilityof soil moisturein this area evapotranspiration rates estimatedfrom soil moistureobser- wasquite low. Attempts to comparein situ and aircraft surface vationswere about two thirdsof thosemeasuredby eddy corre- moisture observations at the SSA-OBS site were abandoned lation.Reasonsfor the discrepancies are not clear at thispoint. due to the periodic presenceof surfacewater, and moisture Nijssenet al. [thisissue]describethe testingof a distributed storagein the mossand peat layer which are not measuredby hydrologicalmodel, which will ultimatelybe applied to sub- the TDR techniques.However, gravimetricsurfacesoil and catchments in the SSA and NSA, and to smaller areas of 5 moss/peatmoisture transectscollected for calibration of the  28,754  SELLERS ET AL.: BOREAS IN 1997  of the studyregionswith respectto the boreal aircraft measurementsshowedthat large spatialdifferencesin resentativeness soil and other surface moisture (e.g., moss) contentsexist forestbiome as a whole (Figure 16 and Plate 2). In supportof thesegoals,RSS investigatorsacquiredmeawithin the "footprint" of the SSA-OBS site, which must be taken into account when relating the SSA-OBS tower flux surementsof reflectance,emittance,and backscatteringproperties of forest canopies and background,and atmospheric measurementsto ground data. Daviset al. [thisissue(b)] performeda sensitivityanalysisto scatteringfrom the leaf to the regional scale. In parallel, a examinethe effectsof coniferheight and canopydensityon the significantamount of ancillaryvalidation data on forest stand timing and rates of snowablation.The analysisassumedradi- and soil biophysicalpropertieswas acquiredin situ (Table 4). ation to be the first-order  cause of snowmelt  consistent with  These  combined  data sets are a valuable  resource  for devel-  the results of Hardy et al. [this issue]. Forest maps of tree oping and testing new algorithmsfor mapping terrestrial bioheightand canopydensitycombinedwith genericconifercan- physicalparameters;specifically,they will prepare the way for opy propertiesprovidedboundaryconditionsfor the calcula- the next generation of Earth-observingsatellite instruments tions,which showedthat canopydensityand tree height are of such as MODIS, MISR, and RADARSAT. The data will be more or less equal importancein controllingsnow ablation producedon CD ROM by the BOREAS Information System rates. The calculatedtime seriesof snow depth showedclose (BORIS) andeventuallyarchivedat the Oak RidgeData Analagreementwith surveydata from jack pine sites,but measure- ysisand Archiving Center (DAAC). The RSS investigatorsalso engaged in algorithm developments showedslower melting than the model calculationsin ment and testingand producedmultiyear site-level,studyarea black sprucestands. level, and regional-scalemaps of radiation and biophysical Hardy et al. [thisissue]comparedmodelpredictionsof snow parameters.A numberof land coverparametermaps(Plate 2) grain growth,compaction,and melting,with measurements of have been producedas part of BOREAS to provide inputsfor snowdepth at the SSA-OJP site (Figure 15b). They adjusted ecosystemor climate models and to evaluate the expected meteorologicalmeasurementsfrom Shewchuk[this issue]to performanceof future spacebornesensors.The articlessummimic conditionsat the forest floor and over a nearby open marized below represent a significant cross section of area. Levine and Knox [this issue]presentresultsfrom a soil BOREAS remote sensingscience activities and range across and snowphysicssimulationmodel that includesa description the entire multiscaledesignof BOREAS. of the effectsof the overlyingvegetationcanopy.The simula6.4.1. Ground and aircraft measurements of biophysical tions were run for the SSA-OBS, SSA-OJP, and NSA-OJP  and optical characteristics and understory and canopy reflec-  sites. The results showed good fits to measured snowpack tance. Remote sensingalgorithm development,as with prothicknessand soil temperatureprofilesand alsopredictedthe cessmodel development,involvesa scalingstrategyfrom the extendedfrozenperiodin the NSA and the formationof an ice leaf level, where electromagneticproperties can be carefully lens at the SSA-OBS site. measured, to the pixel level (meters to kilometers) where 6.3.3. Landscape-scaleprecipitation and soil moisture dy- ground-basedmeasures are impractical and remote sensing namics. An importantcauseof spatialvariabilityin soilmois- algorithmsmust function autonomously.Translatingdata and ture, and hence in surfacemoisture and energy fluxes,is the knowledgeamong these scalesinvolvesa mix of measurement space-timevariabilityof precipitation.Schnuret al. [thisissue] and modeling in the form of radiative transfer (RT) models operateda truck-mountedC-band precipitationradar, located that can accuratelycompute reflectanceas a function of canto the south of the SSA from May to September1994. Com- opy structure,optical and biophysicalcharacteristics,etc. The parisonsof radar-derived precipitation fields with estimates validatedRT modelsare then usedfor algorithmdevelopment from a gagenetwork showedthat in termsof area averages,the to infer canopycharacteristics over larger areasgivensatellite radar estimated slightly higher precipitation accumulations. reflectance values. The algorithms take the form of either Simulationsperformed by usinga spatiallydistributedhydrol- simplefunctionalrelationsbetweenvegetationindicesand bioogy model showedthat spatial averagesas well as local esti- physicalcharacteristicsdevelopedby usingRT simulationsor matesof surfaceenergyfluxeswere quite sensitiveto the useof more sophisticateddirect inversionapproaches.The BOREAS radar-basedprecipitationimagesascomparedwith a gage-only experimentwas designedin the contextof this generalscaling product. However, when the effectsof spatialresolutionwere and algorithm developmentstrategy,and the remote sensing isolated(by usingonly the radar precipitationproduct at dif- papersin this specialissuereflect this approach. An important unresolvedissuegoinginto BOREAS was the ferent spatial resolutions), the sensitivityof area-averaged (over the 40 x 50 km southern modeling subarea) energy accuratein situ characterizationof canopybiophysicalparamfluxes to the precipitation product was generally less than eters over plots large enough to validate processmodels and about 10%. However, local differences were much more sen- remote sensingalgorithms. BOREAS staff and PIs designed and implemented an approach which combined destructive sitive to the spatial resolution of the precipitation product. sampling,allometry,and opticalmethods(hemisphericalpho6.4. Remote Sensing Science tography,photosyntheticallyactive radiation (PAR) sensors, Remote sensing served three critical roles in BOREAS. multibandphotography)to gencrategroundestimatesof canFirst, it provided local- to regional-scaleland cover data and opy architecture, leaf area index (LAI), biomass density solar radiation forcingswhich allow us to investigatethe pro- (BMD), and the fraction of PAR absorbedby the vegetation cessesgoverningforest-atmosphereinteractionson timescales canopy (FPAR) over 30 x 30 m plots and 100-m transects ranging from minutes, with the GOES satellite, to decades, within the BOREAS tower and auxiliarysites(Figure 2). Sevwith the Landsat seriesof satellites.Second,ground, aircraft, eral activitiesreported here deal with groundmeasurementsof and satellite data (Table 4) were used to develop and test these biophysicalcharacteristics. ecosystem-atmosphere processmodels.Finally,remote sensing Chenet al. [thisissue]useddirect and indirecttechniquesto imageswere processedto define the overall context and rep- estimateLAI for the BOREAS tower and auxiliarysites.They  SELLERS ET AL.: BOREAS IN 1997  28,755  GOES  AVHRR, LANDSAT, SPOT  SSM/I, ERS-1, Radarsat  Optical Studies Hall et al. (Classificationand Biomass;Landsat) Steyaertet al., (Classificationand Fire; AVHRR) Li et al. (Fire Detection;AVHRR) Cihlar et al. (Parameter maps; AVHRR) Mahrt et al. (Ts)  Czajkowski etal.(Ts)  "'  Markham etal.(Aerosols)  Radar  Studies  Way et al. (Freeze, Thaw; ERS-1)  Radiation  Fluxes  Gu and Smith (S, PAR; GOES) Kaminskyand Dubayah (Rn) Li et al. (APAR; AVHRR)  Airborne Optical RSS Radar Ranson et al. (Classification,biomass;RD)  Russellet al. (Images; RC) Bicheronet al. (Images; RC) Miller and O'Neill (Aerosols; RP)  ,-d••• Loechel etal. (Radiances, RH) Passive Microwave  ••-  Chang et al. (Snow; RT)  Models Ni et al. (RT Model)  Goel etal.(RTModel) Privetteet al. (PARABOLA, RT Models)  Local Scale Optical Studies Chenet al. (LAI) Fournieret al. (LAI, Architecture)  '•'•ll•l?,••'-'-:;' !• Middletonet Kucharik etal. (LAI, Architecture) I i; •':;;'*%•" t• r•'• al. (Leaf Spectra)  Milleret al. (GroundReflectance) Bubieret al. (MossReflectance)  Figure 16. Schematic summarizing papersin section6.4; remotesensingscience.(RC, RD, RP, RH, RT refer to aircraft platforms;seeTable 2).  