• The CO₂ efflux of adult trees is supplied by recent photosynthates and carbon (C) stores. The extent to which these C pools contribute to growth and maintenance respiration (RG and RM, respectively) remains obscure. • Recent photosynthates of adult beech (Fagus sylvatica) and spruce (Picea abies) trees were labeled by exposing whole-tree canopies to ¹³C-depleted CO₂. Label was applied three times during the year (in spring, early summer and late summer) and changes in the stable C isotope composition (δ¹³C) of trunk and coarse-root CO₂ efflux were quantified. • Seasonal patterns in C translocation rate (CTR) and fractional contribution of label to CO₂ efflux (FLabel-Max) were found. CTR was fastest during early summer. In beech, FLabel-Max was lowest in spring and peaked in trunks during late summer (0.6 ± 0.1, mean ± SE), whereas no trend was observed in coarse roots. No seasonal dynamics in FLabel-Max were found in spruce. • During spring, the RG of beech trunks was largely supplied by C stores. Recent photosynthates supplied growth in early summer and refilled C stores in late summer. In spruce, CO₂ efflux was constantly supplied by a mixture of stored (c. 75%) and recent (c. 25%) C. The hypothesis that RG is exclusively supplied by recent photosynthates was rejected for both species.