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[1] This paper presents a Lagrangian budgeting approach to quantify the uptake of CO 2 by vegetation at horizontal scales of several tens of kilometers. For this purpose, CO 2 and meteorological parameters were measured from a small aircraft during four flights in June 2001 over a flat homogeneous and productive temperate forest in the Landes region (southwestern France). Additional CO measurements were made in order to identify and quantify the potential influence of anthropogenic emissions on the net CO 2 flux derived from the measurements. For one of four flights, Lagrangian conditions were nearly perfectly fulfilled. On average, the CO 2 mixing ratio in the boundary layer decreased at a rate of 0.11 ppm km À1 , yielding an average CO 2 uptake by the forest of 16 ± 2.5 mmol m À2 s À1 between 1230 and 1430 UT. Our result is about 15% smaller than the local net ecosystem exchange measured by eddy covariance at a tower north of the flight domain and about 12% higher than a regional estimate based on remote sensing data for the whole experimental area. The contribution of anthropogenic emissions to the regional CO 2 budget was estimated from the CO measurements to be to <0.5 mmol m À2 s À1 .

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