compareresultsfrom their indirectoptical techniquewith those from a similar manufactured  instrument  and estimates  derived from direct destructivesamples.They found that the manufactured  LAI instrument underestimated  the destructive  of canopyelementsand crownform, data whichprovidecritical inputsfor their and other RT models.With their model they are able to computeand validatearchitecturalvariables suchasgapsizeasa functionof viewzenith.Their datasetswill  amongstandstructure, sampleestimates of LAI becauseitsalgorithmassumed a spa- be usedto explorethe relationships tially randomdistributionof leavesto computeLAI asa func- light regimes,canopyreflectance,and ecologicalfunctionin tion of canopygapfraction.Chenet al. [thisissue]developeda general. Kuchariket al. [this issue]utilized a two-bandvisibleand clumpingindexto correctfor thiserror andusedit to estimate camera,the multithe LAI of severalBOREAS forestsites.Chenet al. [thisissue] near-infrared(NIR) digitalcharge-coupled of found that LAI rangedfrom I to 4 for jack pine and aspen bandvegetationimager(MVI), to map the nonrandomness standsand from I to 6 for blackspruce.They alsoreport errors canopygaps,i.e., the spatial"clumping"of leaves.As menof the order of 25%, a magnitudeconsistentwith direct de- tionedabove,the spatialdistributionof leavesis a key paramstructivesamplingtechniques.The difficultieswith ground eter for opticalLAI-measuringdeviceswhichrelate gap fraccharacterization of biophysical variablesmustbe kept in mind tion to LA!; the distribution may also be critical to canopyCO2assimilation modelsandRT modwhen usingthem to evaluatesatellite-based remote sensing parameterizing els relatingcanopyreflectanceto biophysical parameters.Asalgorithms. wereimagedfrom belowwith Fournieret al. [thisissue]combinehemispherical photogra- pen andbalsampoplarcanopies phy, standmapping,destructivesamplingand modelingto the MVI to generatean NDVI image, which permitted the characterizethe architectureof vegetated boreal landscape identification of sunlit and shaded foliage, discrimination elements.They employ a variety of destructiveand optical amonglive foliage,twigsandbranches,and sky-clouddiscrimof techniquesto characterizethe three-dimensional distribution ination.Analysesof the imageryshowedthat an assumption  28,756  SELLERS ET AL.: BOREAS IN 1997  spatially random foliage leads to underestimatesof LAI in aspenby about 45%, which then leads to underestimatesin CO2 assimilationrates by about 39%. In supportof this,Dang at al. [this issue]show that the relationshipbetween canopy photosyntheticcapacityand spectralvegetationindex can vary considerablydependingon the scalingalgorithm used. An important and sometimesneglectedcomponentof forest remote sensingstudiesis the bidirectionalreflectanceof understoryvegetation.Millar at al. [this issue]describea multiteam effort to measure nadir understoryreflectancefor the tower  flux sites. Shaded  and sunlit reflectance  measurements  were made for black spruceand jack pine sitesnear solarnoon throughout the year, capturing the variability due to species and seasonalphenologybetween and within the tower flux sites.Temporal-spectralvariationswere observedwhich correlated with changesin vegetationtype and phenology.The data were shownto be usefulfor improvingregressionrelationships between leaf area index and vegetation indices. Multiband optical propertiesof mosseswere found to be a critical input for understandingthe relationshipbetween canopy reflectanceand biophysicalproperties.In addition to the leaf opticalpropertiesmeasuredby Middlatonat al. [thisissue], discussedin section6.1.1, Bubiar at al. [this issue]conducted high spectralresolutionmeasurementsof mossground cover on the BOREAS wetlandssites.They found that high spectral  The advantagesof multiangle observationsfor forest-type classificationare also addressedby Bicharonat al. [this issue] who describean analysisof bidirectional reflectance data acquired from the airbornePOLDER (Polarizationand Directionality of Earth Reflectances)instrument.The instrument wasflown aboardthe NASA C-130 aircraft over fivevegetated areas representativeof BOREAS land cover. Using multiple spectralbandsand three view directionsgreatly improved the discrimination  of forest cover.  6.4.2. Radiative transfer models and algorithm development. The most direct approach to using RT models in in-  ferring biophysicalcharacteristics of the land coveris through mathematicalinversionof the RT model given a set of reflectancemeasures.However, this approachis often limited by the number of parametersin the RT model, which in many cases exceedsthe number of remote sensingmeasurementsnecessaryfor inversion.To circumventthis problem, sensitivityanalyses often reveal which parameters can be held fixed with minimum impact to the inversion,thereby reducingthe number of bands required for inference. BOREAS investigatorsand staff acquired validation data setswhich are ideal for evaluating different direct inversion techniques.Privattaat al. [this issue] describesome of the above-canopymeasurementsacquired in BOREAS with the PARABOLA instrument,a three-bandradiometer capableof resolution reflectance data could be used to discriminate the rapidly acquiring reflectance measurementsover the entire brown and sphagnummossesassociatedwith bogs, fens, and viewing hemisphere.PARABOLA was used to collect BRDF water-saturated conifer stands from the feather mosses assodata setsat the BOREAS SSA aspen,black spruce,and jack ciated with more productivestandsgrowingon mineral soils. pine flux tower sites(SSA-OA, SSA-OBS, SSA-OJP) usinga SinceHardan at al. [this issue]report that drainageclassis a 100-m-long tram-mounted system elevated several meters strongdeterminantof carbonstoragerate, the resultsof Bubiar abovethesecanopies.The observedBRDFs were usedto evalet al. [this issue]provide a basisfor designingremote sensing uate the performancesof 10 different RT modelsin predicting algorithms that can map these important classesover large albedo and nadir reflectanceover nine land cover typesrangareas. ing from grasslandsat the FIFE site to the forest data setsof Moving upscalefrom ground to aircraft,Loachalat al. [this BOREAS. Their findings provide an important lesson for issue]report on a seriesof helicopter-basedreflectancemealarge-arearemote sensingalgorithms.They showthat simpler surements acquired over the BOREAS tower and auxiliary sitesthroughoutthe 1994 growingseason.They comparedred RT models using fewer parameters may be both faster and and NIR reflectance measurementsto ground measuresof more robust for inversion than complex RT models, even overstoryLAI. Their resultsshowthat reflectancein individual thoughthe simplermodelsdo not representthe BRDF aswell. bands as well as common vegetation indices based on these They attribute this to the fact that with simpler models,more bands are poorly correlated with LAI, confirmingthe results accurate inversionsare possiblewith sparseremote sensing data sets. obtainedby Hall at al. [1995] over similarspeciesin the SupeGoal at al. [thisissue]demonstratea computer-based simurior National Forest in Minnesota.Hall at al. [1995] analyzed lation approach for investigating the utility of reflectancethe difficultiesfound by Loachalat al. [thisissue]and proposed and evaluated alternative approachesthat demonstrateskill based remote sensingalgorithms and vegetation indices to for those situationswhere vegetationindicesperform poorly. infer canopybiophysicalcharacteristics.They use a sophistiHall at al. [this issue]successfully applied thesetechniquesto cated computergraphics-basedmodel to simulatethe relationshipbetweenred and NIR reflectanceand canopybiophysical black sprucecoversin BOREAS (section6.4.2). characteristics at variousview and illumination angles,includBidirectional reflectanceimages similar to those to be obtained from the MISR sensor to be flown aboard EOS AM-1 ing the "hot spot"where the view and solarilluminationangles were acquired over the BOREAS SSA and NSA sitesfrom the are identical. They varied the magnitudesof the biophysical airborneadvancedsolid-statearrayspectroradiometer (ASAS) characteristicsused as parametersin the canopyreflectance mountedon the NASA C-130. Russallat al. [thisissue]report model (crowndimensions,crownspacing,leaf size,etc.) over on these spectral and bidirectional reflectancedata acquired realisticrangesto simulatethe covarianceamongtheseparamover the BOREAS SSA tower sites.They found that bidirec- etersand reflectance.This permitsthem to investigatehow the tional reflectancedistributionfunction(BRDF) measurements relationshipbetweenreflectanceand eachof the canopymodel at 26ø backscatterin the solar principal plane were best for parameterssuchas leaf sizeis affectedby knownvariationsin discriminationamongspecies.They alsoconfirma large num- the remainderof the canopymodelparameters.They show,for ber of previous studies showingview angle to significantly example,that for deciduouscanopies,the ratio of NIR reflecaffect NDVI and its relationship to biophysicalproperties. tance for a givenview angle to hotspotreflectanceat a high Their measurementsshow that view angle effects are much Sun zenith angle is the best index for estimatingleaf size since more important for conifer standsthan for aspen. it minimizesthe effectof variabilityin the other modelparam-  SELLERS ET AL.: BOREAS IN 1997  eters. For conifers they show that leaf size is difficult to estimate exceptfor sparselyspacedcanopies. Ni et al. [this issue]further developgeometricaloptical canopy reflectance models to include light interception effects within tree crownsfrom the nonrandomspatialdishibutionsof leaf material typicalof needleleafcanopies[seeChenet al., this issue;Fournieret al. this issue;Kuchariket al., this issue].For a specifiedcanopyleaf area index,leaf clumpingincreasesgap probabilitywithin a tree crown,increasinglight transmissionto the canopyfloor; however, this is partially offset by reduced light transmissivitythrough the clumps.The primary motivation for the work of Ni et al. [this issue]was to improve the radiative computations for snowmelt models, an important problem for properly parameterizingalbedo in weather forecastand climate models[seeBettsand Ball, this issue],as well as in initiatingphotosynthesis in carbonmodels[seeFrolkinget al., 1996].Consistentwith this motivationthey test their model by comparing its estimates of downwelling solar flux through the canopy profile to measured values. These comparisons showtheir modified model to agreewell with measuredvalues; in addition, they show that the vertical distribution of radiation is quite different than would be predictedby Beer's law, pointing out the importanceof proper representationsof tree morphologyto both photosynthesisand snowmeltmodels. 