At eight different dates during the 2000 growing season, δ13C and δ18O were determined in the phloem of adult beech trees growing in natural beech stands in south-west Germany differing in stand density and local climate. In addition, stand transpiration, precipitation, photosynthetic active radiation, relative air humidity, water pressure deficit of the air, air and soil temperature, soil water potential, and sugar concentration of the phloem sap were determined directly and evaporation and canopy stomatal conductance were modelled. All parameters were related to δ13C. The study aimed to identify the time integral within which the δ13C of organic compounds transported in the phloem is an indicative measure of these environmental influences. δ13C of soluble carbon transported in the phloem was well correlated with mean stomatal conductance in a two-day integral prior to phloem sampling but did not depend on either light intensity or soil water availability. A strong positive relationship between δ13C and δ18O pointed to observed variation in δ13C of phloem sap being a result of variation in stomatal conductance. Bulk leaf δ13C was a poor indicator of changes in environmental conditions during the growing season. From these results we conclude that the analysis of δ13C in soluble carbon transported in the phloem is a reliable indicator of short-term changes in Ci/Ca. In contrast, the δ13C of structural carbon in beech foliage represents an integration of a range of factors that mask short-term influences responsible for Ci/Ca.