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 (delta(13)C) of leaves and current-year stems were examined in beech trees over one year. The delta(13)C of both tissues were equal in the bud stage and started to diverge during growth, with values decreasing by 2.5 and 4.5parts per thousand for stems and leaves, respectively. The dynamics of the delta(13)C 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 delta(13)C values. Later in the season, the delta(13)C of leaf soluble sugars progressively decreased from the end of May and the delta(13)C of stem sucrose was at least 1.5parts per thousand higher than that of leaves. The delta(13)C of CO2 respired by stem tissue increased during stem growth and exhibited large seasonal variations (from -22.1 to -26.3parts per thousand). These values generally fell between those of starch and total organic matter. The results of the study showed that the delta(13)C 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.