6,4.3. Landscape-scaleland cover and biophysical characteristics algorithms. A number of BOREAS investigations developedand producedmultiyear site-level,studyarea level, and regional-scalemaps of radiation and biophysicalparameters. This parameter set was defined by joint efforts between the BOREAS modelinggroup and the RSS investigators,identifying those land cover classesand radiation parametersconsideredto be essentialas inputsto the processmodelsand also spectrallyseparableusing remote sensingmultispectral,multidate reflectancemeasurements(see Figure 17). Hall el al. [this issue]developeda physicallybasedapproach to integratedland cover classificationand biophysicalparameter estimationusing LandsatThematic Mapper (TM) data. They utilized geometric RT models to compute the expected spectralsignaturesof the land coverclassesas a function of the distribution of biophysicalparameterswithin each class.The algorithm does not depend on unrealisticstatisticalassumptions, such as multiw•riatc normal distributions,to represent classsignatures.Further, it can compute the signaturesas a function of changing view and illumination conditions and other w•riablesthat give rise to global variations in the signatures, thus rendering the signaturesmore invariant to changing global conditions. Using this algorithm, they classify the BOREAS SSA and estimatethe proportionsof the land cover classesused in modeling. In addition, they estimate biomass density(BMD) for black sprucein the studyarea. BMD is an importantparameterfor ecosystemmodelssinceaboveground carbon is related to AN}'}' and litter fall [Gower et al., this issue].BMD is usuallyrelated to maintenancerespiration,and in conifer wetlands, BMD may be related to below-ground drainage and age class. As Harden et al. [this issue] show, drainage and age classcorrespondto varying annual carbon storage rates. The Hall et al. [this issue] map for wetland conifers in the SSA showsBMD to be bimodally distributed  28,757  BOREAS studyregion at 1 km resolutionusingan algorithm developedby Lovelandet al. [1991] (Plate 2a). The algorithm utilized monthlycompositesof normalizeddifferencevegetation index(NDVI) observations acquiredby the advancedvery high resolutionradiometers(AVHRR) carriedon the NOAA meteorologicalsatellites.The resultingland coverproductwas evaluatedby comparisonto the Hall et al. [thisissue]30-m TM land cover product to investigatethe effects of scale on land coveridentification.The Steyaert et al. [thisissue]map showed that wetland conifer dominatesthe BOREAS study region. Wetland conifer is an important landscapeelement in carbon sequestration[Goweret al., this issue;Hardenet al., this issue]. Steyaertet al. [this issue]also discoveredanother result key to understandingthe regionalcarbonflux; the boreal ecosystemis more fire prone than previouslybelieved,with nearly 30% of the area having been cleared by fire in the last 25 years. Ransonet al. [thisissue]report on the resultsof usingSIR-C, the X-band syntheticaperture radar carried aboard the NASA DC-8, and LandsatTM to classifythe BOREAS SSA and map abovegroundwoody biomass.Plot-level measurementsof leaf, branch,bole size,and angledistributionswere usedto develop relationshipsbetweenbackscattercoefficientsin variousbands, then the relationshipswere applied to imagery covering the -2 SSA. Pixel-levelBMD accuracieswere estimatedat 1.6 kg m  overa rangeof biomass from0 to 15 kgm 2. Thistechnique could be a valuable mappingtechniquefor the boreal ecosystem which is often obscuredby cloudsor smoke. Liet al. [this issue(b)] usedthe 3.0-/•m channelof AVHRR single-date images to detect fire events during 1994 in the BOREAS region. This work complementsthe historical firescarmapsof St•yaertet al. [this issue].The Li et al. [this issue (b)] 1994fire map showsthat 99 firesoccurredin the BOREAS region during the summer of 1994, consuming about 20,000  km•, or about2% of thetotalBOREASregion.Their accuracy assessmentshowed that they detected 87% of the grounddetectedfires and, in addition, identified many fires missedby conventionalmethods.Compositeddata were not effective in fire detection, which limits the usefulnessof this technique when smokefrom fire and cloudsobscurethe region. Cihlar et al. [this issue]developedand applied an AVHRR preproccssingalgorithm to the NOAA 11 1994 growingseason AVHRR record to compute 10-daycompositesof surface reficctance  in bands 1 and 2 and emitrance  in band 4 based on  data with improvedcalibration,registration,and correctionfor atmosphericand angular reflectanceeffects. The composited data were  also screened  for cloud  and snow contamination  using the multidate algorithm developedby Cihlar et al. [this issue]. Clear-sky values in bands 1 and 2 were corrected for fixed and assumed densities of molecular, aerosols, and water  vapor absorption and scattering. Band 4 was corrected for atmosphericwater vapor effects to generate surface temperatures usingsplit-windowtechniques[Collet al., 1994]. Where no clear-skypixelsexistedin a 10-dayperiod during the growing season,replacementpixelsfor bands1 and 2 were substituted from adjacentcompositedperiodsusinglinear interpolation. Because the composited data set consists of observationsat multiple view angles,Cihlar et al. [this issue] also apply land-cover-dependentcorrectionsto bands 1 and 2 withthelowermode(1-6 kgm •) consisting of poorlydrained for view-anglevariations in surface reflectance.The preproareassuchas bogsand fens and the upper mode (8 to 10 kg cessed,composited data set was then evaluated by using TM m 2) consisting of moderately to betterdrainedareaswith a data and further processedusingalgorithmsdevelopedat the slightlylower water table. CanadianCentre for Remote Sensing(CCRS) to producereSteyaertet al. [this issue] mapped the 1000 x 1000 km gionalmapsof seasonalmultidate NDV1, surfacetemperature,  28,758  SELLERS ET AL.: BOREAS IN 1997  LAI, greenFPAR, PAR, albedo,and APAR (absorbedPAR) over the 1000 x 1000 km BOREAS studyregion. These parameter maps are being used in various processstudiesbut need further validation and comparisonwith other products (see also Plate 2b). Two papersdeal with the combineduse of satellitethermal and optical data to infer certain thermodynamicpropertiesof the vegetatedland surfaceand environmentalvariablesin the atmosphericboundarylayer. Mahrt et al. [this issue]examine the use of radiometrictemperatureto infer aerodynamictemperature, a key variable in sensibleheat transfer between the surface and the atmosphere.In essence,their work confirms similar research done in other ecosystems[e.g., l/ining and Blad, 1992;Hall et al., 1992], in that radiometrictemperature cannot be used to accuratelyinfer aerodynamictemperature for sensibleheat computation when using Monin-Obukhov similaritytheory. In the boreal environmentthe warm sphagnum mossbackgroundcontributessignificantlyto radiometric temperaturebut insignificantly to sensibleheat transfer(aerodynamictemperature).They show,however,that the difference in thesetwo temperaturesdecreaseswith red reflectance correspondingto densercanopiesand, subsequently, low visible backgroundfraction.Czajkowskiet al. [thisissue]usedthe AVHRR surfaceNDVI versusradiometrictemperaturein an attempt to infer surfaceair temperature.They found that most of the air temperaturevaluesinferred in thiswayfell within _+5 K of ground-measuredtemperatures,but there were observed differencesas large as _+15K. They concludethat there is a consistentbias of 3.2 K causedby errors in the split-window estimatesof surface temperaturesand subpixelclouds and standingwater.The main advantageof thisapproachwouldbe more explicit spatial information on surfaceair-temperature distribution;however, the frequent presenceof clouds and smokein the regionlimits opportunitiesfor applyingthistech-  menting model predictionswith direct measuresof the water statewill be a valuableadjunctto landscape-scale carbonflux  prediction.From ground measures,Way et al. [this issue] showedthat transitionsin soiland stemthawingrelate directly to the stateof soil respirationand canopyphotosynthesis, respectively.Using ERS-1 satellite radar images,they showed that shiftsin backscatteringcrosssectioncorrelatewell to soil thaw and possiblyto canopythaw as two independenttransitions (Plate 2c). 6.4.4. Radiationand atmosphericeffects. Five papersfocuson radiativecomponentsof the surfaceenergybalanceand  the effectsof atmosphericaerosolson downwellingand upwelling radiation retrieved from satellite observations.The net radiation R,• absorbedby the surfacethrough radiative exchangewith the atmosphereis a criticalparameterin carbon, water, and energycycling.To infer R,•, the componentsof the surface shortwaveand longwave radiation balance must be quantified.  