We aimed to quantify the separate effects of photosynthetic and postphotosynthetic carbon isotope discrimination on ~d13C of the fast-turn-over carbon pool (water soluble organic carbon and CO2 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~ d13C in total and water-soluble organic matter of the foliage plus ~d13C and ~d18O 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 ~d13C of stem-emitted CO2. In the canopy (foliage and twigs), the ~d13C of soluble organic matter varied diurnally with amplitudes of up to 1.9ppt. The greatest 13C 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. 13C 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. CO2 emitted from the stem was enriched by 2.3–5.2ppt compared with phloem organic matter. When day-to-day variation was addressed, water-soluble leaf ~d13C and twig phloem ~d18O were strongly influenced by ci/ca and stomatal conductance (Gs), respectively. These results show that both photosynthetic and postphotosynthetic carbon isotope fractionation influence ~d13C of organic matter over time, and over the length of the basipetal transport pathway. Clearly, these influences on the ~d13C of respired CO2 must be considered when using the latter for partitioning of ecosystem CO2 fluxes or when the assessment of ~d13C in organic matter is applied to estimate environmental effects in ci/ca.
