Leaf-to-ecosystem water use efficiency and asynchronous of CO2 and H2O vapor fluxes: A comparison between Californian dry and wet ecosystems on multiple timescales

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Siyan Ma, Environmental Sciences, Policy, and Management, University of California Berkeley, Berkeley, CA, Elke Eichelmann, School of Biology and Environmental Science, University College Dublin, Dublin, Ireland and Dennis D. Baldocchi, Environmental Sciences, Policy, and Management, University of California, Berkeley, CA

Background/Question/Methods
Water use efficiency (WUE), the amount of carbon assimilation with a unit water investment, is an index for evaluating carbon-water interactions of a leaf, canopy, or ecosystem. As CO2 and H2O vapor fluxes are measurable with eddy-covariance systems above an ecosystem, leaf-level WUE definitions are often quantified and discussed with ecosystem-level alternatives. However, the WUE definitions can cause different magnitudes, leading to conflict conclusions. It is also unclear on which timescale (i.e., diurnal, seasonal, or annual) ecosystem-level WUE values are comparable to the results based on leaf-level definitions in dry and wet conditions.
To understand WUE variations given extreme dry and wet extents, we conducted this study at a savanna-type dryland and a restored wetland in California. Based on the CO2 and H2O vapor fluxes measured with leaf-level gas exchange devices and EC towers, we compared instantaneous WUE (WUEET or WUET as water expense counted with total evapotranspiration (ET) or partitioned transpiration (T), respectively), intrinsic WUE (iWUE, a leaf-level definition with the focus on stomatal conductance, gs), and marginal WUE (uWUE, a maximized WUE based on an ecosystem-level algorithm reflecting the leaf-level stomatal optimization theory).
Results/Conclusions

We observed that annual precipitation at the two study sites were 526±226 mm during the study period, but annual transpiration at the wetland site could be up to 700-800 mm, two folds of that at the dry savanna. WUE values were unstable and could be misleading in dry conditions since an increase in the magnitude of WUE did not always imply more carbon assimilation with less water investment. In general, WUEET and WUET had similar diurnal and seasonal patterns, but the pattern disagreed with those suggested by iWUE and uWUE in dry conditions. Their interannual variations were also divergent. Asynchronous CO2 and H2O vapor fluxes suggested that transpiration be more sensitive to water deficit than photosynthesis. The insight will be useful for improving strategies of water use management to mitigating adverse effects of climate change on ecosystem function.