DESPITE THE NEED TO UNDERSTAND TERRESTRIAL BIOSPHERIC CARBON FLUXES TO ACCOUNT FOR CARBON CYCLE FEEDBACKS AND PREDICT FUTURE CO2 CONCENTRATIONS KNOWLEDGE OF THESE FLUXES AT THE REGIONAL SCALE REMAINS POOR. THIS IS PARTICULARLY TRUE IN MOUNTAINOUS AREAS WHERE COMPLEX METEOROLOGY AND RELATIVE LACK OF OBSERVATIONS LEAD TO SIGNIFICANT UNCERTAINTIES IN CARBON FLUXES. YET MOUNTAINOUS REGIONS ARE ALSO WHERE SIGNIFICANT FOREST COVER AND BIOMASS ARE FOUND I.E. AREAS THAT HAVE THE POTENTIAL TO SERVE AS TERRESTRIAL CARBON SINKS. WE PROPOSE TO BUILD UPON THE FOUNDATION THAT OUR TEAM HAS LAID IN DEVELOPING AND TESTING A PROTOTYPE OF THE COMPLEX TERRAIN CARBON MONITORING SYSTEM (AKA CMS-MOUNTAINS ) OVER THE WESTERN U.S. WHERE WE HAVE CONSTRUCTED: PHYSIOLOGICAL DATASETS AND KNOWLEDGE ON THE CONTROLS ON LEAF-LEVEL TO STAND-LEVEL SOLAR-INDUCED FLUORESCENCE (SIF) AND THEIR RELATIONSHIPS TO GROSS PRIMARY PRODUCTIVITY (GPP) COMMUNITY LAND MODEL (CLM) CONFIGURED FOR THE WESTERN U.S. AT HIGH SPATIAL RESOLUTION WITH THE ABILITY TO ASSIMILATE SIF OBSERVATIONS AND SIMULATE PHYSICALLY CONSISTENT BIOMASS QUANTITIES AN ADVANCED ASSIMILATION SYSTEM USING THE NCAR DATA ASSIMILATION RESEARCH TESTBED (DART) WITH CLM AT ITS CORE WE PROPOSE TO FURTHER REFINE CMS-MOUNTAINS AND DELIVER PRODUCTS TO KEY STAKEHOLDERS BY ADDRESSING THESE OBJECTIVES: OBJECTIVE #1: EXTRACT FINE-SCALE INFORMATION REGARDING GPP IN COMPLEX TERRAIN USING HIGH RESOLUTION SIF AND MODIS REFLECTANCE DATA COMBINED WITH FLUX TOWER DATA. OBJECTIVE #2: USE FINE-SCALE SIF TO IMPROVE PHOTOSYNTHETIC PHENOLOGY WITHIN CLM. OBJECTIVE #3: ASSIMILATE FINE-SCALE SIF AND OTHER SATELLITE DATA WITHIN CLM TO PRODUCE REGIONAL CARBON STOCK AND FLUX ESTIMATES OVER THE WESTERN U.S. OBJECTIVE #4: CONSTRUCT FOREST HEALTH EARLY WARNING CAPABILITIES TO ENGAGE AND SUPPORT STAKEHOLDERS. WHILE THE PROJECT WILL FOCUS ON THE WESTERN U.S. WITH SPECIAL ATTENTION TO CALIFORNIA S SIERRA NEVADA REGION AND THE COLORADO ROCKIES REGION THE DEVELOPED FRAMEWORK WILL BE OF GENERAL APPLICABILITY. IN FACT WE ANTICIPATE THE COMPLEX TERRAIN CMS THAT WILL EMERGE FROM THIS WORK TO ULTIMATELY BE APPLIED TO OTHER REGIONS OF COMPLEX TERRAIN AROUND THE WORLD DRIVEN BY REMOTE SENSING DATA IN THE ABSENCE OF IN-SITU MEASUREMENTS. PROJECT APPROACHES: SATELLITE REMOTE SENSING: SOLAR-INDUCED FLUORESCENCE (SIF) FROM GOME-2 TROPOMI AND OCO-2/-3 WILL BE USED TO CONSTRAIN CLM. WE WILL ALSO INVESTIGATE THE USE OF NEW PRODUCTS FROM ECOSTRESS TO CONSTRAIN LAND SURFACE TEMPERATURE (AND THEREBY EVAPOTRANSPIRATION AND GPP). FIELD DATA ANALYSIS: IN 2017 WE INSTALLED AND CONTINUE TO RUN A TOWER-BASED CUSTOM SPECTROMETER AT THE NIWOT RIDGE AMERIFLUX CORE FLUX TOWER IN COLORADO. SEVEN FLUX TOWERS IN THE SIERRA NEVADA MOUNTAINS OF CALIFORNIA ARE ALSO CURRENTLY RUNNING (RUN BY SOUTHERN SIERRA CRITICAL ZONE OBSERVATORY AND THE NATIONAL ECOLOGICAL OBSERVATORY NETWORK). WE WILL CONTINUE USING FLUX TOWER DATA FROM COLORADO AND CALIFORNIA TO PROBE THE MECHANISTIC LINKAGE BETWEEN SIF AND GPP. BIOSPHERIC MODELING: BUILDING UPON OUR TEAM S PREVIOUS PROTOTYPE CMS AND DOE PROJECTS IN WHICH THE COMMUNITY LAND MODEL (CLM) WAS APPLIED TO THE WESTERN U.S. WE WILL ADOPT THE LATEST CLM RELEASE (CLM 5) AS THE MAIN MODELING PLATFORM AND LEVERAGE FIELD DATA OBSERVATIONS TO IMPROVE THE MECHANISTIC REPRESENTATION OF SIF. DATA ASSIMILATION: WE WILL CONTINUE USING NCAR S DATA ASSIMILATION RESEARCH TESTBED (DART) A STATE-OF-THE-SCIENCE ENSEMBLE KALMAN FILTER DATA ASSIMILATION SYSTEM WIDELY ADOPTED BY THE EARTH SYSTEM SCIENCE COMMUNITY. DART WILL BE EXTENDED TO ENABLE NONLINEAR DATA ASSIMILATION METHODS.