COMPARING NIGHT CO2 STORAGE AND ADVECTION AT FIVE FORESTED SITES

The estimation of carbon sequestration by forests requires accurate flux measurements
of CO2 exchange during both day and night. The eddy covariance method allows
such measurements and is now widely spread. However, results from many sites show
that during stable nights the fluxes measured by this method differ from the expected
biotic flux. A likely explanation of these differences is that some of the carbon dioxide
produced by respiration is stored in the air below the measurement system or lost by
advection. The relative importance of these processes varies largely from site to site.
In this presentation, results from different measurement campaigns held at five european
forested sites will be shown. The site comparison notably shows that the importance
of the storage flux depends on both turbulence and advective conditions. Criteria
to determine the potential storage importance at each site are proposed.
A comparison between the advection characteristics at the different sites will also be
presented. The advection importance and the balance between vertical and horizontal
advection will be shown to depend on meteorological conditions but also to vary
greatly from site to site according to their topographic (slope, distance from the ridge)
and vegetative (leaf area density distribution, source heterogeneity) characteristics.
Finally, some methodological points are underlined and recommendations for future
measurements are given.