Terrestrial Ecosystem-Atmosphere Exchange of CO2, Water and Energy from FLUXNET; Review and Meta-Analysis of a Global in-situ Observatory



After more than a century of theoretical development and more recent advances in instrumentation, modern observational micrometeorology includes a global network of long-term continuous measurement sites (FLUXNET). Each site uses eddy covariance methods to monitor terrestrial ecosystem-atmosphere exchanges of CO2, water vapor, momentum and heat. Currently FLUXNET includes more than 500 sites, covering all major plant functional types and climates. However, there is a strong spatial bias toward northern hemisphere mid-latitude locations, particularly Europe, North America and East Asia. Tropical ecosystems are particularly poorly represented, despite playing important roles in carbon and water cycles. Synthesis of FLUXNET data shows terrestrial ecosystems in general provide a weak sink of atmospheric CO2 at the annual timescale, but with large spatial and temporal variability, ranging from strong sinks to strong sources. The net ecosystem exchange of CO2 (NEE) that is measured at FLUXNET sites is driven by two much larger opposing CO2 exchange processes, ecosystem photosynthesis and respiration, which tend to co-vary in strength yet are sensitive to separate environmental drivers. Therefore, NEE is not explained well spatially by plant functional type or climate regime and is sensitive to inter-annual climate variability, especially seasonal rainfall, pests or human management approaches. FLUXNET data has been widely used in development of remote sensing, surface-vegetation-atmosphere transfer models and inversion models. These in general capture seasonal cycles and annual budgets quite well, but are weaker at explaining spatial and interannual variability and susceptible to inaccuracies in simulated inputs such as precipitation. Although research has focused primarily on ecosystem functioning and carbon cycling, hydrologic aspects of micrometeorology have received considerable attention and several studies have investigated the surface energy balance and its lack of closure among FLUXNET sites. The contribution of thousands of rigorous experimentalists and the collaborative vision of FLUXNET, despite its relative youth, has already produced significant advances in micrometeorology and ecosystem sciences. Future growth of FLUXNET sites in geographically under-represented areas will make a more complete understanding of terrestrial ecosystems. Meantime, there are many geographic questions ripe for assessment using existing FLUXNET datasets which will, in time, build a global compendium of terrestrial bio-micrometeorology.