The Cretaceous physiological adaptation of angiosperms to a declining &amp;lt;i&amp;gt;p&amp;lt;/i&amp;gt;CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;: a modeling approach emulating paleo-traits

Julia Bres,Nicolas Viovy,Nicolas Vuichard,Pierre Sepulchre

doi:10.5194/bg-18-5729-2021

Abstract

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.

Highlights

Vegetation plays a pivotal role in the climate system as it controls water and energy fluxes at the interface between land surfaces and the atmosphere through albedo (Charney et al, 1975; Port et al, 2016; Brovkin et al, 2009), roughness length and evapotranspiration capabilities (e.g., Bathiany et al, 2010; Betts et al, 1997; Kleidon et al, 2000; Gibbard et al, 2005)
Leaf hydraulic and photosynthetic capacities have not been altered for the temperate needleleaf evergreen plant functional types (PFTs), as they correspond to gymnosperms
We show that the simulated vegetation cover, transpiration rate and water use efficiency are sensitive to the paleovegetation trait prescribed

Summary

Introduction

Vegetation plays a pivotal role in the climate system as it controls water and energy fluxes at the interface between land surfaces and the atmosphere through albedo (Charney et al, 1975; Port et al, 2016; Brovkin et al, 2009), roughness length and evapotranspiration capabilities (e.g., Bathiany et al, 2010; Betts et al, 1997; Kleidon et al, 2000; Gibbard et al, 2005). Evapotranspiration, i.e., the sum of soil evaporation, vegetation evaporation and vegetation transpiration fluxes, is a key term in the continental hydrological cycle and has been shown to control moisture convergence and convection and thereby precipitation patterns in the tropics either in present-day-like configurations (Sun and Barros, 2015; Fraedrich et al, 1999) or for past climates (e.g., Braconnot et al, 1999; Brovkin et al, 2006). As shown recently for the extinct vegetation of the late Paleozoic, fossil plants provide invaluable information regarding “paleo-traits” that can be in turn included in land surface models (White et al, 2020; Richey et al, 2021)

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