# Does Below-Above Canopy Air Mass Decoupling Impact Temperate Floodplain Forest CO2 Exchange?
## 1 Introduction
Friction velocity is a measure of mechanical turbulence development and allows to discard measurements from weak turbulent periods which are potentially not fully representative for the whole ecosystem, and to replace them by gap-filled estimates obtained under well-developed turbulence conditions [27]. The estimation of a filtering threshold is site-specific and empirically determined by evaluating the relation between u* and nighttime above canopy CO\({}_{2}\) fluxes under the assumption that nighttime above canopy CO\({}_{2}\) fluxes become independent of u* at a certain threshold [28; 29; 30]. The problem of flux determination under poorly developed turbulence has been known for many years, however, all determining factors are not yet fully understood. In recent years, several case studies approached this topic by addressing canopy decoupling in forests. They aimed, inter alia, for testing and validating the applicability of the EC technique, assessing the spatial variability of subcanopy fluxes, evaluating the factors controlling these fluxes, or the improvement of the NEE estimates [20; 22; 31; 32; 33]. Some of these studies led to the conclusion that using the single-level u* filtering approach is not sufficient to define well-developed turbulent mixing between below and above canopy air masses [13; 20; 21; 34]. Especially at tall and dense canopies, continuous decoupling of the sub-canopy layer from the overstory and above canopy air masses may occur due to stable stratification across the canopy and the mechanical barrier induced by the forest canopy [35]. Recently, as an alternative to the single-level u* filtering approach, a two-level filtering strategy has been proposed [20; 32]. It can be applied both on the relation between u* above and u* below the canopy as well as on the relation between the standard deviation of the vertical wind speed above (\(\sigma\)w_a) and below (\(\sigma\)w_b) the canopy. Linear relation between u* above and below canopy (or \(\sigma\)w_a and \(\sigma\)w_b) reflects conditions when below and above canopy air masses are fully coupled. [PERSON] et al. [20] proposed ways to identify thresholds for both \(\sigma\)w (or u*) above and below the canopy, above which full coupling is assumed. However, as micrometeorological conditions differ spatially and temporarily, individual threshold determination approaches might be applicable, even though general strategies are proposed [20; 32]. For comparability reasons, the focus in the current study is on the u* thresholds as it gives the possibility to directly assess the decoupling effect in comparison to the most widely used standard method single-level u* filtering. The application of concurrent EC below and above canopy at the studied floodplain forest promises new insights on the phenomenon of canopy decoupling and its potential influence on above canopy derived EC fluxes, as the study site is in very flat terrain. In this sense, the study site represents ideality. Our specific aims are (1) to interpret the annual CO2 exchange corresponding to environmental conditions over the study years at the floodplain forest in Lanzhot and (2) to evaluate and assess different single- and two-level flux filtering strategies with regards to decoupling and its impact on above canopy derived forest CO2 exchange. We hypothesized that conventional single-level EC flux filtering strategies like the u* filtering might not fully capture the forest CO2 exchange at the studied ecosystem.