Rn = S{1 - a} + Lwd- Lwu,  (1)  whereS isthe downwelling shortwave radiationenergyflux(W  m-2), a is the albedo,andL,•d andL,•u aretheincidentand emittedlongwave radiationat the surface(W m-2), respectively. In addition to S the downwellingphotosynthetically activeradiationflux (PAR) is alsoa key variablein the surface energybalanceof vegetativesurfacessinceit regulatesphotosynthesisand the stomatalcontrol of evapotranspiration. Gu and Smith [this issue] describe the modification and evaluationof a publishedalgorithmfor inferring S and PAR fluxesfrom GOES 7 imagery.Their mapswere generatedfor the entire BOREAS studyregion at 30-min intervalsand at 1 km resolution.Their modifiedalgorithmincorporatesa radiative transfer model to estimate aerosol transmittance  and re-  flectanceand accountsfor columnwatervapor.Satelliteresults nique. were compared with measurementsacquired in situ from The final two papers in this sectiondeal with the remote ground-basedradiometersand agreedto within about 2 to 7% sensingof snowand soil temperatures,two importantvariables for S and PAR fluxesrespectively,with a relativeprecisionof that other BOREAS studieshave shown stronglyinfluence around20% over clear, cloudy,and smokydays. interannualvariationsin carbon uptake by wetland conifers Kaminskyand Dubayah[thisissue]explorethe relationship [e.g., Frolkinget al., 1996]. Timing of snowmeltis critical in betweenR,• and surfaceshortwavefluxesusing1 year of 15determining the course of surface albedo, subsequentsoil min surface flux data from nine BOREAS surface stations to thawing,and the initiation of photosynthesis by wetlandconi- determinehow effectivelyR,• couldbe inferred from a knowlfer. Changet al. [thisissue]usebrightnesstemperaturediffer- edgeof the shortwavecomponentalone.Their analysisshows ences in the 18- and 37-GHz bands from microwave multiband that if the term S( 1 - a) in (1) wasknownprecisely,thenR,• radiometers  flown aboard the Twin Otter  to determine  snow  could be estimated with an rms error of from 18 to 36 W m -2.  water equivalent(SWE) and lake ice thickness.They evaluate two algorithmsthat differ primarilyin the parameterizationof meansnowgrainsize.In nonforestedareasthe two algorithm's SWE estimatesdiffer by from 4 to 10 mm from groundmeasuresfor a snowpackof about 40 mm SWE. In forestedareas the differencesare largerwith estimatedSWE valuesgenerally lower than ground observations.Overall, differencesin ob-  A multifrequencyradiometer, the TIROS N vertical sounder  ing SWE estimatesfor forest cover.  AVHRR  Completingthe pictureof the beginningof activephotosynthesis,Wayet al. [thisissue]exploitthe sensitivity of microwave backscatteringcrosssectionto dielectric differencesbetween frozenandliquid moistureto map soilandcanopyfreeze/thaw boundariesin the borealspringtime.Currently,soilandcanopy temperaturesare modeled as a function of soil and canopy thermalproperties,moisture,and air temperature.Thesevariablesare not easilyobtainedat the landscapescale;thusaug-  photosynthetically active radiation absorbedby the surface, APARsfc, a key variable in both surfaceenergybalanceand photosynthesis calculations.They comparetheir estimatesto  (TOVS), was used to obtain atmospherictemperatureand water vapor profiles over the FIFE site for input to atmosphericradiativetransfermodelsto estimateL,• d [Breonet al., 1990]. The satellite-derivedestimateswere compared to ground measuresof L,• d. Breon et al. [1990] estimatedthe error on a pixellevelto be about20 W m-2, underbothclear servcd and inferred SWE in the SSA with its denser forests are and cloudyconditions. larger than for the NSA, indicatingthe importanceof correctLiet al. [this issue (a)] apply an algorithm to process red and NIR  reflectance data to infer the amount of  flux tower measurements  and determine  their estimates to  havea biasof about10 W m-2. TheymapAPARs• for the 1000 x 1000 km BOREAS region and showthat surfacevariationsin cloudiness andsurfacealbedocausesignificant spatial variation in APAR,•c.  SELLERS ET AL.: BOREAS IN 1997  Miller and O'Neill [this issue] used the compact airborne spcctrographicimager (CASI) instrument to measure shortwave upwelling and downwellingirradiance at multiple altitudes  to examine  the effects  of airborne  smoke  from  forest  fires on downwelling radiation. They report on an episode where smoke from wildfires  reduced  surface insolation  at noon  from 800 to 680 W m 2 and PAR from 320 to 250 W m -2  They found also that smoke from distant fires, consistingprimarily of sulfate aerosols,had much higher single-scattering albedos  and lower  attenuation  coefficients  than aerosols  from  nearby fires from which the soot had not settled out. Markham et al. [thisissue]alsoverified the important impact of forest fire on atmosphericopacityfrom analysisof aerosol optical depth measurementsacquired by a network of solar radiometersscatteredthroughoutthe BOREAS region. They alsoreport on water vapor columnabundanceestimates.Their results show a dramatic interannual variation in atmospheric opacitydue to smoke.In 1994 and 1995 the numerousfires in  28,759  Of thesefactors,experiencewas probablythe mostsignificant in terms of augmentingefficiency. BOREAS is a large enterprise and therefore inevitably servesas a test case for what "big science"(i.e., large-scale, interdisciplinary,coordinated)projectscan and cannot do. So far, the indicationsare that asin FIFE and similarprojects,the sum will be much greater than the parts in terms of scientific dividends.A data set is being constructedwhich will link our understandingof small-scalebiologicalprocessesto zonal climate variations and the global carbon cycle. The multiscale designof the experimentwill permit rigoroustestingof the scaleintegrationtechniquesthat will be employedin the process. Most of the papers in this specialissuereport on data acquisitions,data analyses,or the resultsof preliminarymodeling studies.On the basisof previousexperience(see,for example, Sellersand Hall [1992] and Hall and Sellers[1995]), another couple of years will see integrated studiescarried out using multiple data sets from all the contributing disciplines in BOREAS. These follow-on studies are expected to directly addressthe goalsof BOREAS and are discussedin section7.5. However, even at this early stage in the analysisphase of the project, some important scientificfindingshave emerged. The papersin this specialissueand the synopses of chapter 6 are organized by scientific subdisciplineand spatial scale.  the BOREAS region reportedby Liet al. [this issue(b)] and Steyaertet al. [this issue]resultedin a much wider variation in aerosolopticaldepthsacrossthe regionthan in 1996where few fires were detected. Atmospheric optical depth varied widely over the area from as low as 0.02 (at 500 nm) under clear-sky conditionsto as much as 4.5 (at 500 nm) becauseof smoke. The scientific review in this final section reverts to the structure Data from Markham et al.'s [thisissue]measurementsare used outlined in the introductionand in Figure 1, namely, physical widely in atmosphericcorrection algorithms to correct satelclimatesystem,carbonand biogeochemistry, and ecology,with an lite-measured radiance to surface reflectance. Such corrections additionaldiscussion of progressmadein remotesensingscience. are critical for use in physicallybasedclassificationand biophysicalparameter retrieval algorithmsof the typesdeveloped 7.1. Physical Climate System by Hall el al. [this issue]. BOREAS results have already had a direct impact on climate modelingand numericalweather prediction(Figure 17). Observationsfrom the mesoscalemeteorologicalnetwork and 7. Scientific Summary and Future Research tower flux sites have provided new insight into the radiation Directions and energy budgets of the forest and how these affect the Planning for BOREAS was initiated in 1990. Starting in regional climate. 1993, the scicncc team and staff set in place a significant infrastructure  in the  Canadian  wilderness  and  mounted  10  major intensiveficld campaigns(IFCs): one in late 1993,five in 1994,and four in 1996.Thc campaignswere complementedby a monitoring program of data acquisitionwhich spannedthe period 1993-1996. Each IFC, and the BOREAS experimentas a whole,wasconductedaccordingto detailedexperimentplans which translatedscientificgoalsinto operationalrealities. 1994 saw the most intcnsc and wide-rangingactivitiesin the field, includingover 350 airborne scienceflights; 1995 was a year of analysis,assessment, and planning;and 1996 sawthe redeployment of many field teams to fill critical gaps in the 1994 data set.  When snow free, the BOREAS  coniferous sites were ob-  served to have the lowest growing season albedos for any vegetated surface that we know of, about 8%. The winter albedo  of the forest  was measured  to be much lower  than the  value used by a number of AGCMs. Most NWP models treat snow albedo effects over the forest in the same way as for grass-coveredor agriculturalsurfaceswhere the vegetationcan be completely covered by snow to give a very high surface reflectance.In fact, forestvegetationusuallyprojectsabovethe snow,and even bare deciduoustrees can present a very dark surface to the solar flux, which is also incident at a glancing anglefor much of the winter (Figure 7a) [Bettsand Ball, this issue].Shortwaveradiation is therefore efficientlyintercepted by the forest and largely convertedinto sensibleheat flux and outgoing longwave radiation during the winter. Omission of this effect in the European NWP model, which carried a winter albedo of around 0.80 as opposedto a value of around 0.25 as observedin the field, resulted in the systematicunderestimation of near-surfacewinter air temperaturesby up to 15øC during the BOREAS 1996 winter field campaign(Figure 7b). Since then, the model has incorporated more representative winter albedovalueswhich hasresultedin great improvements in the predictionof the near-surfaceair temperature,reduced lower tropospherictemperaturebiases,and improvedforecast  In terms of the personneland resourcesinvolved,BOREAS is roughly3 times the sizeof the FIFE experiment[Sellerset al., 1992; Hall and Sellers,1995] and was also