Proxy-Derived photosynthetic carbon isotope discrimination in trees since the Industrial revolution: observations, predictions and drivers.

Under elevated CO₂, photosynthesis stimulation is a widely documented response in laboratory, field experiments and short-term observations, however, multi-decadal proxy records indicate that such response is not universally observed in forested ecosystems, nor it is attributable to secular growth of atmospheric CO₂. We investigated photosynthetic carbon isotope discrimination derived from carbon isotope measurements of tree rings (Δ¹³C) as a proxy of tree’s physiological response to changes in atmospheric CO₂. We aimed to characterize temporal trends and environmental drivers of Δ¹³C since the onset of the Industrial revolution. We used a large set of chronologies across a variety of climate regions and biomes covering the period from 1900 to 2012 CE.

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.