Floodplain forest CO2 exchange – a micrometeorological point of view

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. They are characterized by a high production level and biodiversity.

In this study we focus on the quantification of CO₂ exchange attributable to the floodplain forest in Lanzhot, Czech Republic. This quantification is a critical requirement in order to estimate the CO₂ balance on a local and regional scale. Lanzhot is a floodplain forest located in South Moravian Region of Czech Republic (48.6815483 N, 16.9463317 E). It’s a 122 years old, mixed deciduous-broadleaf forest. Main species are english oak, narrow-leaved ash and hornbeam. Mean ground water level reaches depth of 2.7 m.

To evaluate the ecosystem-atmosphere CO₂ exchange we apply the eddy covariance (EC) method, which became a key method for measurements of energy and greenhouse gas exchange between ecosystems and the atmosphere. In recent years, case studies focused on testing and validating the applicability of the EC technique above forest ecosystems. The majority of these studies led to the conclusion that above forest canopy derived CO₂ fluxes might be biased due to missing below canopy respiration components in the above canopy signal during periods of insufficient mixing (decoupling) across the canopy. As standard flux filtering methods like the u_* filtering may not account for decoupling sufficiently, additional below canopy EC measurements were suggested to tackle this problem. The key assumption behind such two-level measurements is, that quantities like u_* or the standard deviation of vertical wind velocity exhibit a linear relation during periods of full coupling between below and above canopy air masses.

In this study, we assess different single- and two-level flux filtering strategies with regards to decoupling and its impact on the above canopy derived CO₂ fluxes. The analysis is focused on one year of concurrent below and above canopy EC measurements. Our starting hypothesis was that conventional single-level EC flux filtering strategies like the u_* filtering might not be sufficient to fully capture the forest CO₂ exchange at the studied ecosystem. Initial results suggest that decoupling occurs regularly at the studied floodplain forest. The implication on the above canopy derived EC CO₂ fluxes, however, appears to be negligible. We attribute this to the open canopy and flat EC tower surrounding terrain which inhibits horizontal removal of below-canopy respired CO₂. Overall, our study underlines the need of explicit decoupling investigations at each forest ecosystem, as decoupling is strongly site specific, depending on canopy properties, site meteorology and tower surrounding topography.