The vertical axis has a different interpretation depending on the solution curve. Solutions are defined in terms of a nondimensional VPD ( V P D m/ g *), but to aid physical interpretation, the horizontal axis is additionally provided in terms of dimensionalized VPD assuming m=1/2 and g *=110Pa 1/2 (average of all plant functional type g 1). Solutions corresponding to inflection points between concave up and concave down ET‐VPD curves (equation (11)) for three specific scenarios. This response is otherwise challenging to assess in an environment where many processes coevolve together.Įcohydrology ecosystem modeling evapotranspiration land‐atmosphere interaction stomatal conductance vapor pressure deficit. These results, derived from previous literature connecting plant parameters to plant and climate characteristics, highlight the utility of our simplified framework for understanding complex land-atmosphere systems in terms of idealized scenarios in which ET responds to VPD only. The ET response varies due to (1) climate, with tropical and temperate climates more likely to exhibit a positive ET response to increasing VPD than boreal and arctic climates (2) photosynthesis strategy, with C3 plants more likely to exhibit a positive ET response than C4 plants and (3) plant type, with crops more likely to exhibit a positive ET response, and shrubs and gymniosperm trees more likely to exhibit a negative ET response. The theory suggests that depending on the environment and plant characteristics, ET response to increasing VPD can vary from strongly decreasing to increasing, highlighting the diversity of plant water regulation strategies. We use Penman-Monteith, combined with semiempirical optimal stomatal regulation theory and underlying water use efficiency, to develop a theoretical framework for assessing ET response to VPD. We examine which effect dominates the response to increasing VPD: atmospheric demand and increases in ET or plant response (stomata closure) and decreases in ET. While increased evapotranspiration (ET) in response to increased atmospheric demand seems intuitive, plants are capable of reducing ET in response to increased VPD by closing their stomata. Increasing vapor pressure deficit (VPD) increases atmospheric demand for water.
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