Methodological challenges for understory eddy-covariance measurements

Understory eddy-covariance measurements provide valuable insights into ecosystem CO₂ exchange processes, particularly in understanding the interplay between understory and overstory exchange processes. However, their placement deep within the canopy presents some methodological challenges not typically encountered in standard eddy-covariance measurements above the canopy, where surface layer assumptions are generally applicable.

Key challenges arise from the violation of these surface layer assumptions in common flux correction and quality control procedures. Traditional frequency response corrections for flux calculations often rely on idealized cospectra derived from surface layer theory. These assumptions do not hold within the canopy, where spectra and cospectra exhibit distinct characteristics. Furthermore, commonly used turbulence-based quality control metrics, like the integral turbulence test, rely on surface layer scaling relationships to compare measured and modeled fluxes. The application of these relationships within the canopy is questionable due to the altered turbulence structure. For net ecosystem exchange (NEE) measurements, conventional filtering methods, such as friction velocity (u_*) filtering, aim to identify periods when measured fluxes are expected to closely represent the true NEE. However, the low fluxes and turbulence characteristic of the understory environment complicate the reliable application of these filtering approaches.

This study critically examines and revises established correction and quality control procedures specifically for understory eddy-covariance measurements. We investigate the impact of these revised methods on understory CO₂ exchange estimates using data from an understory site in Tyrol, Austria (At-Mmg). Our results are further compared with the total net ecosystem exchange estimated by an above-canopy eddy-covariance system over the past three years.