On geologic timescales, plant carbon isotope fractionation responds to precipitation similarly to modern plants and has a small negative correlation with pCO2

Leaf carbon isotope fractionation (Δleaf) is sensitive to environmental conditions and can provide insights into the state and evolution of leaf gas-exchange in response to climate and environment factors. In modern plants, water availability is the strongest environmental predictor of Δleaf across sites that experience relatively uniform and low concentrations of CO2 in the atmosphere (pCO2). Growth chamber experiments show Δleaf of modern...