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Gravitational flows generated on a gentle slope in stable conditions were analysed at a forested site at Vielsalm in Belgium. There were two distinct situations at the site, one corresponding to vertical convergence, characterised by a negative vertical velocity at the canopy top and horizontal velocity divergence below the canopy, the other corresponding to an equilibrium situation without any vertical movement. The causes of these two distinct flow patterns were analysed. These measurements combined with those of the horizontal CO2 concentration gradient below the canopy supported the dilution hypothesis suggested by Aubinet et al. (2003): the horizontal CO2 concentration gradient is negative in convergence situations but slightly positive in equilibrium conditions. The existence of such patterns allows us to confirm the coherence of advection observations made at the site. However, the sum of turbulent CO2 flux, changes in CO2 storage and advective terms were shown to greatly overestimate the expected net ecosystem exchange in the convergence conditions. The most probable cause was identified as being a poor estimate of the vertical profile of the vertical velocity component. [ABSTRACT FROM AUTHOR]

A dataset from two campaigns conducted at the Vielsalm experimental site in Belgium was used as a basis for discussing some methodological problems and providing intermediate results on estimating CO2 advection. The analysis focused on the horizontal [CO2] gradient and on the vertical velocity w, the variables most affected by uncertainty. The sampling error for half-hourly horizontal [CO2] gradients was estimated to be 1.3 μmol mol−1. Despite this important random error for half-hour estimations of [CO2], the mean horizontal [CO2] gradients in advective conditions were shown to be representative at the ecosystem scale and to extend only to the lowest part of a drainage sub-layer, which developed in the trunk space. By contrast, under daytime conditions, this gradient was shown to be more sensitive to local source heterogeneities. The estimation of the short-term averaged vertical velocity ( $$\bar{w})$$ was the greater source of error when computing advection terms. The traditional correction methods used to obtain $$\bar{w}$$ are discussed and a (co)sine correction is tested to highlight the instrumental origin of the offset in w. A comparison of measurements by sonic anemometers placed close together above the canopy showed that the uncertainty on $$\bar{w}$$ was 0.042 m s−1, which is of the same order of magnitude as the velocity itself. In addition, as the drainage sub-layer is limited to the lowest part of the canopy, the representativeness of $$\bar{w}$$ is questionable. An alternative computation using the divergence of the horizontal wind speed in the trunk space produced a $$\bar{w}$$ estimation that was four times lower than the single-point measurement. However, this value gives a more realistic estimate of the vertical advection term and improves the CO2 budget closure at the site. [ABSTRACT FROM AUTHOR]

SummaryA closed-dynamic-chamber system (CDCS) was used to measure the spatial and seasonal variability of the soil CO2 efflux (Fs) in beech and in Douglas fir patches of the Vielsalm forest (Belgium). First the difference between natural and measured soil CO2 efflux induced by the presence of the CDCS was studied. The impact on the measurements of the pressure difference between the outside (natural condition) and the inside of the chamber was found to be small (0.4%). The influence of wind disturbance in the closed chamber was tested by comparison with an open-chamber system characterized by a different wind distribution. A very good correlation between the two systems was found (r2 = 0.99) but the open system yielded slightly lower fluxes than the closed one (slope = 0.88 ± 0.05). A measurement procedure has been developed to minimize the effect of the other sources of perturbation. The spatial and seasonal evolution of the soil CO2 efflux was obtained by performing regular measurements on 29 spots in the beech patch over a period of 12 months and on 24 spots in the Douglas fir patch over 8 months. For each spot, the experimental relationship between Fs and soil temperature was compared with the fitted line for an Arrhenius relationship with a soil temperature-dependent activation energy. Soil temperature explains 73% of the seasonal variation for all the data. The spatial average of the soil CO2 efflux at 10 °C (Fs10) in the beech patch is 2.57 ± 0.41 μmol m-2 s-1, approximately twice the average in the Douglas fir patch recorded at 1.42 ± 0.22 μmol m-2 s-1. The litter fall analysis seems to indicate that soil organic matter quality and quantity may be one the reasons for this difference. Finally the annual soil CO2 efflux was calculated for the beech and Douglas fir patches (870 ± 140 and 438... [ABSTRACT FROM AUTHOR]