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

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 CO 2 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 CO 2 impacts.

Plant responses to rising vapor pressure deficit

“…As a result, iWUE will saturate or even decline as VPD continues to rise. Thus, the overall relationship between iWUE and VPD is likely hyperbolic (Zhang et al , 2019), and the sensitivity of photosynthesis to VPD will likely be weaker than the sensitivity of conductance to VPD.…”

High Vapor Pressure Deficit Decreases the Productivity and Water Use Efficiency of Rain‐Induced Pulses in Semiarid Ecosystems

“…In response to increasing VPD, plants decrease stomatal conductance to reduce water loss (Buckley, 2019;Collatz et al, 1991;Leuning, 1995). These plant responses can decrease ecosystem-scale canopy conductance, GEP, and ET (Anthoni et al, 1999;Ding et al, 2018;Grossiord et al, 2020;Novick et al, 2016;Sulman et al, 2016;Wharton et al, 2009;Zhang et al, 2019). Additionally, physical manifestations of high VPD (warmer temperatures; increased evaporative demand) may influence ET by increasing rates of soil and interception evaporation and may stimulate temperature-dependent autotrophic and heterotrophic respiration during periods of sufficient water availability.…”

North American temperate conifer (Tsuga canadensis) reveals a complex physiological response to climatic and anthropogenic stressors

“…Increasing temperatures under climate change will influence atmospheric water demand and soil moisture differently (Novick et al ., 2016; Ficklin & Novick, 2017) and, thus, greater understanding of the influence on these variables on g s is needed (e.g. Yi et al ., 2019; Zhang et al ., 2019). Trends over time in dominant climate variables were characterised through linear regression analysis and differences among periods were examined through analysis of variance.…”