Short‐term variation in the isotopic composition of organic matter allocated from the leaves to the stem of Pinus sylvestris : effects of photosynthetic and postphotosynthetic carbon isotope fractionation

We aimed to quantify the separate effects of photosynthetic and postphotosynthetic carbon isotope discrimination on δ 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 δ 13 C in total and water‐soluble organic matter of the foliage plus δ 13 C and δ 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 δ 13 C of stem‐emitted CO 2 . In the canopy (foliage and twigs), the δ 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 δ 13 C and twig phloem δ 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 δ 13 C of organic matter over time, and over the length of the basipetal transport pathway. Clearly, these influences on the δ 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 δ 13 C in organic matter is applied to estimate environmental effects in c i / c a .