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Abstract:
The terrestrial carbon and water cycles are intimately linked: the carbon cycle is driven by photosynthesis, while the water balance is dominated by transpiration, and both fluxes are
controlled by plant stomatal conductance. The ratio between these fluxes, the plant wateruse
efficiency (WUE), is a useful indicator of vegetation function.
WUE can be estimated using several techniques, including leaf gas exchange, stable isotope
discrimination, and eddy covariance. Here we compare global compilations of data for each
of these three techniques.
We show that patterns of variation in WUE across plant functional types (PFTs) are not
consistent among the three datasets. Key discrepancies include the following: leaf-scale
data indicate differences between needleleaf and broadleaf forests, but ecosystem-scale
data do not; leaf-scale data indicate differences between C3 and C4 species, whereas at
ecosystem scale there is a difference between C3 and C4 crops but not grasslands; and
isotope-based estimates of WUE are higher than estimates based on gas exchange for
most PFTs.
Our study quantifies the uncertainty associated with different methods of measuring
WUE, indicates potential for bias when using WUE measures to parameterize or validate
models, and indicates key research directions needed to reconcile alternative measures of
WUE.
