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

Temperature and light responses of CO2 efflux of Fagus sylvatica (beech) current-year 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 20degreesC (R-20) were very high during the growing season, reaching a maximum of 17 170 mumol 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.