Seasonal, daily and diurnal variations in the stable carbon isotope composition of carbon dioxide respired by tree trunks in a deciduous oak forest

The stable C isotope composition (delta(13)C) of CO(2) respired by trunks was examined in a mature temperate deciduous oak forest (Quercus petraea). Month-to-month, day-to-day and diurnal, measurements were made to determine the range of variations at different temporal scales. Trunk growth and respiration rates were assessed. Phloem tissue was sampled and was analysed for total organic matter and soluble sugar (13)C composition. The CO(2) respired by trunk was always enriched in (13)C relative to the total organic matter, sometimes by as much as 5 parts per thousand. The delta(13)C of respired CO(2) exhibited a large seasonal variation (3.3 parts per thousand), with a relative maximum at the beginning of the growth period. The lowest values occurred in summer when the respiration rates were maximal. After the cessation of radial trunk growth, the respired CO(2) delta(13)C values showed a progressive increase, which was linked to a parallel increase in soluble sugar content in the phloem tissue (R = 0.95; P < 0.01). At the same time, the respiration rates declined. This limited use of the substrate pool might allow the discrimination during respiration to be more strongly expressed. The late-season increase in CO(2) delta(13)C might also be linked to a shift from recently assimilated C to reserves. At the seasonal scale, CO(2) delta(13)C was negatively correlated with air temperature (R = -0.80; P < 0.01). The diurnal variation sometimes reached 3 parts per thousand, but the range and the pattern depended on the period within the growing season. Contrary to expectations, diurnal variations were maximal in winter and spring when the leaves were missing or not totally functional. By contrast to the seasonal scale, these diurnal variations were not related to air temperature or sugar content. Our study shows that seasonal and diurnal variations of respired (13)C exhibited a similar large range but were probably explained by different mechanisms.