Contribution of advection to the carbon budget measured by eddy covariance at a steep mountain slope forest in Switzerland

Abstract. We calculated the contribution of advection to the C budget measured by the eddy covariance (EC) technique for a steep and forested mountain site (CarboEurope site CH-Lae, Lägeren, Switzerland) during the growing season 2007 (May to August). Thereby we followed two approaches: (1) the physical correction of the EC data for directly measured advection terms and (2) the u_∗ filter approach that replaces periods with u_∗ below a site-specific threshold with empirically modelled fluxes. We found good agreement between the two approaches in terms of daily (linear regression slope: 0.78 ± 0.04, intercept: 0.68 ± 0.29 μmol m⁻² s⁻¹, adj. R²=0.78) and seasonal sums of gross fluxes (difference ≤ 12%), when using a u_∗ threshold of 0.3 m s⁻¹ and correcting EC for horizontal advection only. Incorporating also vertical advection into the mass balance equation resulted in unrealistic and highly erratic fluxes. However, on a daily basis vertical advection cancelled out to nearly zero. The u_∗ filter seems to account primarily for respiration fluxes, which are mainly affected by horizontal advection. We could confirm our corrections by a cross-validation with independent approaches, such as soil respiration chamber measurements, light curves and energy budget closure. Our results show that flux measurements on steep sites with complex topography are possible. Actually, sloping sites seem to have the advantage over flat sites that advection measurements can be reduced to a simplified two-dimensional measurement approach due to the two-dimensional characteristics of the wind field at those sites.