Plot-scale vertical and horizontal transport of CO2 modified by a persistent slope wind system in and above an alpine forest

Data from the flux tower site Renon/Ritten, Italy, located at 1735 m. a.s.l. on a south exposed steep (118)forested alpine slope, is analyzed. In spite of the complex terrain, a persistent slope wind systemprevailed at the site during most of the ADVEX campaign from April to September 2005. We describe indetail how CO 2 is transported parallel to the slope and how this transport affects net ecosystem exchange(NEE) in the diurnal course. The local slope wind system may be strongly modified by two different largescale synoptic situations. The ‘‘Tramontana’’, a persistent strong wind from the north, amplified thedrainage  flow  during  nighttime  and  suppressed  the  upslope  flow  above  the  forest  canopy  duringdaytime. Vice versa, we observed periods with continuing flow from the south, which supported the localdaytime upslope flow and partly suppressed the nighttime downslope flow. This led to periods of severalhours with opposite flow directions in and above the canopy. Depending on the prevailing situation, thetrunk space is coupled and/or decoupled with/from the roughness sublayer above the forest canopy. Inparticular, vertical and horizontal mixing of CO 2 was strongly dependent on the dominating wind fieldwith essential impact on the horizontal advective flux of CO 2 . The most common ‘‘Local’’ situation,dominated by the slope wind system, showed positive horizontal and vertical advection (with typicalvalues around 7 and 3 mmol m 2 s 1 , respectively) together with downslope winds at night and slightlynegative horizontal advection (typical values around 2 mmol m 2 s 1 ) together with upslope windsduring the day. This pattern was amplified at night when the wind was consistently (day and night) blowing downslope (the “Tramontana” situation) and, vice versa, attenuated during the night, when the wind was blowing permanently upslope (the “Southerlies” situation). Taking into account these advective fluxes would significantly reduce the reported annual CO2 uptake of this forest. Related effects are expected to occur at flux tower sites with similar topography and vegetation.