conductedover a much larger, sparselypopulated area. Operations often involved coordinated activitiesby over 200 people and up to 10 aircraft at a time over the two studyareas.The BOREAS data set is estimatedto be roughly5 timesthe sizeof the FIFE data set and is considerablymore complex.Likewise,the sciencein BOREAS spans a wider range of disciplinesthan in FIFE. However, the managementand staff head count for BOREAS is almostthe same as it was for FIFE, which was possibledue to the experienceof many of the veteran staffersand scientists scores over the North Pacific and North Atlantic. As of Deinvolved,improved procedures,and the use of better technol- cember 1996, the European NWP model included these ogyfor communications,data collection,and data compilation. changesin its operational forecasts,and there are plans to use  28,760  SELLERSET AL.: BOREASIN 1997 AtmosphericGCMs  Regional/Mesoscale ß Deep, Dry, Boundary Layers ß Reduced  Cloudiness  and  Precipitation  Global  Satellite/airborne  RSS  Surface, Upper A•r Meteorology  ß Surface condition ß Radiation ß Land cover  Radiation  Mesoscale  Regional  Local  Scale  Heat,H20,"[:fluxes ,•  ß Low Evaporative Fractions  ß Very Low Summer Albedos ß Low Winter  Albedos  Local Scale  H20J/'  Plot Scale  /'1• //'  Canopy physiology  ad•abon budget  Low Canopy Conductances Due To:  ß Frozen Roots in Spring  Plot Scale  ß Low PhotosyntheticRates in Summer  Figure17. Schematic summarizing gainsin thephysical climatesystem science areadueto BOREAS;see section 7.1.  satellitedata to track snow-relatedalbedodynamicsduringthe springand fall.  to 3000 m), and reducedcloudiness.In the EuropeanNWP model, the systematic overestimation of the latent heat flux  The low evapotranspiration ratesobservedat the BOREAS forest sites,particularlythe coniferouscovers,are not representedcorrectlyin mostatmospheric models.In the middleof  over the boreal forest resulted in overpredictionof precipitation and cloudinesswithin the region during the BOREAS  thegrowingseason, the depressed evapotranspiration ratesare  1994 growingseason[Sellerset al., 1995b].Again, BOREAS resultshavehad an immediateimpacton NWP modeling,and  due to low stomatal conductances associated with the low it is expectedthat the BOREAS datawill be extensively used in climatemodelsin photosynthetic ratesof thesespecies, whichare adaptedto to improvelandsurfaceparameterizations nutrient-poorenvironments. In the springthe veryhighsensi- the near future [Sellerxet al., 1997]. ble heat fluxes observed over the forest seem to be due to late  thawingof the soil, so the root systems remainfrozen and canopytranspirationis cut off. In wetlandareasthe forest canopyinterceptsand processes almostall of the available energy,so the wet soil or moss-covered surfaceplaysonly a minor role in the surfaceenergybalance.Interceptionand partitioningof mostof the radiationfluxby the forestcanopy thusdelaysthe warmingandthawingof the underlyingsoil,so the bulk of net radiation is shunted into sensible heat flux. Airborne flux measurements in BOREAS have also shown  how the thermal inertia of the lakesgreatlyreducestheir contribution to the surface heat fluxes. All these effects combine  to make the boreal forest,which includeslarge proportionsof lakes,wetlands,and fens, a surprisinglystrongsourceof sensible heat and a weak source of latent heat, comparedwith  temperategrassland sites[Sellers and Hall, 1992]and tropical forests[Shuttleworth et al., 1984a,b]. This situationoften resultsin the generationof a very dry warm lower troposphere with a deepandturbulentatmospheric boundarylayeroverthe forest during the growingseason,more typical of a lowerlatitude arid zone than would be expectedfor a high-latitude biome suppliedwith plentiful water. These phenomenawere directlyapparentto flux aircraftcrewsin BOREAS 1994who, in makingthe transitfrom the agriculturalregionnear Prince Albert northward into the forest, often encounteredgreatly increasedturbulence,deeper boundarylayers(sometimesup  The flux aircraftprovideda mesoscale pictureof the surfaceatmospherefluxeswhich complementsthat providedby the tower flux measurements.In general, aircraft-to-aircraftand aircraft-to-towercomparisons were very encouragingand reinforcethe perceptionthatthe regiongivesoff muchlesslatent heatthanpreviously thought.As in otherfieldexperiments, the flux aircraft seemto systematically underestimatethe surface fluxes,most likely becauseof problemswith capturingvery  longwavelength turbulentstructures; seeDe,sjardim • e! al. [this issue]and paperscited by Selle•:¾ and tIall [1992]. AircraftmeasuredCO: andwatervaporfluxeswere foundto be closely relatedto spectralvegetationindices[Ogtmjemiyo et al., this issue],which concurswith previousfindingsfor grasslands [Cihlaret al., 1992]andwith theory[Selle•:• e! al., 1992b].This latest result for boreal forest holds considerablepromise for future modelingwork. Lakes were seen to act as important  playersin the regionalenergybalance,and more attention needsto be paid to their role in the regionalclimate.It seems that the lakes act as large storesfor heat and give off little sensibleheat flux duringthe growingseason.They may alsobe associatedwith generatingmesoscalecirculationsduring periods of weak synopticflow. A number of BOREAS investigationsdeal with the problemsof modelingsnowinterceptionand snowmeltin the forest; the resultsof these will doubtlessimprove the realism of winter-springtransitionsas modeledin AGCMs. Also, satcl-  SELLERS ET AL.: BOREAS IN 1997  28,761  Regional Estimates  Regional Carbon Budget  RegionalCO2, RSS  Vegetation  From Aircraft  ß Correlationof AirborneCO2  Classification  and Vegetation Properties  Fluxes With RSS Data  From Satellite  ß Classification, Fire Extent  Data  ß Interannual Variability Flux Measurements  Drivers of Photosynthesis and Respiration  Integrated Models  ß PAR  ß Temperature  CO2  Flux Tower Scale Integrations  ß Water  Balance  ß Ecosystem State  ß Carbon SequestrationRates for Different Cover Types ß Weather and Interannual Variations in Carbon Fluxes  ß Scaling-Up Studies  Process Studies  Canopy physiology Soil/Moss/Stem Respiration Trace Gas Exchanges Litterfall  Models  Chambers, etc.  RootSystemDynamics  Carbon Cycle Components ß Low PhotosyntheticRates  ß Canopy Profilesof PAR, N, Photosynthesis,Conductance  ß Controlson Soil, CO2, CH4 Emissions  Slow Carbon Cycle  Figure 18. Schematicsummarizinggainsin the carboncyclesciencearea due to BOREAS; seesection7.2.  lite-borne radar has been shownto provide usefulinformation about these transitionswhich are important for both climate and carbon models. Correct representationof soil moisture and surfacehydrologicalprocessesin AGCMs and regional hydrologicalmodelsis obviouslycrucial, and it has been observedthat small variationsin the hydrologyof this biome can have significanteffectson the carbonand trace gas budgets. The BOREAS hydrologicalstudieswere well integratedwith tower flux measurementsand have been used as an independent checkon the growingseasonlatent heat fluxesas well as furthering our understandingof soil moisture controls on evapotranspiration. The radiation budget of the forest can be estimatedwith good accuracyfrom satelliteobservationsprocessedby algorithmswhichwere refined usingBOREAS data. However, the  been used to partially validate global data setsgeneratedfor globalmodels[Meesonet al., 1995;Sellerset al., 1996b]. 7.2.  Carbon and Biogeochemistry  The BOREAS field program has provided us with a comprehensivedata set covering almost all componentsof the plant and soil physiologicalcontrols related to the energy, water, and carboncyclesin the region (Figure 18). BOREAS resultshave alreadyshedconsiderablelight on someaspectsof the missingglobal carbonproblem. Could the boreal ecosystem with its low productivitiesbe sequesteringa significant  portionof the 1-3 Gt C yr-• whichis unaccounted for in the annual global carbon budget? The deciduousand coniferousforests of the boreal ecosys-  temoccupy12-20millionkm2, depending on thedatasources  used[seeWhittaker,1975;DeFriesand Townshend, 1994;Sellers shown to be more difficult than first thought; the radiometer et al., 1996b].Dependingon whichestimateof the borealforest intercomparison effort of HodgesandSmith[thisissue]showed area we use, the globalboreal forest need only sequesterbethat a commonlyused instrument systematicallyunderesti- tween50 and80 g C m-2 yr-1 to explaina 1 Gt C yr- • global mates the net radiation flux as its transparent dome cover sink,and this doesnot includecarbonuptake by high northern deterioratesover time (see also Smith et al. [1992]). Surface latitude tundra, which has been implicatedas a carbonsink by measurements have shown that smoke and associated aerosols Myneni et al. [1997]. What do the BOREAS plot-level carbonflux measurements are importantfactorsin reducingthe incidentsolarflux during the growingseason.The large-scalevegetationclassification of canopyand soilsindicatewith respectto theseflux magniproductsgeneratedfrom satellitedata will alsobe essentialto tudes? Measured annual NEE values at the BOREAS tower correctlydefine the contributionsof different surfacetypesto sitesrangefrom 130g C m-2 yr-• (a moderatesink)in the the regionalfluxesof heat, water, momentum,as well as car- southernold aspen(SSA-OA) site[Blacket al., 1996]to a weak bon, discussed below.The BOREAS albedo data have already carbonsourceof 50 g C m-2 yr-1 at the northernold black in situ measurement  of net radiation  fluxes over the forest was  28,762  SELLERS ET AL.: BOREAS IN 1997  to 50g C m-2 yr- • overthelastcentury. Modelsimulations for  The trace gas studiesin BOREAS showedthat drier