Towards physiologically meaningful water-use efficiency estimates from eddy covariance data

Research output: Contribution to journalArticle

Abstract

Intrinsic water-use efficiency (iWUE) characterizes the physiological control on the simultaneous exchange of water and carbon dioxide in terrestrial ecosystems. Knowledge of iWUE is commonly gained from leaf-level gas exchange measurements, which are inevitably restricted in their spatial and temporal coverage. Flux measurements based on the eddy covariance (EC) technique can overcome these limitations, as they provide continuous and long-term records of carbon and water fluxes at the ecosystem scale. However, vegetation gas exchange parameters derived from EC data are subject to scale-dependent and method-specific uncertainties that compromise their ecophysiological interpretation as well as their comparability among ecosystems and across spatial scales. Here, we use estimates of canopy conductance and gross primary productivity (GPP) derived from EC data to calculate a measure of iWUE (G1, “stomatal slope”) at the ecosystem level at six sites comprising tropical, Mediterranean, temperate, and boreal forests. We assess the following six mechanisms potentially causing discrepancies between leaf and ecosystem-level estimates of G1: (i) non-transpirational water fluxes; (ii) aerodynamic conductance; (iii) meteorological deviations between measurement height and canopy surface; (iv) energy balance non-closure; (v) uncertainties in net ecosystem exchange partitioning; and (vi) physiological within-canopy gradients. Our results demonstrate that an unclosed energy balance caused the largest uncertainties, in particular if it was associated with erroneous latent heat flux estimates. The effect of aerodynamic conductance on G1 was sufficiently captured with a simple representation. G1 was found to be less sensitive to meteorological deviations between canopy surface and measurement height and, given that data are appropriately filtered, to non-transpirational water fluxes. Uncertainties in the derived GPP and physiological within-canopy gradients and their implications for parameter estimates at leaf and ecosystem level are discussed. Our results highlight the importance of adequately considering the sources of uncertainty outlined here when EC-derived water-use efficiency is interpreted in an ecophysiological context.

Details

Authors
  • Jürgen Knauer
  • Sönke Zaehle
  • Belinda E. Medlyn
  • Markus Reichstein
  • Christopher A. Williams
  • Mirco Migliavacca
  • Martin G. De Kauwe
  • Christiane Werner
  • Claudia Keitel
  • Pasi Kolari
  • Jean Marc Limousin
  • Maj Lena Linderson
Organisations
External organisations
  • Max Planck Institute for Biogeochemistry
  • Western Sydney University
  • Clark University
  • Macquarie University, Sydney
  • University of New South Wales
  • Albert-Ludwigs University Freiburg
  • University of Sydney
  • University of Helsinki
  • University of Montpellier
  • Friedrich Schiller University Jena
Research areas and keywords

Subject classification (UKÄ) – MANDATORY

  • Oceanography, Hydrology, Water Resources

Keywords

  • aerodynamic conductance, canopy gradients, eddy covariance, energy imbalance, intrinsic water-use efficiency, Penman–Monteith equation, slope parameter, surface conductance
Original languageEnglish
Pages (from-to)694-710
Number of pages17
JournalGlobal Change Biology
Volume24
Issue number2
Publication statusPublished - 2018 Feb 1
Publication categoryResearch
Peer-reviewedYes