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

Area of Science:

• Ecology
• Biogeochemistry
• Environmental Science

Background:

• Intrinsic water-use efficiency (iWUE) is crucial for understanding terrestrial ecosystem carbon and water exchange.
• Leaf-level measurements of iWUE have spatial and temporal limitations.
• Eddy covariance (EC) technique offers ecosystem-scale, continuous flux data but introduces uncertainties.

Purpose of the Study:

• To calculate ecosystem-level iWUE (G1, stomatal slope) using EC data from six forest sites.
• To assess six mechanisms causing discrepancies between leaf and ecosystem-level G1 estimates.
• To improve the ecophysiological interpretation of EC-derived iWUE.

Main Methods:

• Utilized EC-derived canopy conductance and gross primary productivity (GPP) to estimate ecosystem-level G1.
• Evaluated the impact of non-transpirational water fluxes, aerodynamic conductance, meteorological deviations, energy balance non-closure, GPP partitioning, and within-canopy gradients.
• Analyzed data from tropical, Mediterranean, temperate, and boreal forests.

Main Results:

• Energy balance non-closure was the primary source of uncertainty in G1 estimates, especially with inaccurate latent heat flux.
• Aerodynamic conductance effects on G1 were adequately represented by simple models.
• G1 showed lower sensitivity to meteorological variations and filtered non-transpirational water fluxes.

Conclusions:

• Accurate estimation of ecosystem-level iWUE requires careful consideration of EC data uncertainties, particularly energy balance closure.
• Understanding these uncertainties is vital for reliable ecophysiological interpretations and cross-ecosystem comparisons.
• Future research should focus on refining EC data processing to minimize biases in iWUE estimations.