Seasonal patterns of carbon allocation to respiratory pools in 60‐yr‐old deciduous ( Fagus sylvatica ) and evergreen ( Picea abies ) trees assessed via whole‐tree stable carbon isotope labeling

Summary• The CO 2 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 ( R G and R M , respectively) remains obscure. • Recent photosynthates of adult beech ( Fagus sylvatica ) and spruce ( Picea abies ) trees were labeled by exposing whole‐tree canopies to 13 C‐depleted CO 2 . Label was applied three times during the year (in spring, early summer and late summer) and changes in the stable C isotope composition (δ 13 C) of trunk and coarse‐root CO 2 efflux were quantified. • Seasonal patterns in C translocation rate (CTR) and fractional contribution of label to CO 2 efflux ( F Label‐Max ) were found. CTR was fastest during early summer. In beech, F Label‐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 F Label‐Max were found in spruce. • During spring, the R G 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, CO 2 efflux was constantly supplied by a mixture of stored ( c. 75%) and recent ( c. 25%) C. The hypothesis that R G is exclusively supplied by recent photosynthates was rejected for both species.