Angiosperms leaf evolution and the Cretaceous continental hydrological cycle : accounting for paleotraits in paleoclimate numerical simulations

Land cover, and thereby vegetation, can alter climate through biogeochemical and biogeophysical effects. Specifically, plants mediate radiative and turbulent fluxes between the surface and atmosphere and contribute to defining temperature and precipitation patterns in continental areas. In recent decades, pioneering works based both on fossil records and climate modelling have shown that vegetation parameterization is pivotal for accurately simulating past climates. Here, we focused on the Cretaceous, during which the radiation of angiosperms was accompanied by a physiological revolution characterized in the fossil record by an increase in the density of leaf veins and, ultimately, an unprecedented rise in their stomatal conductance. Emulating such an evolution of leaf traits, quantifying their consequences on plant productivity and transpiration, and identifying the associated feedbacks in the Cretaceous climate is a very challenging task. We addressed this triple problem by embedding the reconstruction of physiological paleotraits from the fossil record within the IPSL-CM5A2 earth system model, which land surface scheme allows for the interaction between stomatal conductance and carbon assimilation.

We built and evaluated vegetation parameterizations accounting for the increase in stomatal conductance during angiosperm radiation, which is consistent with the fossil record, by altering the hydraulic and photosynthetic capacities of plants in a coupled fashion. These experiments, combined with two extreme atmospheric pCO2 scenarios, show that a systematic increase in transpiration is simulated when vegetation shifts from a proto-angiosperm state to a modern-like state, and that its magnitude is related to primary productivity modulated by light, water stress, and evaporative demand. Under a high pCO2 scenario, only stomatal conductance plays a role, and the feedback of vegetation change consists mainly of more intense water recycling and rainfall over the continents. At low pCO2, the effect of the high stomatal conductance on transpiration is enhanced by the development of vegetation cover, resulting in more transpiration and higher precipitation rates at all latitudes. Enhanced turbulent fluxes lead to a surface cooling that outcompete the warming linked to the lower surface albedo. Our results suggest a larger impact of angiosperms on climate when atmospheric pCO2 is decreasing, and stresses the importance of accounting for fossil-based paleotraits in paleoclimate simulations.