THE MAGNITUDE OF CARBON FLUXES THROUGH ARID AND SEMI-ARID ECOSYSTEMS IS CONSIDERED MODEST BUT INTEGRATED OVER THE ~40% OF THE GLOBAL LAND SURFACE COVERED BY THESE ECOSYSTEMS THE TOTAL CARBON STORED IS ALMOST TWICE THAT IN TEMPERATE FOREST ECOSYSTEMS. UNDERSTANDING WHERE CARBON IS STORED IN THESE ECOSYSTEMS AND HOW CARBON SEQUESTRATION AND EXCHANGE WITH THE ATMOSPHERE WILL RESPOND TO CLIMATE CHANGE IS CRUCIAL GIVEN THE EXTENT OF THESE ECOSYSTEMS AND THAT THE SOUTHWESTERN US IS PREDICTED TO EXPERIENCE WARMING OF UP TO 4##C AND SUBSTANTIAL DECLINES IN PRECIPITATION OVER THE NEXT CENTURY. A CRITICAL TOOL FOR POLICY MAKERS DETERMINING SAFE LEVELS OF GREEN HOUSE GASES IN THE ATMOSPHERE ARE EARTH SYSTEM MODELS CAPABLE OF PREDICTING THE RESPONSE OF THE BIOSPHERE TO CLIMATE CHANGE GIVEN THE POTENTIAL FOR CARBON CYCLE FEEDBACKS EXACERBATING GREENHOUSE-HOUSE-GAS INDUCED CLIMATE CHANGE IS LARGE.HOWEVER A RECENT COMPARISON OF THE CARBON DYNAMICS FORECAST BY A NUMBER OF DIFFERENT EARTH SYSTEM MODELS EXHIBITED GREAT VARIABILITY ON DECADAL TIMESCALES INDICATING CONSIDERABLE UNCERTAINTIES REMAIN.THE PURPOSE OF THIS PROPOSAL IS TO REDUCE UNCERTAINTIES REGARDING REGIONAL CARBON DYNAMICS IN THE SOUTHWESTERN US BY COMBINING A MORE ACCURATE ESTIMATE OF VEGETATION STRUCTURE USING FULL-WAVEFORM LIDAR TO DIRECT MEASUREMENTS OF ECOSYSTEM-ATMOSPHERE CARBON EXCHANGE ACROSS A NETWORK OF EIGHT EDDY COVARIANCE TOWER SITES IN TX AND NM. WE WILL DO THIS WITHIN A MODEL-DATA FUSION (MDF)FRAMEWORK TO IMPROVE ESTIMATES OF REGIONAL CARBON STOCKS AND FLUXES AND PRODUCE A LAND SURFACE MODEL (LSM) CAPABLE OF MAKINGPREDICTIONS ABOUT THE RESPONSE OF C DYNAMICS IN SEMI-ARID ECOSYSTEMS TO CHANGING CLIMATE AND DISTURBANCE.LIDAR REMOTE SENSING HAS BECOME A CRITICAL INSTRUMENT FOR OBTAINING INFORMATION ON VEGETATION STRUCTURE AND A NUMBER OF STUDIES HAVE TESTED THE USE OF WAVEFORM LIDAR FOR CHARACTERIZING STRUCTURE AND ESTIMATING ABOVE GROUND BIOMASS VIA HEIGHT-DIAMETER ALLOMETRIC RELATIONSHIPS IN TEMPERATE AND TROPICAL FORESTS. HOWEVER IN SEMI-ARID ENVIRONMENTS THE DOMINANT SPECIES EXHIBIT MORE COMPLEXALLOMETRIC RELATIONSHIPS DUE TO THE MULTI-STEMMED ARCHITECTURE AT THE BASE OF THE TREE. IN THIS PROPOSAL WE WILL INVESTIGATE HOW LASER WAVEFORM DERIVED PARAMETERS CAN PROVIDE A MORE ACCURATE ESTIMATION OF VEGETATION STRUCTURE (ABOVE GROUND BIOMASS HEIGHT AND LEAF AREA) ACROSS A RANGE OF SEMI-ARID ECOSYSTEMS PROVIDING A CRITICAL TEST OF THE POTENTIAL OF SPACE-BASED FULL-WAVEFORM LIDAR IN THESEENVIRONMENTS. WE WILL ALSO INVESTIGATE THE IMPACTS OF SENSOR RESOLUTION AND SPATIAL AGGREGATION ON MEASUREMENT UNCERTAINTY AND HOW THIS IS PROPAGATED INTO MODEL UNCERTAINTY WHEN USED IN MODEL INITIALIZATIONS.THIS RESEARCH RESPONDS DIRECTLY TO THE NRC REPORT RESTRUCTURING FEDERAL CLIMATE RESEARCH TO MEET THE CHALLENGES OF CLIMATE CHANGE (2009) BY: (I) COLLECTING DATA NEEDED TO DOCUMENT UNDERSTAND AND PREDICT THE EFFECTS OF CLIMATE CHANGE WHICH WILL BE MADE AVAILABLE THROUGH THE FLUXNET DATA BASE SUPPORTING THE INTEGRATIVE ANALYSIS MADE POSSIBLE BY AMERIFLUX; AND (II) HELPING TO SUPPORTTHE DEVELOPMENT OF A NEW GENERATION OF COUPLED EARTH SYSTEM MODELS THROUGH PROVIDING MORE ACCURATE OBSERVED LAND SURFACE STATE FOR MODEL INITIALIZATION AND TESTING FURTHER DEVELOPING AND CONSTRAINING MODEL PARAMETERIZATIONS. BY ASSESSING CARBON STOCKS AND FLUXES ACROSS A RANGE OF BIOMES (NATURAL AND MANAGED FOREST AND RANGELAND) WHERE ECOSYSTEM SERVICES ARE UNDER THREAT FROM DISTURBANCE ANDPEST OUTBREAK DUE TO GLOBAL CHANGE RELATED DROUGHT OR WOODY ENCROACHMENT WE ARE ADDRESSING IMPORTANT SCIENTIFIC-SOCIETAL ISSUES RELEVANT TO CARBON CYCLE SCIENCE IN THE SOUTHWESTERN US.