<p>The eddy covariance (EC) method, nowadays the standard method for determining forest ecosystem-atmosphere turbulent exchange, faces a major threat in its application: the air masses below the canopy are regularly decoupled from the air masses above the canopy. Consequently, the EC measurements above the canopy like e.g. H<sub>2</sub>O and particularly CO<sub>2</sub> fluxes can be biased due to missing signals from below-canopy processes. This decoupling is strongly site dependent and influenced by meteorological conditions, canopy properties and tower-surrounding topography. It can be verified and addressed by subsequent EC measurements below and above the canopy. Specifically, the correlation of σ<sub>w</sub> below and above the canopy gives information about the coupling state as this correlation is linear during periods of full coupling.</p> <p>The current study aims to address the decoupling issue on a global scale. For this purpose, approximately 30 forest sites from around the world will be analyzed in a standard way with regards to decoupling. The study sites cover manifold vegetation types and climate zones, all sites are equipped with concurrent below and above canopy EC measurements. Preliminary results highlight the dependence of decoupling on meteorological conditions, canopy properties and tower surrounding topography. Nevertheless, the final goal of this action is to derive global relations between these influence factors and decoupling which will be applicable in a general way on each forest site worldwide. Highest quality turbulent fluxes will be the outcome and the accuracy of EC derived forest water and carbon budgets will improve.</p>

Abstract. Natural wetlands constitute the largest and most uncertain source of methane (CH4) to the atmosphere and a large fraction of them are found in the northern latitudes. These emissions are typically estimated using process (“bottom-up”) or inversion (“top-down”) models. However, estimates from these two types of models are not independent of each other since the top-down estimates usually rely on the a priori estimation of these emissions obtained with process models. Hence, independent spatially explicit validation data are needed. Here we utilize a random forest (RF) machine-learning technique to upscale CH4 eddy covariance flux measurements from 25 sites to estimate CH4 wetland emissions from the northern latitudes (north of 45∘ N). Eddy covariance data from 2005 to 2016 are used for model development. The model is then used to predict emissions during 2013 and 2014. The predictive performance of the RF model is evaluated using a leave-one-site-out cross-validation scheme. The performance (Nash–Sutcliffe model efficiency =0.47) is comparable to previous studies upscaling net ecosystem exchange of carbon dioxide and studies comparing process model output against site-level CH4 emission data. The global distribution of wetlands is one major source of uncertainty for upscaling CH4. Thus, three wetland distribution maps are utilized in the upscaling. Depending on the wetland distribution map, the annual emissions for the northern wetlands yield 32 (22.3–41.2, 95 % confidence interval calculated from a RF model ensemble), 31 (21.4–39.9) or 38 (25.9–49.5) Tg(CH4) yr−1. To further evaluate the uncertainties of the upscaled CH4 flux data products we also compared them against output from two process models (LPX-Bern and WetCHARTs), and methodological issues related to CH4 flux upscaling are discussed. The monthly upscaled CH4 flux data products are available at https://doi.org/10.5281/zenodo.2560163 (Peltola et al., 2019).

To investigate the energy, matter and reactive and non-reactive trace gas exchange between the atmosphere and a spruce forest in the German mountain region, two intensive measuring periods were conducted at the FLUXNET site DE-Bay (<i>Waldstein-Weidenbrunnen</i>) in September/October 2007 and June/July 2008. They were part of the project "ExchanGE processes in mountainous Regions" (EGER). Beyond a brief description of the experiment, the main focus of the paper concerns the coupling between the trunk space, the canopy and the above-canopy atmosphere. Therefore, relevant coherent structures were analyzed for different in- and above canopy layers, coupling between layers was classified according to already published procedures, and gradients and fluxes of meteorological quantities as well as concentrations of non-reactive and reactive trace compounds have been sorted along the coupling classes. Only in the case of a fully coupled system, it could be shown, that fluxes measured above the canopy are related to gradients between the canopy and the above-canopy atmosphere. Temporal changes of concentration differences between top of canopy and the forest floor, particularly those of reactive trace gases (NO, NO₂, O₃, and HONO) could only be interpreted on the basis of the coupling stage. Consequently, only concurrent and vertically resolved measurements of micrometeorological (turbulence) quantities and fluxes (gradients) of trace compounds will lead to a better understanding of the forest-atmosphere interaction