Field experiments using CO2 enrichment: a comparison of two main methods

The dramatic increase in global atmospheric carbon dioxide over the past century is hypothesized to have significant impacts on the earth system. To understand the effects of elevated CO2 on terrestrial ecosystems, two main methods have been used to simulate an increase of CO2 in a semi-controlled field setting: 1) Open Top Chambers (OTC); and 2) Free Air CO2 Enrichment (FACE). The OTC method has been applied to study the components of forest ecosystems at small scale by manipulating seedlings or isolated juvenile trees, but is not able to address ecosystem processes as a whole. For technical reasons, OTC cannot be used to consider scaling issues, interaction with the boundary layer, and competition among species. To address these issues FACE technology was developed. FACE enables longer-term studies in larger plots, and allows studies of plant processes such as leaf area and canopy development, canopy energy balance and canopy gas exchange. In this review, I synthesize results from literature, in particular from meta-analysis techniques applied either to OTC or FACE. The results are qualitatively similar: CO2 enrichment leads to reduced stomatal conductance and leaf nitrogen, and enhanced photosynthesis and production. However, photosynthesis and crop yield were lower in FACE experiments than OTC, while starch content was higher. These results provide support for ecosystem model simulations, and help fill the gap between individual plants, forest and regional ecosystem. Neither OTC nor FACE can provide a clear indication of the regional-scale feedbacks between atmosphere and vegetation that might be expected under elevated CO2. To address this issue, further research is needed.