Do stomata respond to variations in atmospheric humidity?

A growing literature argues that ecosystem-scale evapotranspiration is more sensitive to drying of the atmosphere because of stomatal regulation by plants than to reductions in surface soil moisture. Past studies analyzed observations, for which it is difficult to conclusively control for potential relations between plant physiology, measurable state variables like vapor pressure deficit (VPD) or soil moisture, and the ecosystem-scale water flux. Here, we analyze natural mechanism-denial experiments at non-vegetated but hydrologically active salt flats. At these sites, any apparent sensitivity of the ecosystem-scale surface conductance (a bulk measure of how the land surface influences evapotranspiration) to VPD cannot be due to stomatal closure. Over the salt flats, we find a relation between VPD and surface conductance similar to the one commonly attributed to stomatal closure, and reproduce similar relations using a parsimonious boundary layer model that excludes plants. We conclude that observational studies likely overstate the sensitivity of ecosystem-scale surface conductance to atmospheric drying and understate the importance of variations in surface soil moisture. This finding has broad implications for future ecosystems because anthropogenic trends in soil moisture are uncertain and spatially heterogeneous, whereas ubiquitous atmospheric drying is expected due to global warming.

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