FLUXNET: A New Tool to Study the Temporal and Spatial Variability of Ecosystem–Scale Carbon Dioxide, Water Vapor, and Energy Flux Densities

Abstract

FLUXNET is a global network of micrometeorological flux measurement sites that measure the exchanges of carbon dioxide, water vapor, and energy between the biosphere and atmosphere. At present over 140 sites are operating on a long-term and continuous basis. Vegetation under study includes temperate conifer and broadleaved (deciduous and evergreen) forests, tropical and boreal forests, crops, grasslands, chaparral, wetlands, and tundra. Sites exist on five continents and their latitudinal distribution ranges from 70°N to 30°S. FLUXNET has several primary functions. First, it provides infrastructure for compiling, archiving, and distributing carbon, water, and energy flux measurement, 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.ornl.gov/FLUXNET/.) Second, the project supports calibration and flux intercomparison activities. This activity ensures that data from the regional networks are intercomparable. And third, FLUXNET supports the synthesis, discussion, and communication of ideas and data by supporting project scientists, workshops, and visiting scientists. The overarching goal is to provide information for validating computations of net primary productivity, evaporation, and energy absorption that are being generated by sensors mounted on the NASA Terra satellite. Data being compiled by FLUXNET are being used to quantify and compare magnitudes and dynamics of annual ecosystem carbon and water balances, to quantify the response of stand-scale carbon dioxide and water vapor flux densities 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 C0 2 exchange of temperate broadleaved forests increases by about 5.7 g C m~2 day-1 for each additional day that the growing season is extended; 2) the sensitivity of net ecosystem C0 2 exchange to sunlight doubles if the sky is cloudy rather than clear; 3) the spectrum of C0 2 flux density exhibits peaks at timescales of days, weeks, and years, and a spectral gap exists at the month timescale; 4) the optimal temperature of net C0 2 exchange varies with mean summer temperature; and 5) stand age affects carbon dioxide and water vapor flux densities.

Analysing surface energy balance closure and partitioning over a semi-arid savanna FLUXNET site in Skukuza, Kruger National Park, South Africa

“…The seasonal fluctuations of energy fluxes are affected by the seasonal changes in the solar radiation, air temperature, precipitation, and soil moisture (Baldocchi et al, 2001;Arain et al, 2003). These climatic variables influence vegetation dynamics in an ecosystem, as well as how solar radiation is partitioned.…”

“…They provide a distinct contribution to the study of environmental, biological and climatological controls of the net surface exchanges between the land surface (including vegetation) and the atmosphere (Aubinet, et al, 1999;Baldocchi et al, 2001). The accuracy of these data is very important because they are used to validate and assess performance of land surface and climate models.…”

The SPARSE model for the prediction of water stress and evapotranspiration components from thermal infra-red data and its evaluation over irrigated and rainfed wheat

“…Both have been grown in a semiarid climate (central Tunisia and Morocco). Surface temperature data were acquired with a nadir-looking Apogee thermoradiometer, while energy fluxes were measured according to classical FLUXNET recommendations (Baldocchi et al, 2001) with Campbell ™ CSAT sonic anemometers and Krypton fast-response hygrometers. Observed and simulated latent heat flux values (half-hourly averages in W m −2 ) are compared at midday (local standard time) in all sky conditions.…”

Regional representativeness assessment and improvement of eddy flux observations in China

“…The technique is closely related to physiological and ecological processes and can reflect the seasonal and interannual variability of carbon fluxes (Baldocchi et al, 2001;Chen et al, 2013). Since being established in the early 1990s, eddy covariance flux towers have provided continuous measurements of the ecosystemlevel net exchange of carbon (Wofsy et al, 1993).…”

“…Since being established in the early 1990s, eddy covariance flux towers have provided continuous measurements of the ecosystemlevel net exchange of carbon (Wofsy et al, 1993). The FLUXNET is the global network of flux towers (Baldocchi et al, 2001). There are currently approximately 541 towers registered on the FLUXNET (http://wwweosdis.ornl.gov/FLUXNET/), which are distributed in different ecosystems and climate zones all over of the globe (Baldocchi, 2008).…”