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

Knauer, Jürgen; Zaehle, Sönke; Medlyn, Belinda E.; Reichstein, Markus; Williams, Christopher A.; Migliavacca, Mirco; De Kauwe, Martin G.; Werner, Christiane; Keitel, Claudia and Kolari, Pasi, et al. (2018) In Global Change Biology 24(2). p.694-710
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... (More)

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.

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published
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aerodynamic conductance, canopy gradients, eddy covariance, energy imbalance, intrinsic water-use efficiency, Penman–Monteith equation, slope parameter, surface conductance
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Global Change Biology
volume
24
issue
2
pages
17 pages
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Wiley-Blackwell
external identifiers
  • scopus:85041375135
ISSN
1354-1013
DOI
10.1111/gcb.13893
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English
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yes
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9b177af0-d85b-4720-9e6c-df0f0f619373
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2018-02-12 08:05:20
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2018-02-13 03:00:11
@article{9b177af0-d85b-4720-9e6c-df0f0f619373,
  abstract     = {<p>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 (G<sub>1</sub>, “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 G<sub>1</sub>: (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 G<sub>1</sub> was sufficiently captured with a simple representation. G<sub>1</sub> 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.</p>},
  author       = {Knauer, Jürgen and Zaehle, Sönke and Medlyn, Belinda E. and Reichstein, Markus and Williams, Christopher A. and Migliavacca, Mirco and De Kauwe, Martin G. and Werner, Christiane and Keitel, Claudia and Kolari, Pasi and Limousin, Jean Marc and Linderson, Maj Lena},
  issn         = {1354-1013},
  keyword      = {aerodynamic conductance,canopy gradients,eddy covariance,energy imbalance,intrinsic water-use efficiency,Penman–Monteith equation,slope parameter,surface conductance},
  language     = {eng},
  month        = {02},
  number       = {2},
  pages        = {694--710},
  publisher    = {Wiley-Blackwell},
  series       = {Global Change Biology},
  title        = {Towards physiologically meaningful water-use efficiency estimates from eddy covariance data},
  url          = {http://dx.doi.org/10.1111/gcb.13893},
  volume       = {24},
  year         = {2018},
}