Tracing carbon uptake from a natural CO2 spring into tree rings: an isotope approach

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Tree Physiology, 23:997–1004

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Tracing carbon uptake from a natural CO₂ spring into tree rings: an isotope approach

Matthias Saurer (1, 2), Paolo Cherubini (3), Georges Bonani (4) and Rolf Siegwolf (1)

1. Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland / 2. Author to whom correspondence should be addressed (matthias.saurer@psi.ch) / 3. Swiss Federal Research Institute WSL, CH-8903 Birmensdorf, Switzerland / 4. Institute of Particle Physics, ETH Hönggerberg, CH-8093 Zürich, Switzerland / Received November 22, 2002; accepted March 18, 2003; published online September 1, 2003

Summary

We analyzed ¹⁴C, ¹³C and ¹⁸O isotope variations over a 50-year period in tree rings of Quercus ilex L. trees growing at a natural CO₂ spring in a Mediterranean ecosystem. We compared trees from two sites, one with high and one with low exposure to CO₂ from the spring. The spring CO₂ is free of ¹⁴C. 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 CO₂ springs for elevated CO₂ 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 ¹⁴C-calculated CO₂ concentration in the canopy to determine the ¹³C discrimination of the plants growing under elevated CO₂ by calculating the effective canopy air ¹³C/¹²C isotopic composition. The trees near the spring showed a 2.5‰ larger ¹³C 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 CO₂ indicated reduced photosynthetic capacity or increased stomatal conductance. The latter assumption is unlikely as inferred from the ¹⁸O data, which were insensitive to CO₂ concentration. In conclusion, we found evidence for a downward adjustment of photosynthesis under elevated CO₂ in Q. ilex in this dry, nutrient-poor environment.

Keywords: carbon isotope ratio, dendrochronology, elevated carbon dioxide, oxygen isotope ratio, Quercus ilex, radiocarbon analysis.