Within the framework of the European Land Arctic Physical Processes project and as part of the Danish Research Council's Polar Program, a study on trace gas exchange in a high-arctic ecosystem was conducted in NE Greenland, May--August 1997. On the basis of carbon dioxide flux measurements from three dominant surface types, this paper reports on the upscaling of such measurements from canopy to landscape level. Over a three-week period starting in mid-July, the different surfaces revealed large differences in the CO2 flux with uptake rates ranging from 0.7 g C m-2 d-1 over the dwarf shrub heath to 3.0 g C m-2 d-1 over denser parts of the fen, while willow snowbed revealed intermediate uptake rates. The carbon dioxide exchange could be simulated by a CO2 model, combining photosynthesis and soil respiration routines, for which the parametrization depended on the vegetation type. Results from the simulation were supported by a sensitivity analysis based on a three-dimensional footprint model where it was shown that the CO2 uptake was strongly related to the measured leaf area index. The CO2 model was used to calculate the spatial distribution in Net Ecosystem Exchange (NEE) on the basis of Landsat satellite data acquired at the peak of the growing season and stratified according to vegetation type. It was found that there was a reasonable agreement between the satellite-based flux estimate (-0.77 g C m-2 d-1) and the CO2 flux found by areal weighting of the eddy correlation measurements (-0.88 g C m-2 d-1) for the specific study day. Finally, the summer season NEE was calculated for the whole Zackenberg Valley bottom. In June, there was a valley-wide carbon loss of 8.4¿2.6 gCm-2 month-1, whereas the valley system accumulated 18.8¿6.7 gCm-2 season-1 during the growing season (July--August). ¿ 2000 American Geophysical Union