sites (aspen,jack pine) with fast nitrogencyclingin the soilscould act as strongsinksfor CH4, while many of the wetter siteswere strongsources,particularlythe beaver ponds and fens. Some  conifer wetlandsfor the years 1968 to 1989 yield a calculated  intermediate  spruce(NSA-OBS) site for 1996 [Gouldenet al., this issue].A numberof chambermeasurements[Trumboreand Harden,this issue]showthat the boreal wetland peatshave accumulatedup  sites switched from source to sink status based on  variationin carbonfluxfroma loss(source) of 50g C m-2 yr- • variationsin the water table. Surprisingly,chamber measureto a gain(sink)of -140 g C m-2yr-• [Frolking etal.,1996].His ments made at fen siteswere found to scaleup well to match 1994modelvalueof -51 g C m-2yr -• for theNSA-OBSsite eddy correlation measurementsof CH 4. CO fluxes from a agreeswell with the measuredvalue of Gouldenet al. [this issue]. Thus although the boreal forest is characterizedby low  aboveground productivity of between 50 and100g C m-2 yr- • [e.g.,Goweret al., this issue],equal or greaterannualrates of fine root productioncombinedwith long carbonturnoverrates of 500 years [Trumboreand Harden, this issue] in the cold anaerobic soils of the boreal forest wetlands appear to be sufficientto explain a significantsink of global carbon when extrapolatedover the large area occupiedby the circumpolar boreal ecosystem. It shouldbe emphasizedthat at this early stagein the analysis,inferencesof boreal carbon flux at the global scalefrom the BOREAS sitescan only serveto showthat we cannotrule out the boreal ecosystemas an important global carbon sink. Clearly, the dynamicsand strength of carbon sequestration differ by land cover type, climate, soils,topography,and many other factors that are more aptly captured using satellite extrapolations combined with ecosystemprocessmodels. For example,small chambermeasurementshighlightedthe large spatialvariabilityin soilandmosscarboneffluxeswhichappear to be highly dependent on soil moisture content and litter quality. Beaver pondswere observedto be very strongsources of CO2. The observedlags between carbon uptake by photosynthesis,which is confinedto the short growingseason,and carbonlossby respiration,whichcontinuesthroughoutmostof the year, emphasizethe need to correctlymodel the different controls on carbon sinks and sourcesin large scale models. Cihlar et al. [thisissue]usedsatellitedata analysesto showthat fires burned over 3% of the BOREAS region during 1994. Steyaertet al. [this issue] show that nearly one third of the BOREAS studyarea hasbeenburnedwithin the last 25 years. Carbon  contributions  from  fire disturbance  must be factored  into global estimates.A more reliable estimateof the magnitude of the global sinkwill be made after scalingstudiesin the BOREAS follow-on program has been completed. By partially elucidating the mechanismsunderlying atmosphere-biospherecarbon exchange,BOREAS tower flux and chamber measurementsare also leading to a better understandingof the magnitude,trends,and interannualvariationin  range of fire siteswere measured,but more work needsto be done before an estimateof regional CO fluxescan be made. The meteorologicaltrendsfor 1961-1990 analyzedby Chapman and Walsh[1993] showthat air temperatureincreasesare most rapid over high-latitudecontinentalinteriors,with documented temperature trends as high as 1.25øC per decade. These  observed  trends are also consistent  with AGCM  simu-  lations showing high-latitude seasonal air temperature increaseswith increasingatmosphericCO2 concentrationsand inferencesaboutthe extensionof the boreal growingseasonby about six days over the last 15 years (see also Keelinget al. [1996] and Myneni et al. [1997]). If the increasedwinter and springtemperaturesare coupledwith relatively cool summers, as it would appear, global warming could initially lead to increasedproductivity,hence increasedcarbon sequestrationat these latitudes. However, BOREAS soil carbon flux data show  that longer warmer growingseasonscould also lead to more rapid loss of older deeper soil carbon stores which would partially offset gainsfrom productivityincrease. The biometric studiesshowedthat the abovegroundnet primary productivity(ANPP) of the boreal forest is relatively small and the abovegroundcarbon stocksare low compared with temperate and tropical ecosystems.However, large amounts of carbon are stored in wetlands, and these stores are  sensitiveto climate change.With regard to the annual carbon balance,we are presentedwith a picture of an ecosystemoperatingvery closeto the marginsof carbonprofit and loss,with growingseasonlength and summertimetemperaturesperhaps being the deciding factors. Sequestrationof carbon by any ecosystemis a nonequilibrium process.In the longer term, changesin fire frequency,climate regime and vegetationsuccessionpatterns, and warming and drying of boreal soilswith associatedincreasesin respirationcould greatly alter or even reverse  the  "boreal  carbon  sink"  scenario  described  above.  Follow-on analyses of ecological data collected during BOREAS will provide valuable insight into the mechanisms driving future changes. 7.3.  Ecology  The future courseof carbon dynamicsin the boreal zone is  globalcarbonflux.For example,measurements andmodeling thought to depend on variationsin the physicalclimate in the have shownthat interannual variation in the timing of snowmelt is a major factor in determining annual carbon uptake rate. BOREAS measurementshave shownthat carbonuptake does not begin until the snowmelts and the root zone thaws. Other measurementsshowthat carbon respirationlossis not appreciablebelow about 17øC;thus immediatelyafter snowmelt and soil thaw, cool air temperaturesare conduciveto rapid carbon uptake. However, as the soil warms up and air temperaturesrise above 17øC,soil respirationis enhancedand the net ecosystemcarbon lossincreasesrapidly. These observations are supported by BOREAS modeling studieswhich indicate that years with early springsand cool, wet summers are associated with strongerannualcarbonuptakerates [Frolking et al., 1996].  near term and on ecologicalresponsesin the midterm and long term. Will warming drive the boreal ecosystemto a higher productivity state, populated by plant communitiesthat can sequesterhigherlevelsof carbonthan the presentborealcommunities, or will a new low-productivity equilibrium be achievedlimited by the paucity of the boreal soils?Paleoclimate studiesrevealthat this ecosystem migratedto muchmore southerlylatitudesduring the last glaciation,then retreated to its present northerly position as the ice receded. Will the anticipatedrapid warming induce ecologicalinstabilities,resulting in rapid carbonlossfrom the great reservesof carbonin the largely anaerobicconifer wetlands before other plant forms caneither fix the carbonin their livingbiomassor act asagents in stabilizingthe soil? To understandthese longer-term dy-  SELLERS ET AL.: BOREAS IN 1997  28,763  Satellite  RSS  AVHRR,  ES  LANDSAT, SPOT  Regional/Mesoscale  SSM/I, (DMSP),  ERS-1  Biophysical Parameters VegetationClassification Fire Extent, Frequency Regional  Freeze/Thaw  ,•  i  ....i  ß  !  -•  Scale  Models  Intercomparisons Airborne Optical and Microwave  Mesoscale/Local  (Activeand Passive)RSS  Scale  :•:•'..' ........ •:•.'.*.'•:•i• •.:....  BiophysicalParameters VegetationClassification MultiangleTechniques  Study Area  Scale  RT Models  ,• • In Situ/Plot  In-Situ Ra. diometr[c and  Scale  ß BiophysicalParameters (LAI, FPAR....) ß Reflectances, emittances  Local  Scale  ß In Situ Verification  Figure19. Schematic summarizing gainsin ecology andremotesensing science dueto BOREAS;see sections 7.3 and 7.4.  effecton the namics,a numberof questionsmustbe addressed. Where is inganddryingclimate,couldhavean enormous the carbonbeingscquestered: in the canopy,litter or below carbonand water budgetsof the region.This speculationis groundbyfinerootturnover? Whatis the relativeimportance supportedby the tower flux and flux aircraftmeasurements of the variousboreallandcovertypesin thisprocess? How will carriedout at largerspatialscales,both of whichreconfirmed andphotosynthesis the source/sink strengthdependon climate,particularlygrow- the tightlinksbetweenevapotranspiration ingseason length,andwill respiration beginto dominateover during the growingseason.The flux aircraft observations carbonupphotosynthesis in response to globalwarming;renderingthe showedthat mesoscalepatternsof photosynthetic ecosystem a net sourcerather than a net sink of carbon'? takewerecloselyrelatedto spectralvegetationindexinformaWouldincreasing firefrequency leadto increased or decreased tion, which can also be used to estimate large-scalecanopy The highesttranspirationand carbonuptake carbonsinkstrengths overall?DatacollectedduringBOREAS conductances. rates were observed over the deciduous stands near the southwill be usedto addressthesequestions. by the The ecophysiological studiesin BOREASclarifiedmanyof ern edgeof the biome,whichwere alsocharacterized the linksbetweenvegetationtype and stateand the carbon, highestspectralvegetationindexvalues.The towerfluxmea-  energyandwaterbudgets of the region,whilebiometric and surementsshoweda wide variation in carbon fluxesamong the  remotesensing studieshavelaid the groundwork for carrying differentcovertypesstudiedin BOREAS, but mostof them out regionalsurveys of ecosystem stateusingremotesensing seemed to be weak sinks on an annual basis.Given that up to data(Figure19).The ecophysiological workhasbeencovered 3% of the boreal forest is consumedby natural fire eachyear, for an equilibrium borealcarbonbudalready,so thisdiscussion is limitedto a reviewof progress it is entirelyreasonable of weaksinkscoveringmostof the regioncounmadein understanding ecological processes on relativelylong getto consist teredby catastrophic fire eventswithinrelativelysmallareas, timescalesand over large spatialscales. The leaf-scaleandcanopy-scale measurements conducted in their recoveringto a strongersinkas newgrowthdevelops.It BOREASpresentuswitha consistent pictureof an ecosystem should also be borne in mind that in a warming and drying wouldchangealongwith physiological operating at a relatively lowlevel:thedeciduous canopies were climate,fire frequency Observedto have much lower photosynthetic capacitiesthan  theirtemperatecounterparts, andtheconiferous species were observedtOhaveevenlowercapacities, abouthalf thoseof the  and specieschanges. 