The physical and chemical environment of forests will change in the future. How forests will react to new conditions is not known yet. In order to get an idea of the sensitivity of present forests to possible atmospheric changes, it is helpful to investigate the physiological response of forest ecosystem to a change of key environmental parameters.In order to estimate the response of a mountain spruce forest to different atmospheric conditions during the summer a six-layer non-steady-state SVAT model (SLODSVAT) was used. Eight scenarios were used for modelling energy and mass exchange during an eleven day summer period, combining different combinations of microclimatic conditions. All atmospheric scenarios were examined for three various CO 2 mixing ratio levels: 350ppm (current condition), 450ppm and 550ppm. A scenario 0 assuming the current climatic features at different CO 2 contents was considered as well. Structural and physiological adaptation of the forest to the new atmospheric conditions were not taken into account. For all scenarios the modelling results show increased net CO 2 flux into the forest with increasing ambient CO 2 concentration. Maximum net CO 2 uptake was simulated for dry climate scenarios. Transpiration and evapotranspiration rates had similar trends independently of the ambient CO 2 concentration used: at cold and wet conditions they decreased, while at warm and dry conditions transpiration and evapotranspiration rates increased. The influence of CO 2 concentrations on transpiration rates is of minor importance if compared to changes of temperature, water vapour pressure, cloud amount and atmospheric precipitation as considered in this investigation.