Carbon isotope composition of current-year shoots from Fagus sylvatica in relation to growth, respiration and use of reserves

Temporal variations in the stable carbon isotope composition (δ¹³C) of leaves and current‐year stems were examined in beech trees over one year. The δ¹³C of both tissues were equal in the bud stage and started to diverge during growth, with values decreasing by 2·5 and 4·5‰ for stems and leaves, respectively. The dynamics of the δ¹³C and content of non‐structural sugars were also assessed. The beginning of the growth period was characterized by a decrease in starch content and high starch δ¹³C values. Later in the season, the δ¹³C of leaf soluble sugars progressively decreased from the end of May and the δ¹³C of stem sucrose was at least 1·5‰ higher than that of leaves. The δ¹³C of CO₂ respired by stem tissue increased during stem growth and exhibited large seasonal variations ( from ?22·1 to ?26·3‰). These values generally fell between those of starch and total organic matter. The results of the study showed that the δ¹³C of stems is altered by two apparent fractionation steps: one during sugar transfer from leaves to stems and one during stem respiration. These results may have implications for analysis of isotopic signals in tree rings and forest ecosystems.