Photosynthetic carbon isotope discrimination in trees: observations, predictions, and drivers

Category: Time:
Wednesday, September 13, 2023 - 12:00 to 13:00
Room: URL: Speaker:
Soumaya Belmecheri
Associate Professor, Laboratory of Tree-Ring Research, University of Arizona
Pamela Pelletier
Calendar Status:

Abstract: Under elevated CO2, photosynthetic carbon isotope discrimination is expected to increase in response to photosynthesis stimulation driven by the growth of atmospheric CO2. While this response is widely documented in laboratory, field experiments and short-term observations, long-term proxies indicate that such response is not universally observed in forested ecosystems. Here, the historical trends of  photosynthetic carbon isotope discrimination derived from carbon isotope measurements of tree rings (Δ13C) are investigated from a large set of chronologies across a variety of climate regions and biomes. The Δ13C response to CO2 is predicted from a recent meta-analysis of paleo and elevated CO2 data to detect and quantify the magnitude of Δ13C change-if any- driven solely by increases in atmospheric CO2. The deviation of observed tree-ring Δ13C from the predicted in response to CO2 only is then assessed. The majority of tree-ring chronologies (~80%) exhibited a negative deviations from the expected Δ13C if driven by a CO2 stimulation of photosynthesis (A). Chronologies with negative deviations were negatively correlated with vapor pressure deficit (VPD), and correspond to sites with a  maximum of 30% increase in VPD over the period of record. The widespread negative Δ13C deviations are consistent with a reduction of stomatal conductance (gs) or A having not increased as much as expected for a given CO2-driven stimulation of A. The documented trends in tree-ring records of carbon isotope discrimination, together with lower sensitivity of gross primary production recently reported in mature forests under elevated CO2 challenges the type and magnitude of tree’s response to increasing or elevated CO2 and its attribution as the driver of the terrestrial Carbon sink.