The Cretaceous physiological adaptation of angiosperms to a declining <i>p</i>CO₂: a modeling approach emulating paleo-traits

Abstract

The Cretaceous evolution of angiosperm leaves towards higher vein densities enables unprecedented leaf stomatal conductance. Still, simulating and quantifying the impact of such change on plant productivity and transpiration in the specific environmental conditions of the Cretaceous remain challenging. Here, we address this issue by combining a paleo proxy-based model with a full atmosphere–vegetation model that couples stomatal conductance to carbon assimilation. Based on the fossil record, we build and evaluate three consistent proto-angiosperm vegetation parameterizations under two end-member scenarios of pCO2 (280 and 1120 ppm) for the mid-Cretaceous: a reduction of leaf hydraulic or photosynthetic capacity and a combination of both, supported by a likely coevolution of stomatal conductance and photosynthetic biochemistry. Our results suggest that decreasing leaf hydraulic and/or photosynthetic capacities always generates a reduction of transpiration that is predominantly the result of plant productivity variations modulated by light, water availability in the soil, atmospheric evaporative demand and pCO2. The high pCO2 acts as a fertilizer on plant productivity that strengthens plant transpiration and water-use efficiency. However, we show that proto-angiosperm physiology does not allow vegetation to grow under low pCO2 because of a positive feedback between leaf stomatal conductance and leaf area index. Our modeling approach stresses the need to better represent paleovegetation physiological traits. It also confirms the hypothesis of a likely evolution of angiosperms from a state of low leaf hydraulic and photosynthetic capacities at high pCO2 to a state of high leaf hydraulic and photosynthetic capacities linked to leaves with more and more veins together with a more efficient biochemistry at low pCO2.