# Does Below-Above Canopy Air Mass Decoupling Impact Temperate Floodplain Forest CO2 Exchange?
## 4 Discussion  
The highest NEE in 2018 in comparison to the other study years was generated by an exceptionally warm spring (with earlier leaf development) and a dry summer [9].  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**u* above the** **Canopy (u*\({}_{a}\))** & **u* below (u*\({}_{b}\))** & **C Exchange (g m\({}^{-2}\))** & **Percentage of Gaps after** **Two-Level u* Filtering in Relation** **for the 2019 Year** & **Two-Level u* Filtering (\%)** \\ \hline u*\({}_{a}\) = 0.43 & – & \(-188.9\) & – \\ u*\({}_{a}\) = 0.43 & u*\({}_{b}\) = 0.02 & \(-185.5\) & 1.6 \\ u*\({}_{a}\) = 0.43 & u*\({}_{b}\) = 0.03 & \(-181.3\) & 6.0 \\ u*\({}_{a}\) = 0.43 & u*\({}_{b}\) = 0.04 & \(-216.8\) & 11.9 \\ u*\({}_{a}\) = 0.43 & u*\({}_{b}\) = 0.05 & \(-201.0\) & 18.0 \\ u*\({}_{a}\) = 0.43 & u*\({}_{b}\) = 0.06 & \(-209.0\) & 23.5 \\ u*\({}_{a}\) = 0.43 & u*\({}_{b}\) = 0.07 & \(-203.7\) & 27.8 \\ u*\({}_{a}\) = 0.43 & u*\({}_{b}\) = 0.08 & \(-192.6\) & 31.0 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Sensitivity analysis for u*\({}_{b}\) and its influence on the carbon exchange values for 2019.  
Figure 6: Mean diurnal variation of CO\({}_{2}\) exchange (NEE) based on single- and two-level u* filtered data for the year 2019.  
After this drought year, two years with high amounts of precipitation followed, which, in consequence, led to the highest GPP in 2020 (Figures 2 and 3). However, wet conditions were also associated with high ecosystem respiration and the NEE in 2020 was the smallest among all studied years. Based on our results, we assume that this ecosystem was not characterized by reduced growth after the drought year 2018 and the so-called \"legacy effect\" which is most dominant in dry ecosystems [53].  
Analyzing the influence of decoupling on above canopy derived carbon fluxes, we observed positive below canopy fluxes over the whole course of the year (Figure 4, left). They indicate that decoupling outside of the CUP period would lead to an underestimation of the above canopy derived ecosystem respiration, and ignoring this phenomenon during the CUP period would yield an overestimation of the ecosystem carbon sink strength [54,55].  
The u* two-level decoupling thresholds which we derived are comparable to other sites [20], though differences exhibit the threshold dependency on site-specific characteristics. On average, the two-level filtered data showed a higher respiration signal during nighttime than the single-level filtered data (Figure 6), which would confirm the theoretical assumption that the two-level filtered data are capturing more respiration signals from below canopy as they consider only fully coupled periods. On an annual scale, however, this does not make an impact on the carbon exchange at our site. There might also be filtering effects during daytime which lead to more negative (higher uptake) flux values during daytime periods (Figure 6) and counterbalance the nighttime effects of the two-level filtering. Furthermore, the study site is situated in very flat terrain which inhibits potential horizontal removal from CO\({}_{2}\)-rich below canopy air from the measurement volume. As a consequence, we observe no clear effect of the two-level filtering on the annual carbon exchange estimates (Table 2). At this specific site, contrary to a number of previous studies at other forest sites [20, 21, 22, 32], the single-level u*filtering appears to be sufficient to address decoupling effects on annual carbon exchange values.