Summary. CO2-and H20-gas exchange of 20-to 25-year-old spruce trees from a plantation in the Hunsriick mountains were investigated over a period of 3 years. All measurements were made as pair comparisons, i.e., in each case the gas exchange of a damaged tree and of a relatively healthy tree in its immediate vicinity was measured simultaneously. A second plantation in the Westerwald mountains consisted of 18-year-old apparently healthy spruce trees. Pair comparison at this location meant comparison of two healthylooking trees. The investigations at both locations included diurnal course measurements of photosynthesis and transpiration, and light saturation curves and CO2-saturation curves of photosynthesis. The reduced photosynthesis parameters of the phenotypically damaged trees at the HunsriJck location indicates massive damage to the photosynthetic apparatus. Measurements of H20-gas exchange showed that there are disturbances in stomatal regulation of the needles of damaged trees. As a result, the water use efficiency of these needles proved to be significantly lower. In addition, apparent photorespiration of the damaged trees was decreased, whereas their light-and CO2-compensation points and their dark respiration were increased. In contrast to the Hunsriick plantation, no such effects were detectable when the healthy-looking Westerwald trees were subjected to pair comparison of gas exchange. Reduced photosynthetic capacity and disturbances of the stomatal regulation of the phenotypically damaged Hunsrfick trees may be due to damage in the cellular membranes. Furthermore, a comOffprint requests to: A. Wild parison of three growing seasons led to the conclusion that the gas exchange of spruce trees in their natural habitat is markedly influenced by climatic conditions.

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