Consequences for assuming leaf vapor pressure saturation under drought conditions

In order to close the coupled water and energy balance required for modeling plant transpiration in soil-plant-atmosphere-continuum (SPAC) and earth system models, it is typically assumed that the vapor pressure inside leaf intercellular air space is at full saturation. Differences between saturated vapor pressure and true vapor pressure within the leaf tend to be small relative to the air vapor pressure deficit. It is thus often argued that these differences may be neglected under most conditions, so that the canopy vapor pressure deficit may be regarded as the driving force for transpiration. Yet evidence of significant leaf vapor pressure unsaturation (on the order of 80% in semiarid conifer species) has been identified when high air vapor pressure deficit maintains transpiration, for example due fixed cuticular conductance during drought conditions. Under these conditions, stomatal closure is not sufficient to halt transpiration, which continues through the cuticle, lowering leaf water potentials. Under this scenario, the assumption that leaf internal vapor pressure is at saturation may overpredict evaporative fluxes.

Here, we develop a SPAC model to quantify the potential error in transpiration prediction associated with unconstrained transpiration following drought-induced stomatal closure. We compare predicted transpiration under two different assumptions: the null hypothesis that leaf intercellular air space remains saturated under drought conditions, and the stomatal unsaturation hypothesis, in which leaf internal vapor pressure is in equilibrium with the liquid water inside the leaf tissues, finding discrepancies on the order of 10% between the two assumptions. We test the sensitivity of predicted transpiration to leaf cuticular conductance, air vapor pressure deficit, leaf water potential, and species-specific plant hydraulic limitations in order to assess the validity of assuming leaf vapor pressure saturation under drought conditions.