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

[1]  A. Cescatti,et al.  Drag coefficient and turbulence intensity in conifer canopies , 2004 .

[2]  A. Verhoef,et al.  A system to measure surface fluxes of momentum, sensible heat, water vapour and carbon dioxide , 1997 .

[3]  E. K. Webb,et al.  Correction of flux measurements for density effects due to heat and water vapour transfer , 1980 .

[4]  Dennis Baldocchi,et al.  On Measuring Net Ecosystem Carbon Exchange Over Tall Vegetation on Complex Terrain , 2000, Boundary-Layer Meteorology.

[5]  M. G. Ryan,et al.  Comparing nocturnal eddy covariance measurements to estimates of ecosystem respiration made by scaling chamber measurements at six coniferous boreal sites , 1997 .

[6]  W. Oechel,et al.  FLUXNET: A New Tool to Study the Temporal and Spatial Variability of Ecosystem-Scale Carbon Dioxide, Water Vapor, and Energy Flux Densities , 2001 .

[7]  Dennis D. Baldocchi,et al.  Seasonal variations of CO2 and water vapour exchange rates over a temperate deciduous forest , 1996 .

[8]  Ü. Rannik,et al.  Gap filling strategies for defensible annual sums of net ecosystem exchange , 2001 .

[9]  Larry Mahrt,et al.  Flux Sampling Errors for Aircraft and Towers , 1998 .

[10]  C. Bernhofer,et al.  The Influence of Advection on the Short Term CO2-Budget in and Above a Forest Canopy , 2003 .

[11]  T. Meyers,et al.  Correction Of Eddy-Covariance Measurements Incorporating Both Advective Effects And Density Fluxes , 2000, Boundary-Layer Meteorology.

[12]  Ü. Rannik,et al.  Respiration as the main determinant of carbon balance in European forests , 2000, Nature.

[13]  M. Raupach,et al.  How is the atmospheric coupling of land surfaces affected by topography, complexity in landscape patterning, and the vegetation mosaic? , 1999 .

[14]  Xuhui Lee,et al.  Forest-Air Fluxes Of Carbon, Water And Energy Over Non-Flat Terrain , 2002 .

[15]  D. Baldocchi,et al.  Below-canopy and soil CO2 fluxes in a ponderosa pine forest , 1999 .

[16]  John Moncrieff,et al.  The propagation of errors in long‐term measurements of land‐atmosphere fluxes of carbon and water , 1996 .

[17]  Giorgio Matteucci,et al.  Seasonal net carbon dioxide exchange of a beech forest with the atmosphere , 1996 .

[18]  J. Finnigan,et al.  A comment on the paper by Lee (1998): “On micrometeorological observations of surface-air exchange over tall vegetation” , 1999 .

[19]  Marc Aubinet,et al.  Estimation of the carbon sequestration by a heterogeneous forest: night flux corrections, heterogeneity of the site and inter‐annual variability , 2002 .

[20]  D. Fitzjarrald,et al.  Observing subcanopy CO2 advection , 2004 .

[21]  Xuhui Lee,et al.  On micrometeorological observations of surface-air exchange over tall vegetation , 1998 .

[22]  Peter D. Blanken,et al.  Annual cycles of water vapour and carbon dioxide fluxes in and above a boreal aspen forest , 1996 .

[23]  Ü. Rannik,et al.  Estimates of the annual net carbon and water exchange of forests: the EUROFLUX methodology , 2000 .

[24]  J. Norman,et al.  Correcting eddy-covariance flux underestimates over a grassland , 2000 .

[25]  M. Aubinet,et al.  Horizontal and Vertical Co2 Advection In A Sloping Forest , 2003 .

[26]  Pavel Kabat,et al.  Integrating hydrology, ecosystem dynamics, and biogeochemistry in complex landscapes , 1999 .

[27]  J. William Munger,et al.  Measurements of carbon sequestration by long‐term eddy covariance: methods and a critical evaluation of accuracy , 1996 .

[28]  A. Cescatti.  Modelling the radiative transfer in discontinuous canopies of asymmetric crowns. I. Model structure and algorithms , 1997 .