We aimed to quantify the separate effects of photosynthetic and postphotosynthetic carbon isotope discrimination on d 13 C of the fast-turn-over carbon pool (water soluble organic carbon and CO 2 emitted from heterotrophic tissues), including their diel variation, along the pathway of carbon transport from the foliage to the base of the stem.For that purpose, we determined d 13 C in total and water-soluble organic matter of the foliage plus d 13 C and d 18 O in phloem organic matter of twigs and at three heights along the stem of Pinus sylvestris over a nine-day period, including four measurements per day. These data were related to meteorological and photosynthesis parameters and to the d 13 C of stem-emitted CO 2 .In the canopy (foliage and twigs), the d 13 C of soluble organic matter varied diurnally with amplitudes of up to 1.9%. The greatest 13 C enrichment was recorded during the night/early morning, indicating a strong influence of starch storage and remobilization on the carbon isotope signatures of sugars exported from the leaves. 13 C enrichment of soluble organic matter from the leaves to the twig phloem and further on to the phloem of the stem was supposed to be a result of carbon isotope fractionation associated with metabolic processes in the source and sink tissues. CO 2 emitted from the stem was enriched by 2.3-5.2% compared with phloem organic matter. When day-to-day variation was addressed, water-soluble leaf d 13 C and twig phloem d 18 O were strongly influenced by c i /c a and stomatal conductance (G s ), respectively.These results show that both photosynthetic and postphotosynthetic carbon isotope fractionation influence d 13 C of organic matter over time, and over the length of the basipetal transport pathway. Clearly, these influences on the d 13 C of respired CO 2 must be considered when using the latter for partitioning of ecosystem CO 2 fluxes or when the assessment of d 13 C in organic matter is applied to estimate environmental effects in c i /c a .

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