The global network, called FLUXNET, studies the breathing of vegetation by measuring the exchange of carbon dioxide, water vapor and energy between vegetation and the atmosphere with the eddy covariance method. At present, eddy fluxes and a suite of meteorological, soil and plant variables, are being measured at over 300 sites world-wide, through the auspices and support of regional networks like Ameriflux, CarboEuroflux, AsiaFlux and ChinaFlux. FLUXNET plays the role of integrating and uniting the distributed networks into a global network. When combined as a group, the FLUXNET expands the diversity of biomes, climate regions, disturbance and land use treatment that are associated with its constituent regional networks. The data produced by the network will be used to examine many questions.
The measurements and synthesis are highly relevant to understanding implications of climate change and evaluating efforts to mitigate increasing carbon flux from the land to the atmosphere. It provides a measure of which areas are sequestering carbon and which are losing carbon to the atmosphere.
FLUXNET is an international project involved with measuring the ‘breathing of the biosphere’. Flux networks are proving to be a critical partner in efforts to produce information on trace gas fluxes that are ‘everywhere, all of the time’. This is accomplished by applying the eddy covariance method at more than four hundred tower sites, dispersed across most of the world’s climate space and representative biomes, with sites in North and South America, Europe, Asia and Australia. Key attributes of the eddy covariance method are its ability to measure fluxes directly between the terrestrial biosphere and atmosphere, in situ, without invasive artifacts, at a spatial scale of hundreds of meters and on time scales spanning hours, days and years. Individual flux towers are adept at providing information on the daily, seasonal, annual and interannual variation in trace gas fluxes of a given plant functional type in a specific climate region. They also provide data on how those fluxes may respond to changes in such environmental drivers as light, temperature, soil moisture, and leaf area index. Groups of towers, at the landscape, regional, continental and global scales, allow one to study a greater range of climate conditions and an increased number of treatments—these may differ by plant functional type, biophysical attributes, biodiversity, time since disturbance (e.g., via fire, logging, wind throw, flooding, or insect infestation), or management practices (e.g., fertilization, irrigation, or cultivation practice). And, a global flux network has the potential to observe low probability, but intense events, like disturbance associated with weather and climate episodes, and monitor their recovery. So far, groups of towers have revealed: a) how annual sums of trace gas fluxes co-vary with variations in climate, plant functional type, drought, heat and nitrogen deposition; b) how biophysical variables, like albedo, vary with plant functional type and nutrition; c) how deposition fluxes are changing with pollution control; and d) how trace gas fluxes are modulated by access to groundwater. Networks focusing on net carbon exchange have produced new information on: a) how length of the growing season modulates annual photosynthesis; c) how peak photosynthesis acclimates with temperature; d) how light use efficiency increases with the fraction of diffuse light; e) how photosynthetic capacity adjusts with time of season; f) how rain induces pulses in ecosystem respiration; and g) how net carbon exchange varies as a function of time since disturbance. We have found that an effective network possesses a number of attributes. Key attributes include standards for instrument performance, standards for data quality and assurance, harmonized data processing, regular calibration protocols, centralized data archives and data versioning, data sharing policies and a database that can be interrogated with complex queries that can sample the archive in time and space. A shared database enables investigators to perform new and novel analyzes at unprecedented time and space scales. Within the FLUXNET project, flux and meteorological data are submitted to regional databases and these are connected to a central database (www.fluxdata.org). There, data are: a) checked for quality; b) gaps are filled; c) value-added products, like ecosystem photosynthesis and respiration, are produced; and d) daily and annual sums or averages are computed. The database also contains a cohort of meteorological, ecological, and soil variables. The ability to share data depends upon fostering trust among colleagues and crossing cultural and political that may otherwise hinder data sharing. These hurdles can be crossed by having shared leadership and good communication among parties, through frequent workshops, internet forums, and newsletters. Collaboration is enhanced by writing papers as groups and sharing authorship on papers generated by the network. This work is being done by collaborating with the remote sensing and Earth System modeling communities. Biophysical, biogeochemical and ecological models that diagnose and forecast the state of the land’s trace gas budgets depend upon a network of ‘super-sites’ measuring a suite of site characteristics to identify or quantify important biophysical processes and parameterize mechanistic algorithms. Another group of models need a dense network of less-intensive flux measurement sites that are sampling representative climate and ecological spaces. These scientists digest flux data, remote sensing and climate data and produce maps of trace gas fluxes at regional, continental and global scales. At present, networks of flux data are being used to test and improve upon land-atmosphere flux algorithms that are used in climate models. There is also potential to use trace gas flux measurements to drive prior computations of flux boundary conditions for the next generation of data assimilation models and are coupled to climate and weather models. And, there is potential to supply data that will be used to validate maps of sources and sinks that are being generated by the global network of trace gas concentration monitors and will be generated from inverting the next generation of satellite based CO2 observations.
