Are Trend Analyses from FLUXNET Station Data Realistic?

Extensive studies are available which analyze time series of carbon dioxide and water flux measurements of FLUXNET sites over many years and link these results with climate zones or even climate change. Analysis of the nearly 20 years of measurements of the Waldstein-Weidenbrunnen (DE-Bay) site has shown that several breakpoints, with different causes, are visible in the time series. The trend to a larger carbon uptake is realistic but probably has reasons other than climate change, and we identified several likely causes. There was an affect on small fluxes due to the low bit resolution in the first years, mainly in the carbon rather than in the water fluxes. Under these circumstances the influence of possible mechanical turbulence may have a stronger influence on the flux than does the footprint. The forest was, in the first years, very much affected due to forest decline, and convalesced after a liming. In the last ten years the site was much affected by beetles and wind-throw. The fluxes increased, obviously due to a larger heterogeneity, but this could also indicate a shift of the “area of maximal fluxes” found in Large Eddy Simulations. All these effects also have an influence on the gap filling routine. Furthermore, the inter-annual variation of the carbon and water fluxes, especially of extreme years, is so large that the data are scattered over published climate diagrams. In conclusion: Possible sensor and site-specific effects may be much larger than climate trends. Because the FLUXNET stations were built up 10–20 years ago at nearly ideal forest sites, which are now often much more heterogeneous, a possible site-specific trend should be visible. Therefore, for climate and trend analysis, a careful analysis of the metadata of stations is recommended and not only the download of data from a database. Acknowledgement: This analysis is based on the work of many researchers, a list of whom will be published in Foken (Ed.): Energy and matter Fluxes of a Spruce Forest Ecosystem, Ecological Series, Springer 2016.