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The CO2 and H2O gas exchange of young beech trees (Fagus sylvatica L.) were measured over a growing season. Of particular interest was the adaptation of gas exchange to the low level of photon flux density in the understorey of the old beech. The recorded diurnal courses were subdivided into several classes of irradiance. The most frequent class was from only 30–40 µE * m-2 * s-1. Even at the highest irradiance values, no light saturation in assimilation occurred. The light compensation point lies below 3 µE * m-2 * s-1, because net dark respiration values are very low. Calculated from the initial slope of the light response curves a mean value of 0.02 mol CO2 * mol photons-1 shows a very efficient use of light be the young trees. At the optimal phase of assimilation, the relationship between the daily sum of irradiance and net photosynthesis is highly significantly correlated. Under the local climatic situation, the stomatal opening primarily depends on irradiance. In response to a change in irradiance, stomatal opening also changes rapidly. Therefore, there is only a loose relationship between transpiration rate and vapour pressure saturation deficit. Towards autumn, the transpiration coefficient (E/A-ratio, estimated under light saturation) increases strongly because net photosynthesis decreases simultaneously.