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We analyzed  14 C,  13 C and  18 O isotope variations over a 50-year period in tree rings of  Quercus ilex  L. trees growing at a natural CO 2  spring in a Mediterranean ecosystem. We compared trees from two sites, one with high and one with low exposure to CO 2  from the spring. The spring CO 2  is free of  14 C. Thus, this carbon can be traced in the wood, and the amount originating from the spring calculated. The amount decreased over time, from about 40% in 1950 to 15% at present for the site near the spring, indicating a potential difficulty in the use of natural CO 2  springs for elevated CO 2  research. The reason for the decrease may be decreasing emission from the spring or changes in stand structure, e.g., growth of the canopy into regions with lower concentrations. We used the  14 C-calculated CO 2  concentration in the canopy to determine the  13 C discrimination of the plants growing under elevated CO 2  by calculating the effective canopy air  13 C/ 12 C isotopic composition. The trees near the spring showed a 2.5‰ larger  13 C discrimination than the more distant trees at the beginning of the investigated period, i.e., for the young trees, but this difference gradually disappeared. Higher discrimination under elevated CO 2  indicated reduced photosynthetic capacity or increased stomatal conductance. The latter assumption is unlikely as inferred from the  18 O data, which were insensitive to CO 2  concentration. In conclusion, we found evidence for a downward adjustment of photosynthesis under elevated CO 2  in  Q. ilex  in this dry, nutrient-poor environment.

Tracing carbon uptake from a natural CO 2 spring into tree rings: an isotope approach