Atmospheric dryness reduces photosynthesis along a large range of soil water deficits

Zheng Fu,Philippe Ciais,I Colin Prentice,David Makowski,Paul C Stoy,Hui Yang,Ana Bastos,Xiangzhong Luo,Pierre Gentine,Tomohiro Hajima,Julia K Green

doi:10.1038/s41467-022-28652-7

Abstract

Both low soil water content (SWC) and high atmospheric dryness (vapor pressure deficit, VPD) can negatively affect terrestrial gross primary production (GPP). The sensitivity of GPP to soil versus atmospheric dryness is difficult to disentangle, however, because of their covariation. Using global eddy-covariance observations, here we show that a decrease in SWC is not universally associated with GPP reduction. GPP increases in response to decreasing SWC when SWC is high and decreases only when SWC is below a threshold. By contrast, the sensitivity of GPP to an increase of VPD is always negative across the full SWC range. We further find canopy conductance decreases with increasing VPD (irrespective of SWC), and with decreasing SWC on drier soils. Maximum photosynthetic assimilation rate has negative sensitivity to VPD, and a positive sensitivity to decreasing SWC when SWC is high. Earth System Models underestimate the negative effect of VPD and the positive effect of SWC on GPP such that they should underestimate the GPP reduction due to increasing VPD in future climates.

Highlights

Both low soil water content (SWC) and high atmospheric dryness can negatively affect terrestrial gross primary production (GPP)
Recent studies evaluating the importance of VPD and SWC in controlling GPP or canopy conductance (Gc) have produced conflicting results regarding the relative roles of these two drivers[3,4,5,6,7,8], leaving it unclear how a changing water cycle will impact the carbon cycle
Our aims are (1) to assess under what SWC and VPD conditions is GPP most negatively affected by droughts, (2) to test the hypothesis that GPP reduction induced by partial stomatal closure in response to decreasing SWC is partly compensated by increased photosynthetic rates to maintain carbon fixation, and

Summary

Introduction

Both low soil water content (SWC) and high atmospheric dryness (vapor pressure deficit, VPD) can negatively affect terrestrial gross primary production (GPP). Drought poses an increasing threat to people and ecosystems around the world[1,2] Both decreased soil water content (SWC) and increased atmospheric water demand (vapor pressure deficit, VPD) can negatively affect terrestrial gross primary production (GPP)[3,4,5,6,7,8]. Our aims are (1) to assess under what SWC and VPD conditions is GPP most negatively affected by droughts, (2) to test the hypothesis that GPP reduction induced by partial stomatal closure in response to decreasing SWC is partly compensated by increased photosynthetic rates to maintain carbon fixation, and (3) to evaluate whether Earth System Models (ESMs) capture the relative influence of VPD and SWC on GPP. We used five ESMs that reported daily output, with GPP, VPD, SWC, temperature and incoming shortwave radiation simulated by each model

Objectives

Methods

Results