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 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 C02 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 C02 exchange to sunlight doubles if the sky is cloudy rather than clear; 3) the spectrum of C02 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 C02 exchange varies with mean summer temperature; and 5) stand age affects carbon dioxide and water vapor flux densities

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 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 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 CO2 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 CO2 exchange to sunlight doubles if the sky is cloudy rather than clear; 3) the spectrum of CO2 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 CO2 exchange varies with mean summer temperature; and 5) stand age affects carbon dioxide and water vapor flux densities

Signale für die Klimaentwicklung von Dresden

Dies ist ein Faktenblatt aus dem REGKALM-Vorhaben zum Thema "Signale für die Klimaentwicklung von Dresden"

Quantifying the coupling degree between land surface and the atmospheric boundary layer with the coupled vegetation-atmosphere model HIRVAC

In the present study, the ability of different indices to quantify the coupling degree between a vegetated surface and the atmospheric boundary layer is tested. For this purpose, a one-and-a-half dimensional atmospheric boundary layer model, including a high resolved vegetation canopy, was applied (HIRVAC) and indices, such as the decoupling factor Ω, as well as other measures derived from model out-put were used. The aim of the study was to show that the quite complex coupling and feedback mechanisms can be described with these relatively simple measures. Model results illustrate that the vegetation and the atmosphere are well coupled (expressed by a lower Ω) under conditions of a tall and dense canopy, as well as under strong dynamic forcing. This better aerodynamic coupling leads to an increase in evapotranspiration, as well as an increase in the evaporative fraction. This fact was also shown by the second coupling measure: the relative changes in daily model evapotranspiration. This measure was inspired by the assumption that these changes are primarily dependent on the coupling degree between the surface and the atmosphere, if the other boundary conditions in the model are fixed. A third sensitivity measure was used according to Jacobs and de Bruin (1992). It shows that the sensitivity of evaporative fraction to stomata resistance is much higher with a better aerodynamic coupling. The results of the factor Ω; are in a good agreement with the findings of Jacobs and de Bruin: they stress that it is a valuable strategy to group vegetation into two simple categories (smooth and rough) for the understanding of vegetation-atmosphere coupling.Key words. Atmospheric composition and structure (biosphere- atmosphere interactions) – Hydrology (evapotranspiration; hydroclimatology

COMPARING NIGHT CO2 STORAGE AND ADVECTION AT FIVE FORESTED SITES

peer reviewedThe estimation of carbon sequestration by forests requires accurate flux measurements of CO2 exchange during both day and night. The eddy covariance method allows such measurements and is now widely spread. However, results from many sites show that during stable nights the fluxes measured by this method differ from the expected biotic flux. A likely explanation of these differences is that some of the carbon dioxide produced by respiration is stored in the air below the measurement system or lost by advection. The relative importance of these processes varies largely from site to site. In this presentation, results from different measurement campaigns held at five european forested sites will be shown. The site comparison notably shows that the importance of the storage flux depends on both turbulence and advective conditions. Criteria to determine the potential storage importance at each site are proposed. A comparison between the advection characteristics at the different sites will also be presented. The advection importance and the balance between vertical and horizontal advection will be shown to depend on meteorological conditions but also to vary greatly from site to site according to their topographic (slope, distance from the ridge) and vegetative (leaf area density distribution, source heterogeneity) characteristics. Finally, some methodological points are underlined and recommendations for future measurements are given