Response of ecosystem intrinsic water use efficiency and gross primary productivity to rising vapor pressure deficit

Quan Zhang,Danielle Way,Stefano Manzoni,Richard P Phillips,Lixin Wang,Darren L Ficklin,Kimberly A Novick

doi:10.1088/1748-9326/ab2603

Abstract

Elevated vapor pressure deficit (VPD) due to drought and warming is well-known to limit canopy stomatal and surface conductance, but the impacts of elevated VPD on ecosystem gross primary productivity (GPP) are less clear. The intrinsic water use efficiency (iWUE), defined as the ratio of carbon (C) assimilation to stomatal conductance, links vegetation C gain and water loss and is a key determinant of how GPP will respond to climate change. While it is well-established that rising atmospheric CO2 increases ecosystem iWUE, historic and future increases in VPD caused by climate change and drought are often neglected when considering trends in ecosystem iWUE. Here, we synthesize long-term observations of C and water fluxes from 28 North American FLUXNET sites, spanning eight vegetation types, to demonstrate that ecosystem iWUE increases consistently with rising VPD regardless of changes in soil moisture. Another way to interpret this result is that GPP decreases less than surface conductance with increasing VPD. We also project how rising VPD will impact iWUE into the future. Results vary substantially from one site to the next; in a majority of sites, future increases in VPD (RCP 8.5, highest emission scenario) are projected to increase iWUE by 5%–15% by 2050, and by 10%–35% by the end of the century. The increases in VPD owing to elevated global temperatures could be responsible for a 0.13% year−1 increase in ecosystem iWUE in the future. Our results highlight the importance of considering VPD impacts on iWUE independently of CO2 impacts.

Highlights

Given that plants are constantly challenged to maximize carbon (C) uptake and minimize water loss by regulating stomata, an important metric of ecosystem sensitivity to climate is the intrinsic water use efficiency, defined here as the ratio of C assimilation (An) to stomatal conductance
Response of intrinsic water use efficiency to hydrologic stress As hydrologic stress developed at each site, we observed a nearly universal initial increase in iWUE that was primarily driven by rising vapor pressure deficit (VPD) (see figures 1(a)–(d) for representative sites with contrasting Dryness Index (DI); figures S4 and S5 in the SI for all sites)
Because the partitioning method did not strongly affect results when comparing figures S4 with S5 in SI, here and throughout, we focus on trends in gross primary productivity (GPP) estimated using the so-called ‘nighttime approach’ by Reichstein et al (2005)

Summary

Introduction

Given that plants are constantly challenged to maximize carbon (C) uptake and minimize water loss by regulating stomata, an important metric of ecosystem sensitivity to climate is the intrinsic water use efficiency (iWUE), defined here as the ratio of C assimilation (An) to stomatal conductance (gs). The iWUE reflects plants’ adaptability to changing environmental conditions (Farquhar et al 1982), and because it governs the coupling between carbon assimilation and stomatal conductance, it has received considerable attention (Keenan et al 2013, Lévesque et al 2014, Frank et al 2015, van der Sleen et al 2015). Increasing iWUE has been observed in many ecosystems. This phenomenon has largely been attributed to rising atmospheric CO2 concentrations (Franks et al 2013), a conclusion supported by both observational studies (Ainsworth and Long 2005, Battipaglia et al 2013, Keenan et al 2013, Mastrotheodoros et al 2017) and theoretical models (Knauer et al 2017b). While many factors could explain this disagreement, among them is the possibility that CO2 may not be the only slowly-evolving driver affecting iWUE

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