# Does Below-Above Canopy Air Mass Decoupling Impact Temperate Floodplain Forest CO2 Exchange?
## 1 Introduction  
Floodplain forests play an important role in strong, mutual and continuous interaction between climate and the ecosystems, despite a relatively small total area of coverage in Europe [1]. They are characterized by a high production level and biodiversity [1, 2]. Floodplain forests are able to store huge amounts of soil carbon [3]. Wetlands, to which floodplain forests belong, hold the majority of the global soil carbon pool [4]. Carbon dioxide (CO2) sequestration and storage is the result of the balance of gross primary production (GPP) (taking up CO2 for photosynthesis and producing organic matter), and ecosystem respiration, Reco (or decomposition; generating CO2 or methane from organic matter) [5]. On some wetlands, the decomposition is very slow, and when plant productivity exceeds it, carbon accumulates [6]. In the course of ongoing climate change with more often occurring extreme events, the role of floodplain forests as carbon stores and their role in nutrient cycles to mitigate climate change has become even more crucial [7, 8, 9, 10]. Changing temperature and precipitation regimes due to changing climatic conditions can shift the balance between the rate of Reco and GPP, which can cause wetlands to become a carbon source [11, 12]. The floodplain forest study site near Lanzhot, Czech Republic is a hydrologically controlled wetland ecosystem. Thus, changes in the environmental conditions at the study site might be man-made by artificial flooding, or induced by climate changes. Interpreting the annual CO2 exchange corresponding to environmental conditions in the course of the study at the floodplain forest in Lanzhot was one of our main interests. We focused onthe quantification of Net Ecosystem CO\({}_{2}\) Exchange (NEE) attributable to the floodplain forest near Lanzhot. This quantification is a critical requirement in order to estimate the CO\({}_{2}\) balance on a local and regional scale. Our main method of choice, the eddy covariance (EC) method, has become a key approach to assess the forest ecosystem CO\({}_{2}\) exchange [13; 14]. Through direct, precise and continuous measurements, EC provides the unique possibility to measure ecosystem-atmosphere turbulent exchange of matter and energy fluxes, e.g., CO\({}_{2}\) fluxes. In the case of CO\({}_{2}\), this flux equals NEE under optimal measurement conditions and on longer time scales (e.g., annual). Revealing the interactions between terrestrial biosphere and atmosphere on an ecosystem scale, NEE indicates if there is a carbon uptake or efflux in the studied ecosystem [15]. From NEE and accompanying environmental measurements, GPP and R\({}_{\text{eco}}\) can be derived [16]. Eddy covariance requires certain assumptions and preconditions to be fulfilled, e.g., steady-state conditions, well-developed turbulence, homogenous underlying vegetation and ideally, flat terrain [14]. Additionally, a number of corrections are necessary during the flux calculation process [13; 17]. These preconditions are not always met. For example, low wind conditions and stable stratification across the canopy may lead to decoupling of the air masses close to the ground from the air masses above the canopy due to insufficient turbulent mixing. As NEE measurements are conducted only above the forest, respiration components from within and below canopy might be missing in the above canopy data under such insufficient mixing conditions. This leads to an overestimation of the carbon sink strength of the studied forest ecosystems [18; 19; 20; 21; 22], if the inspired below canopy CO\({}_{2}\) is removed from the probed measurement volume before reaching the above canopy instrumentation. The most common approach to guarantee well-developed turbulence is the single-level friction velocity (u*) filtering approach [23; 24; 25; 26].