Potted seedlings of Austrian spruce (Picea abies (L.) Karst.) provenances from high and low altitude were raised together in a nursery at 1100 m. After an acclimation time of 1 - 3 weeks in climate chambers at 8° and 24°C their gas exchange was measured at various temperatures. The temperature optimum for photosynthesis varies during the vegetation period and was highest in summer. It depends also on the vapor pressure deficit of the air. Low (8°) or high (24°) temperature pretreatment shifted the optimum modulatively to lower or higher temperature, high altitude provenances showing a greater acclimation potential. High altitude provenances had a genetically fixed lower temperature optimum than low altitude provenances in the range of 0.25 - 0.65°C per 100 m of altitude. Plants acclimated to 8° reduced net photosynthesis to about 50 % and doubled dark respiration compared to plants acclimated to 24°. Such acclimation reactions started within hours after a temperature change and were completed within a few days.