Photosynthetic Carbon Isotope Discrimination Increases with Elevated CO2 in a Grassland Ecosystem

Abstract:

Paleoecological reconstructions and land-surface ecosystem models assume that as atmospheric CO₂ changes, photosynthetic carbon isotope discrimination and the ratio of leaf internal to ambient CO₂ concentration (ci/ca) remains unchanged, but this assumption has rarely been tested. Atmospheric CO₂ concentrations are expected to rise to at least 600ppm by the end of the 21st century, with global temperatures and precipitation regimes concurrently changing and interacting to influence plant photosynthetic gas exchange. In the PHACE (Prairie Heating and CO₂ Enrichment) experiment, we examined the influence of elevated CO₂, warming, and summer irrigation on carbon isotope discrimination among C₃ and C₄ graminoid and dicot species to understand species and ecosystem responses to over 7 years of experimental climate change. Carbon isotope discrimination increased for all species when exposed to elevated CO₂, but this effect did not emerge until the third year of the experiment. In addition, though species inherently differed in their carbon isotope discrimination, their ranking remained stable under elevated CO₂. Quantifying the relative importance of factors that influence plant carbon isotope discrimination can guide our understanding of how individual plant species, plant communities, and ecosystems may react to global change. Evidence that plant carbon isotope discrimination shifts with changes in atmospheric CO₂ and temperature has important implications for interpretation of the paleo record from archival organic materials and for modeling carbon cycling processes in future environments.