Environmental drivers of observed photosynthetic carbon isotope discrimination in trees

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. We investigated historical trends of  photosynthetic carbon isotope discrimination derived from carbon isotope measurements of tree rings (Δ¹³C) from a large set of chronologies across a variety of climate regions and biomes. We first predicted Δ¹³C response to CO₂ as reconstructed 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. In a second step we assessed the deviation of observed tree-ring Δ¹³C from the that predicted in response to CO2 only. We found that 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.