UNDERSTANDING THE FEEDBACKS ASSOCIATED WITH CLIMATE CHANGE IN NORTHERN BOREAL FORESTS IS CRITICAL FOR PREDICTING HOW THESE ECOSYSTEMS WILL CHANGE INTO THE FUTURE. A RIGOROUS UNDERSTANDING OF THE PHENOLOGY OF CARBON CYCLING IN EVERGREEN BOREAL FORESTS HAS BEEN CHALLENGING WITH TRADITIONAL VEGETATION INDICES SUCH AS NDVI AND EVI BECAUSE THEY RETAIN FOLIAGE AND REMAIN GREEN YEAR-ROUND BUT THE LIGHT USE EFFICIENCY (LUE) OF PHOTOSYNTHESIS CHANGES RAPIDLY AND DRAMATICALLY. SEASONAL CHANGES IN PIGMENT COMPOSITION ASSOCIATED WITH CHANGING LUE CAN BE DETECTED VIA REFLECTANCE (PRI AND CCI) AND PROVIDE INFORMATION ON PLANT PHOTOPROTECTIVE MECHANISMS BUT THEIR UTILITY FROM SPACE-BORNE PLATFORMS HAS BEEN LIMITED. SOLAR INDUCED FLUORESCENCE (SIF) REFLECTS DYNAMICAL CHANGES ACROSS THE GROWING SEASON DRIVEN BY INTERNAL AND EXTERNAL FACTORS AND THUS MAY PROVIDE A CRITICAL TOOL TO EVALUATE ECOPHYSIOLOGICAL MECHANISMS DRIVING CHANGING GPP. WHILE SPACEBORNE DETECTION OF SIF PRI AND CCI HAS YIELDED IMPORTANT INSIGHTS THE SPATIAL AND TEMPORAL RESOLUTION IS CURRENTLY TOO COARSE TO RESOLVE DETAILED SEASONAL CHANGES. WE PROPOSE TO LINK OBSERVATIONS OF VEGETATION REFLECTANCE AND SIF COLLECTED AT HIGH TEMPORAL RESOLUTION (CONTINUOUS GROUND-BASED HYPERSPECTRAL REFLECTANCE AND SIF WITH CUSTOM PHOTOSPEC SPECTROMETERS) AND HIGH SPATIAL RESOLUTION (AIRBORNE DATA CFIS AVIRISNG) WITH CARBON FLUXES ACROSS A RANGE OF HYDROLOGIC AND CLIMATE REGIMES IN THE NORTHERN BOREAL FOREST. THIS WILL REQUIRE ADVANCING THE MECHANISTIC UNDERSTANDING OF HOW PHYSICAL (CANOPY ARCHITECTURE VIEWING AND ILLUMINATION GEOMETRY) AND ECOPHYSIOLOGICAL (SEASONAL ACCLIMATION OF PHOTOSYNTHESIS AND NEEDLE PIGMENT COMPOSITION) DRIVERS AFFECT TOP-OF-CANOPY SIF AND REFLECTANCE. TO ACCOMPLISH THIS WE WILL LEVERAGE THE STRENGTH OF BOTH SIF AND REFLECTANCE SYNERGISTICALLY TO INTERPRET THEIR RELATIONS TO GPP AND DEPENDENCE ON OBSERVATION GEOMETRY. TOWARDS THIS GOAL WE WILL INSTALL OUR TOWER-BASED SPECTROMETER SYSTEM AT A NEW SITE (DELTA JUNCTION ALASKA) WHICH WILL BE AUGMENTED BY OUR EXISTING NETWORK REPRESENTATIVE OF DIFFERENT CLIMATOLOGICAL AND HYDROLOGICAL REGIMES (SITES NIWOT RIDGE COLORADO; SOUTHERN OLD BLACK SPRUCE SASKATCHEWAN). THESE THREE EVERGREEN-DOMINATED SITES HAVE SUBSTANTIALLY DIFFERENT CLIMATIC RADIATIVE AND HYDROLOGICAL REGIMES WHICH ENABLES US TO CHARACTERIZE MUCH OF THE WIDE RANGE OF EVERGREEN ECOSYSTEMS IN THE ABOVE DOMAIN. TOWER-CANOPY AND AIRBORNE SPECTROSCOPY DATA WILL BE COUPLED WITH IN-SITU MEASUREMENTS AND 3-D RADIATIVE TRANSFER MODELING TO LINK PROCESSES AT A RANGE OF SCALES TO REMOTE SENSING MEASUREMENTS. THIS MULTI-SCALE APPROACH WILL INFORM SATELLITE OBSERVATIONS FROM MODIS OCO-2 AND TROPOMI TO SCALE GPP ESTIMATES ACROSS THE BOREAL REGION. OUR PRIMARY OBJECTIVES ARE AS FOLLOWS: 1) ESTABLISH A QUANTITATIVE FRAMEWORK TO DESCRIBE THE ECOPHYSIOLOGICAL AND PHYSICAL MECHANISMS LINKING SURFACE MEASUREMENTS OF SIF AND VEGETATION REFLECTANCE TO GPP AT ESTABLISHED TOWER SITES AND COINCIDENT AIRBORNE CAMPAIGNS. 2) REDUCE UNCERTAINTIES IN SATELLITE-BASED ESTIMATES OF GPP ACROSS THE BOREAL REGION BY APPLYING A PROCESS-BASED UNDERSTANDING OF BOTH THE TEMPORAL AND SPATIAL DYNAMICS OF SIF AND VEGETATION REFLECTANCE AT FINE SCALES. THIS WORK DIRECTLY RESPONDS TO THE NASA ABOVE PHASE 2 CALL BY IMPROVING OUR UNDERSTANDING OF THE STRUCTURE FUNCTION AND PRODUCTIVITY OF TERRESTRIAL ECOSYSTEMS AND THEIR INTERACTION WITH ATMOSPHERE AND HYDROSPHERE AND BIOGEOCHEMICAL CYCLING. INTEGRATING OBSERVATIONS MADE AT MULTIPLE SCALES IS CRITICAL TO ELUCIDATE ECOSYSTEM FUNCTION AND WILL DIRECTLY INFORM OUR UNDERSTANDING OF HOW VULNERABLE OR RESILIENT BOREAL ECOSYSTEMS ARE TO ENVIRONMENTAL CHANGE. TO ACHIEVE THIS WE WILL DIRECTLY EXAMINE "THE ECOPHYSIOLOGICAL BASIS OF THE RELATIONSHIPS BETWEEN SURFACE AND SATELLITE MEASUREMENTS OF SIF FOR NORTHERN ECOSYSTEMS AND ITS LINK TO ECOSYSTEM PRODUCTIVITY."