7.4. Remote SensingScience  Remote sensing research and development during deciduous sl•ecies. Clearly,large-scale replacement of the cothe nifersbydeciduous species, whichislikelyto occurin a warm- BOREAS producedresultsthat are key to investigating  28,764  SELLERS ET AL.' BOREAS IN 1997  soil and bole thaw, also critical in initiating photosynthesis in the models.Remote sensingalsoshowedthat the role of fire in Landcover Type Canopy Physiology the boreal ecosystemis evenmore importantthan was known Soil Processes Fire Frequency BiophysicalAttributes Scaling Methods previously. Steyaertet al. [thisissue]usedAVHRR visibleand LAI, Biomass Surface-ABL Models Albedo near-infrared data to show that about one third of the study Canopy Conductance/Photosynthesis Estimates regionhasundergonefire over the last 25 years,a much larger proportionthan previouslybelieved;Cihlar et al. [this issue], usingAVHRR data showedthat 3% of the ecosystemburned during 1994 alone. Importantly,both of theseefforts showed Improved Carbon-Energy-Water Regional Fields for that AVHRR data canbe usedto reliablymap the major land Submodels for Global Models Global Models covertypes,making it possiblefor a careful assessment of the Radiative Transfer AlbedoSurfaceConductance, zo global boreal biome. Myneni et al. [1997] showed how the Momentum Transfer Carbon Sink/SourceStrength Heat, Water Exchange Land Use/Land Cover Change 15-yearAVHRR record can be used to monitor the response Carbon Cycle of the biome to climate warming. Finally, remote sensingwas usedduring BOREAS to developseasonallyvarying,regionalscaleparametermapsfor all the major drivingvariablesgoverningcarbon/waterand energyprocesses(Figure 16). These Global Change maps should prove to be invaluablein scalingmodels and • Validation/Forcing Validation/Forcing Model .,. hypotheses from the plot level to the region. , AGCM ! Satellite Retrievals Meteorological i!!!!! Process  Satellite Algorithms  Studies  ! ,  !  Analyses  ,'  t  ,  ,  ,  7.5. I --Freeze/Thaw Radiation Fluxes  i  Surface Model II!!li -Snow Cover  Calculated Time-Series Fields  Comparisons  PhysicalClimate System Parameters  Isotopic Analyses GlobalTracerModels  : ,  ,  ,,  Carbon  Fluxes  Figure 20. Schematic showing how different elements of BOREAS sciencecouldcombineto improvethe performance and realism of global change models; see section 7.5. The resultsof the BOREAS processstudieshaveled to better land surfaceparameterizationsfor AGCMs and alsoimprovedcarbon cyclesubmodels;see two boxesin top left of the figure. The remote sensingscienceand ecologicalresearchwill lead to better classificationschemesand more accuratespecification of biophysicalparametersover the entire boreal zone;seetwo boxes in top right of figure. The combinationof improved models,globalparameterfields,and large-scalevalidationdata sets will result in greatly enhanced simulationsof physical climatesystemfields(temperature,precipitation,etc.) and carbon cycleflows for the biome. These spatiallyresolvedfields can be area integratedand comparedto zonal-scaleinferences aboutthe borealcarboncycleprovidedby atmosphericisotopic analysesand CO2 tracer models;see bottom half of figure. In this way we hope to bridge the gap that currentlyexistsbetween our understandingof small-scalephysiological processes and global-scaleinferencesabout the carbon cycle.  role of the boreal ecosystemin global change(Figure 19). Considerableprogresswas made castingremote sensingalgorithmsinto a more physicallybasedframework[e.g.,Hall et al., this issue],progressthat shouldultimately result in more robust global classificationand biophysicalparametermapping techniques.The improvedradiativetransfermodelsdeveloped during BOREAS should also provide more reliable components in numericalsnowmeltmodels,a key to forecastingthe effects of climate warming on the timing of snowmelt,which we saw was crucial to determining interannual variation and longer-termtrendsin carbonsequestration.In addition, Wayet al. [thisissue]demonstratedthat radar can be usedto monitor  Future  Research  Directions  Over the next few years,BOREAS data setswill be usedto improve many of the critical processsubmodelsneeded for land-atmospheremodels and carbon balance calculations.In particular, the BOREAS follow-on program will encompass effortsto improveradiativetransfermodels,leaf- and canopyscale photosynthesis-conductance models, canopy, root and soil respirationmodels,soil moistureand surfacehydrological schemes,and surface-atmosphere turbulent transfer models. The integrated nature of the BOREAS data setswill permit rigoroustesting of the performanceof these processmodels, togetherand in isolationfrom eachother, over the diurnal and seasonalcycles(Figure 20). With respectto weather and climatemodels,a criticalshortterm task is to applythe BOREAS data set to further improve numerical weather forecasting by improving land surfaceatmospheretransfer codes.Tests of coupledphotosynthesisconductancemodels against BOREAS data have indicated that theseschemesare robustandwill lead to improvedsurface evapotranspirationestimatesfor the AGCMs. Incorporation of better albedovalueshasalreadyimprovedthe performance of a leading operationalforecastmodel; further performance improvementsare expectedwhen satellite-basedestimatesof albedo are incorporated,permitting these modelsto respond seasonallyto the effects of snow and vegetation phenology. BOREAS provided new data on the effects of smoke and associatedatmosphericaerosolson the surfaceradiation balance.Theseeffectswere more widespreadand severeover the biome than was previously thought (see also Sellerset al. [1995b]).GCM radiationcodesmayneedto includethiseffect. A final area of improvementin weather and climate forecasting shouldresult from incorporatingimprovedparameterizations of boundary layer dynamicsutilizing BOREAS radiosonde and aircraft  measurements.  In carbonmodelsthe long-term trends and interannualvariability in carbondynamicsfor the biome appear to be driven primarily by the timing of snowmeltand the summerair temperatures. Incorporation of improved snowmelt models into regional and global carbonbalancemodelsshouldhelp us to understandthesedynamicsand thusbetter simulatethe annual atmosphericCO2 signal. In terms of growingseasoncarbon fluxesthe photosynthesis modelsseem to be realistic,but it is essentialto improve the respirationmodels not only for the  SELLERS ET AL.: BOREAS  abovegroundcomponentsbut also for the roots, litter, and soils.Finally, the effectsof fire and disturbanceare important to understand.Scalingstudiesinvolvingpoint models and remote sensingwill be key to gagingthe effectsof disturbanceat the regional scale. BOREAS has catalyzedseveraladvancesin remote sensing algorithmdevelopmentwhichnowenableusto monitorboreal vegetationby type and stateand to track changesthat may be due to fire, direct human activity,or climate change.Algorithm developments due to FIFE and BOREAS havealreadyled to the productionof AVHRR-derived globalvegetationmapsfor the years 1987 and 1988 [seeMeesonet al., 1995;Sellerset al., 1996a,b, 1997;Randall et al., 1996]. These pilot productswill be expandedto cover the 15-year AVHRR record and will allow usto developtime seriesfieldsof land cover,biophysical parameters,phenology,and snowcover.All thesecanbe com-  pared with the physicalclimate record and to seasonaland interannual variations in atmospheric CO2 concentration. AVHRR data will also be used to monitor changesin the fire disturbanceregime over the same period. The radiometric qualityof the AVHRR data serieswill haveto be enhancedto meet thesetasks;this requiresthe developmentof techniques for improvinglong-term calibration and atmosphericcorrection of the data. The MODIS/MISR  and other sensors to be  launched aboard the EOS-AM platform in mid-1998 should provide significantadditionalcapabilityfor monitoringland vegetation and should be merged with the A VHbtbt data streamto producea seamlessmonitoringdata set into the 21st century.Finally, the useof radar satellitessuchas ERS-1 and JERS-1 will be used to monitor the interannual variability in the freeze-thawboundaryin the boreal ecosystem,shownto be a keyfactorin the interannualvariabilityof the carbonflux.To take advantageof the different attributesof optical and radar sensors,further remote sensingresearchand developmentis required;in particular,data fusionalgorithms,whichcombine optical and microwavesensorsas well as other data suchas topographicdata, couldbc developedto providericher information  about the biomc.  Enhancementof land-atmosphereprocessmodelsand largescaleparameter quantificationusingsatellite data would be considerable  achievcmcnts  for BOREAS.  