FLUXNET: A new tool to study the temporal and spatial variability of ecosystem-scale carbon dioxide, water vapor, and energy flux densities

FLUXNET is a global network of micrometeorological flux measurement site's that measure the exchanges of carbon dioxide, water vapor, and energy betwe en the biosphere and atmosphere. At present over 140 sites are operating on a long-term and continuous basis. Vegetation under study includes temperat e conifer and broadleaved (deciduous and evergreen) forests, tropical and b oreal forests, crops, grasslands, chaparral, wetlands, and tundra. Sites ex ist on five continents and their latitudinal distribution ranges from 70 de greesN to 30 degreesS. FLUXNET has several primary functions. First, it provides infrastructure fo r compiling, archiving, and distributing carbon, water, and energy flux mea surement, and meteorological, plant, and soil data to the science community . (Data and site information are available online at the FLUXNET Web site, http://www-eosdis.oml.gov/FLUXNTET/.) Second, the project supports calibrat ion and flux intercomparison activities. This activity ensures that data fr om the regional networks are intercomparable. And third, FLUXNET supports t he synthesis, discussion, and communication of ideas and data by supporting project scientists, workshops, and visiting scientists. The overarching go al is to provide information for validating computations of net primary pro ductivity, evaporation, and energy absorption that are being generated by s ensors mounted on the NASA Terra satellite. Data being compiled by FLUXNET are being used to quantify and compare magni tudes and dynamics of annual ecosystem carbon and water balances, to quanti fy the response of stand-scale carbon dioxide and water vapor flux densitie s to controlling biotic and abiotic factors, and to validate a hierarchy of soil-plant-atmosphere trace gas exchange models. Findings so far include 1 ) net CO2 exchange of temperate broadleaved forests increases by about 5.7 a C m(-2) day(-1) for each additional day that the growing season is extend ed; 2) the sensitivity of net ecosystem CO2 exchange to sunlight doubles if the sky is cloudy rather than clear; 3) the spectrum of CO2 flux density e xhibits peaks at timescales of days, weeks, and years, and a spectral gap e xists at the month timescale. 4) the optimal temperature of net CO2 exchang e varies with mean summer temperature; and 5) stand age affects carbon diox ide and water vapor flux densities.