Carbon and oxygen isotope composition of organic compounds in the phloem sap provides a short-term measure for stomatal conductance of European beech (Fagus sylvatica L.)

At eight different dates during the 2000 growing season, delta(13)C and delta(18)O 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 delta(13)C. The study aimed to identify the time integral within which the delta(13)C of organic compounds transported in the phloem is an indicative measure of these environmental influences. delta(13)C 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 delta(13)C and delta(18)O pointed to observed variation in delta(13)C of phloem sap being a result of variation in stomatal conductance. Bulk leaf delta(13)C was a poor indicator of changes in environmental conditions during the growing season. From these results we conclude that the analysis of delta(13)C in soluble carbon transported in the phloem is a reliable indicator of short-term changes in C-i/C-a. In contrast, the delta(13)C of structural carbon in beech foliage represents an integration of a range of factors that mask short-term influences responsible for C-i/C-a.