PARENT SESSION
Oral Session #62: Forest Carbon Dynamics.
Presiding: R. Norby
Wednesday, August 7. 1:00 PM to 4:45 PM. Greenlee Meeting Room, TCC.

Ecological relationship between carbon isotope ratios and atmospheric CO₂ concentrations in terrestrial ecosystems.

LIN, GUANGHUI^*,1, VAN HAREN, JOOST¹, PIERCE, DANIELLE, YAKIR, DAN², BERRY, JOSEPH A.³, ¹ Biosphere 2 Center, Oracle, Arizona² Weizmann Institute of Science, Rehovot, Israel³ Department of Global Ecology, Stanford, California

ABSTRACT- The relationship between carbon isotope ratios and atmospheric CO₂ concentrations has been used extensively to investigate plant carbon/water relations at the ecosystem scale. However, the potential ecological information that can be inferred from this relationship during night-time and daytime has not been studied experimentally. In this study, we attempted to address this issue by taking advantage of the mass balance capability of an enclosed rainforest mesocosm (2000 m²) inside Biosphere 2. Net ecosystem exchange (NEE), concentrations, and carbon isotope ratios of atmospheric CO₂ were measured periodically during the times when the rainforest mesocosm was sealed and exposed to various CO₂ concentrations and water regimes. Our results indicate that the relationship between carbon isotope ratios and atmospheric CO₂ concentrations during night-time, as revealed in so-called Keeling plots, can provide information regarding the relative contributions of two distinct respiration components (e.g. soil vs. canopy respiration) to the overall ecosystem respiration. Similarly, this relationship during the daytime can provide values for the actual carbon isotope discrimination at the ecosystem level, which can be used to partition NEE into photosynthetic and respiration components. Partitioning ecosystem CO₂ fluxes is critical for understanding ecosystem responses to environmental change, thus the relationship between carbon isotope ratios and atmospheric CO₂ concentrations provide a useful technique to investigate interactions between terrestrial ecosystem processes and global change, but it is important to distinguish this relationship from nighttime to daytime.

KEY WORDS: carbon isotopes, carbon dioxide, terrestrial ecosystems, carbon/water relations