Importance of advection in the atmospheric CO2 exchanges of an alpine forest

Vertical profiles of turbulence statistics were used together with horizontal and vertical profiles of CO2 concentration to investigate the magnitude of advective fluxes at a sloping alpine site (Renon, BZ, Italy), characterised by typical mountain-valley breeze circulation. The analysis was performed in order to highlight and quantify the contribution of non-turbulent fluxes to the ecosystem carbon exchange under different atmospheric conditions. Both vertical and horizontal advection components consistently showed positive values (CO2 emission) during the night, leading to an increase in the carbon loss from the ecosystem. These results are in contrast with previous experimental analyses on night-time advective fluxes performed in more homogeneous and flat sites, in which the positive vertical advection flux was generally balanced by negative horizontal advection. The occurrence of unbalanced advective components is attributed to the strong horizontal heterogeneities in canopy structure and source distribution. For the calculation of vertical advection, the hypothesis of linear decay of residual vertical velocities in the canopy space was tested against measured profiles of vertical velocities. Results show that this hypothesis does not introduce substantial errors in the estimates of vertical advection at this specific site. Finally, the night-time dependence of turbulent flux, advective fluxes and storage on friction velocity was analysed. While the eddy flux shows a strong dependence on u* at low friction velocity values, the total flux is less dependent on this factor.