The carbon isotope composition of CO2 respired by trunks: comparison of four sampling methods.

The (13)C natural abundance of CO(2) respired by plants has been used in the laboratory to examine the discrimination processes that occur during respiration. Currently, field measurements are being expanded to interpret the respiration delta(13)C signature measured at ecosystem and global levels. In this context, forests are particularly important to consider as they represent 80% of the continental biomass. The objective of this investigation was to compare four methods of sampling the CO(2) respired by trunks for the determination of its carbon isotope composition: three in situ methods using chambers placed on the trunk, and one destructive method using cores of woody tissues. The in situ methods were based either on a Keeling plot approach applied at the tissue level or on an initial flush of the chamber with nitrogen or with CO(2)-free air. In parallel, we investigated the possibility of an apparent discrimination during tissue respiration by comparing the delta(13)C signature of the respired CO(2) and that of the organic matter. The study was performed on six tree species widely distributed in temperate and mediterranean areas. The four methods were not significantly different when overall means were considered. However, considering the individual data, the Keeling plot approach and the nitrogen flush methods gave fairly homogeneous results, whereas the CO(2)-free air method produced more variable results. The core method was not correlated with any of the chamber methods. Regardless of the methodology, the respired CO(2) generally was enriched in (13)C relative to the total organic matter. This apparent enrichment during respiration was variable, reaching as much as 3-5 per thousand. This study showed that, on the whole, the different sampling techniques gave similar results, but one should be aware of the variability associated with each method.

Respiration and photosynthesis characteristics of current-year stems of Fagus sylvatica: from the seasonal pattern to an annual balance.

• Temperature and light responses of CO2 efflux of Fagus sylvatica (beech) current-year stems were measured for 1 yr to estimate their annual carbon balance. • Gas exchanges were determined using infrared gas analysis. Seasonal patterns of a fluorescence parameter ((Fv /Fm )max ), nitrogen and chlorophyll contents were also assessed in stems and leaves, using standard techniques. • Basal respiration rates at 20°C (R20 ) were very high during the growing season, reaching a maximum of 17 170 µmol m-3  s-1 . Light-saturated assimilation followed the same seasonal pattern as R20 . During the winter, chlorophyll content was undiminished compared with the summer, N content was slightly increased, and despite low (Fv /Fm )max values, instantaneous maximum assimilation could account for 80-110% of the respiration. • For an average-size stem (4 mm diameter), the estimated annual respiration was 0.5 g carbon with 55% of this amount attributed to maintenance respiration. The potential assimilation contributed 0.2 g carbon and approximately compensated for the growth respiration. Information on older branches and trunks is now needed for estimations at the tree and stand levels.

Between-tree variations in leaf δ13C of Quercus pubescens and Quercus ilex among Mediterranean habitats with different water availability.

In this study, sun leaf carbon isotope composition (δ13C) of two co-occurring woody Mediterranean species (Quercus pubescens Willd., a deciduous oak, and Q. ilex L., an evergreen one) was investigated on four sites with different water availability. The total range of δ13C values was 4.4 and 3.1‰ for Q. pubescens and Q. ilex respectively. The intra-site variability was about 3‰. Total mean per species was equal. There were significant differences among sites, but at each site means of δ13C were not significantly different between species. A simple physiological model predicts no difference in intrinsic water-use efficiency (WUEi) between evergreen and deciduous oaks. The relationship between site means of δ13C and water parameters suggests that there is a leaf functional adjustment with respect to available water resource. No correlation was found between δ13C and the contents of any mass-based biochemical constituent. Nevertheless there was a significant correlation between δ13C and leaf mass per area of Q. ilex. For both species, there is also a positive correlation between leaf δ13C and individual crown area, i.e. a structural characteristic at tree level. Causal relations between δ13C and plant-environment interactions are discussed.