Global trends of tree-ring carbon isotope discrimination under rising atmospheric CO2 and changing climate

Rising atmospheric CO₂ concentrations are expected to stimulate plant carbon uptake (A) while also reducing transpiration via a decrease in stomatal conductance (g_s), resulting in an increase in the intrinsic water use efficiency (iWUE, i.e. the ratio of A to g_s). While there is overwhelming evidence of a secular iWUE increase in response to rising CO₂ over the 20th-21st century, the magnitude of changes in iWUE reported so far in the literature strongly varies across climatic regions and biomes. Moreover, increasing iWUE has not systematically been translated into tree growth increment at many forested ecosystems, challenging the CO₂ fertilization theory. There is thus a need to track down the key physiological and environmental mechanisms driving changes in iWUE.

Here we estimate the carbon isotopic discrimination (Δ¹³C) - defined as the difference between the stable carbon isotopic compositions (δ¹³C) measured in atmospheric CO₂ and in tree rings – from 147 tree-ring δ¹³C chronologies to: 1) investigate the physiological responses of woody C3 plants to increasing atmospheric CO₂ and, 2) disentangle climate vs CO₂ effects on A and g_s. We specifically study deviations of tree-ring Δ¹³C from the predicted Δ¹³C response to CO₂ as reconstructed from a recent meta-analysis of paleo and elevated CO₂ data. We identify the following: 1) negative deviations from the expected Δ¹³C for most records; 2) no apparent deviation from expected Δ¹³C or; 3) positive deviations, both in a small minority of records ; and 4) an apparent non-linear response with a switch from a more negative Δ¹³C deviations to a Δ¹³C-response consistent with predicted CO₂-effects. The widespread negative Δ¹³C  deviations are consistent with g_s having been reduced or A having not increased as much as expected for a given CO₂-driven stimulation of A. The presented global tree-ring data analyses suggest that a warmer and often drier climate have had a stronger effect on Δ¹³C compared to that of rising CO₂, and a substantial modulation of recent rises in iWUE by climate effects across the globe.