Success in these two  areas is almostcertain,basedon the early resultsand work in progress.However,the real prize for BOREAS would be to incorporatethe improvedprocessmodelsand remote sensing data setswithin large-scaleenergy-water-carbon modelsto calculate surface-atmosphere fluxes of these quantitiesfor the biome over the period of record of the earth-observingsatellites, sayfrom 1980to the presentday (see the bottom half of Figure 20). For this calculationthe surfacestate shouldbe constrainedby satellitedata,while the atmosphericconditions are specifiedfrom meteorologicalanalysesor via direct couplingwith an AGCM. Incorporatingthe role of fire in a carbon cyclemodel representsa real challenge,particularlywhen it comesto predictingchangesin fire frequency,areal extent,and intensityas a resultof climatechange;the satelliteand meteorologicaldata recordsof the last 25 years are essentialresourcesfor addressingthisproblem.Initially, it is very unlikely that theseintegrarivemodelswill correctlysimulatethe value or even the signof the net carbonflux for the biome, but they shouldbe capableof reproducinginterannualvariationsin the flux, superimposedon a relatively invariant bias. The time series of calculated  interannual  variations  in the net carbon  flux can then be comparedwith equivalentnumbersinferred  IN 1997  28,765  from global tracer and isotopicanalysesto shedlight on which processesare responsiblefor perturbationsin the terrestrial carbonbudgetand where, geographicallyand biologically,they operate. In this way we hope that BOREAS will help us to bridge the huge gap that currentlyexistsbetweenglobal-scale inferences about the changingterrestrial carbon budget and our local-scaleunderstandingof the controllingecophysiological processes.When this is done, we can see our way toward constructingusefulpredictivemodelsthat can anticipatefuture interactionsbetweenthe globalphysicalclimate systemand the carbon cycle. Notation  ABL atmosphericboundarylayer. AES AtmosphericEnvironment Services. AFM airborne fluxesand meteorology. AGCM atmosphericgeneral circulationmodel. ANPP above-groundnet primary productivity. APAR absorbedphotosyntheticactive radiation. AR entrainment parameter. ASAS advancedsolid-statearray spectroradiometer. AVHRR advancedvery high resolution radiometer.  BAHC  biosphericaspectsof the hydrological cycle. BMD biomassdensity. BOREAS Boreal Ecosystem-Atmosphere Study. BORIS BOREAS Information System. BP beaver pond. BRDF  bidirectional  reflectance  distribution  function.  CCRS DAAC ECMWF  EOS AM-1  ET FIFE FM FPAR GCM GCTE GEE GOES  Canada Centre for Remote Sensing. Data Analysisand Archiving Center. European Center for Medium-Range Weather Forecasting. Earth Observation System-AM-1 platform. evapotranspiration. First ISLSCP Field Experiment. frequencymodulation. fraction of PAR absorbedby the canopy. general circulation models. global changeand terrestrial ecosystems. grossecosystemexchange. GeostationaryOperational Environmental  Satellite.  GPS  Global PositioningSystem. HAPEX-Sahel HydrologicalAtmospheric Pilot Experiment-Sahel. HYD hydrology. 1FC intensivefield campaign. IGAC International Global Atmospheric Chemistry. IGBP International GeosphereBiosphere Program. ISLSCP  International  Satellite  Land Surface  Climatology Project. LA1  leaf area index.  LSP land surfaceparameterizations. MISR multiangle imaging spectroradiometer.  28,766  SELLERS ET AL.: BOREAS IN 1997  MODIS  moderate resolutionimaging spectrometer.  MVI NASA NCAR  multiband vegetationimager. National Aeronautics and Space  NIR  NMHC NOAA NSERC  normalized differencevegetationindex. net ecosystemexchange. near infrared.  nonmethanehydrocarbon. National Oceanographicand AtmosphericAdministration. Natural Sciencesand Engineering Council.  northern study area. numericalweather prediction. old aspen. old black spruce. old jack pine. photosyntheticallyactiveradiation. portable aparatusfor rapid acquisitionof observations  of land and  atmosphere. Polarization and Directionality of Earth Reflectances.  SART SIR-C SSA SWE TDR TE TF  Fitzsimmons  of Prince  Albert  National  Park  also  Darrel Williams, branch head of 923, NASA/GSFC, is also thanked for  R n surfacenet radiation. RSS remote sensingscience. RT  man and Michael  National Center for Atmospheric  bidirectional  POLDER  Sandra Schussel, D'Arcy Snell, Sarah Steele, John Stewart, Karl Spence,David Terroux, Gillian Traynor, and JasonVogel. Dave Dal-  provided much needed help to BOREAS. All are warmly thanked.  Research  NSA NWP OA OBS OJP PAR PARABOLA  Halliwell, John Martin, Joe Niederleitner, John Norman, Paul Rich,  Administration. Research.  NDVI NEE  thony Young, Barrie Atkinson, Mary Dalman, Tom Gower, David  radiative  transfer.  surface-atmosphereradiative transfer. shuttle imagingradir. southern study area. snowwater equivalent. time domain reflectrometry. terrestrial ecology. tower flux.  TGB TIROS-N TM TOVS VHF WCRP-GEWEX  trace gasbiogeochemistry. multi-frequencyradiometer. thematic mapper. multi-frequencyvertical sounder. very high frequency. World Climate ResearchProgramGlobal Energy and Water Cycle Experiment. YA young aspen. YJP youngjack pine.  maintaininghis tolerance,goodhumor, and goodsensethroughoutthe entire project.The researchaircraftcrewsand managementperformed their tasks with unflagging professionalismand patience. Special thanksgo to GeorgeAlger, Chris Jenison,and Richard Rose (C-130); Gary Shelton(ER-2); Willie Dykes,Charlie Walthall, CharlesSmith, Jeff Sigrist,Ed Melson, Pete Bradfield,and John Schaefer(helicopter); Larry Hill, LawrenceGray, Ted Senese(chieftain);ChrisScofield and Michele Vogt (DC-8); CharlesLivingstoneand Brian Spicer(CV580); Tom Carrol and Rob Posten (aerocommander);Paul SpyersDuran, Henry Boynton,andJerryTejcek (Electra);JohnAitken, Brian Bertrand, Murray Morgan, John Croll, and Charles Taylor (Twin Otter); George Bershinskyand Ernest Gasaway(Kingair); and Ed Dumas and Robert McMillen (for LongEZ). While these (generally unsung)heroesare specificallymentioned,we are fully aware that a tremendousjob was done by many other people under frequently trying conditions.In addition, the crews of the Saskatoon,Prince Albert, and Thompson Flight Service Stations and the staff of ECMWF and NMC who providedBOREAS with invaluableweather predictionproductsduring the field campaignsare warmly thanked. BOREAS benefited immenselyfrom a seriesof independentexternal reviewsconductedby the ScienceAdvisoryGroup (SAG): Mike Unsworth(chairman),Pat Matson, Tzvi Gal-Chen, Joe Landsberg,Jim Ritchie. BOREAS is deeply indebted to thesepeople. BOREAS contributes to both the U.S. and the Canadian Global Change Research Programs.For the United Statesthe effort is beingled by the National Aeronauticsand SpaceAdministrationMission to Planet Earth, with participationfrom the National Oceanic and AtmosphericAdministration, the National ScienceFoundation, the U.S. GeologicalSurvey, the U.S. Forest Service, and the Environmental Protection Agency. ParticipatingCanadian agenciesinclude the Canada Centre for Remote Sensing,Environment Canada, Natural Sciencesand Engineering Research Council, Agriculture and Agri-Food Canada, National ResearchCouncil,Heritage Canada(Parks),CanadianForestService, the Institute for Spaceand Terrestrial Science,and the Royal Society of Canada.The BOREAS project was overseenby programmanagers from the participatingagencieswho made up the BOREAS Coordinating Committee (BCC). The membershipof the BCC is Micheal Allen, Richard Asselin, Carmen Charette, Michael Coughlan, Bruce Hicks, Hank Margolis, Gordon Miller, Jarvis Moyers, Leo SaynWittgenstein,Mac Sinclair,Lowell Smith, Robert Stewart,John Stone, Jeffrey Watson, and Diane Wickland. BOREAS is an element of the International Satellite Land Surface Climatology Project (ISLSCP) whichis part of the World Climate ResearchProgram-GlobalEnergy and Water Cycle Experiment (WCRP-GEWEX). BOREAS is also contributing to three International Geosphere Biosphere Program (IGBP) core projects;BiosphericAspectsof the HydrologicalCycle (BAHC), Global Change and Terrestrial Ecosystem(GCTE), and International Global AtmosphericChemistry(IGAC). References  Acknowledgments. All the BOREAS scientistsand staff contributed to this paper in one way or another. Scientistswho contributed preliminarydata setsare identifiedin the text and figurecaptions.The BOREAS operationsgroupresponsiblefor the designand oversightof the field experimentconsistedof Piers Sellers,Forrest Hall, Dennis Baldocchi,JosephBerry,Andrew Black,JosefCihlar, Barry Goodison, Hank Margolis, Michael Apps, Bob Kelly, Gerry den Hartog, Tom Gower, Mike Ryan, Patrick Crill, Dennis Lettenmaier, and Jon Ranson.The BOREAS principalinvestigatorsare listed in Table 1; many of them provideddata andwritten contributionsfor thispaper.Almost all investigatorstook on project-relateddutiesin addition to their own researchtasksduring the field year. The BOREAS and BORIS staff membersare Valerie Corey, Shelaine Curd, Laura East, Carla Evans, Scott Goetz, Sara Golightley, Saera Haque, Dan Hodkinson, Fred Huemmrich, Fred Irani, David Knapp, David Landis, Elissa Levine, Beth McCowan, Blanche Meeson, John Metcalfe, Karen Mitchell, Amy Morrell, Theresa Mulhern, Alan Nelson, Ross Nelson, Jeffrey Newcomer, Jaime Nickeson,Paula Pacholek,Carey Noll, Don Rinker, Adam Rosenbaum, A1 Schmidt, Richard Strub, Tracey Twine, An-  Amaral, J. A., and R. Knowles,Localizationof methaneconsumption and nitrificationactivitiesin someboreal forestsoilsand the stability of methane consumptionon storageand disturbance,J. Geophys. Res., this issue.  Baldocchi,D. D., and C. Vogel, A comparativestudyof water vapour, energyand CO2 flux densitiesabove and below a temperatebroadleaf and a boreal pine forest, TreePhysiol.,16, 5-16, 1996. Baldocchi,D. D., and P. C. 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