Respiration and photosynthesis characteristics of current-year stems of Fagus sylvatica : from the seasonal pattern to an annual balance

• Temperature and light responses of CO 2 efflux of Fagus sylvatica (beech) currentyear stems were measured for 1 yr to estimate their annual carbon balance. • Gas exchanges were determined using infrared gas analysis. Seasonal patterns of a fluorescence parameter (( F v / F m ) max ), nitrogen and chlorophyll contents were also assessed in stems and leaves, using standard techniques. • Basal respiration rates at 20 ° C (R 20 ) were very high during the growing season, reaching a maximum of 17 170 μmol m − 3 s − 1 . Light-saturated assimilation followed the same seasonal pattern as R 20 . During the winter, chlorophyll content was undiminished compared with the summer, N content was slightly increased, and despite low ( F v / F m ) max values, instantaneous maximum assimilation could account for 80– 110% of the respiration. • For an average-size stem (4 mm diameter), the estimated annual respiration was 0.5 g carbon with 55% of this amount attributed to maintenance respiration. The potential assimilation contributed 0.2 g carbon and approximately compensated for the growth respiration. Information on older branches and trunks is now needed for estimations at the tree and stand levels.