Modeling Leaf-Level Transpiration: Exploring the Consequences of Assumed Saturated Vapor Pressure in Leaves

Understanding the dynamics of water transport through leaf intercellular airspaces (IAS) and its impact on transpiration is crucial for accurate predictions of plant water use and ecosystem response to changing climates. This study investigates the implications of assuming undersaturation of water vapor in the IAS for transpiration predictions and explores potential modifications to standard modeling approaches. A dynamic 1D soil-plant-atmosphere continuum using a stomatal optimization model (SPAC-SOT) framework was used to simulate the response of tree species, P. edulis, to prolonged drought and varying environmental conditions. Comparisons between two model assumptions (saturated vs. undersaturated IAS) reveal notable differences in transpiration and carbon assimilation predictions, particularly when soil moisture and vapor pressure deficit (VPD) are low. Assuming saturation in the IAS leads to an overprediction of transpiration and carbon assimilation as compared with the undersaturated model, suggesting the need for accurate representations of the leaf area index (LAI) and xylem characteristics such as maximum hydraulic conductance (Kp,max) to capture embolism and mortality risk, as well as canopy-level water vapor fluxes influencing global climate conditions. Through sensitivity analysis, it is found that certain plant parameters significantly influence the maximum rate of water loss through transpiration, highlighting the model’s sensitivity to these factors. The LAI parameter emerges as a key factor affecting transpiration under both saturated and undersaturated conditions, while the Kp,max parameter plays a vital role in carbon assimilation. Finally, cuticular conductance, gcut, strongly affects the extent to which model results diverge. The study underscores the importance of leaf-level assumptions in shaping larger-scale water cycles, particularly in the context of ongoing droughts and evolving climates. By refining modeling approaches to incorporate undersaturation in the IAS, our under-standing of plant water use efficiency, drought tolerance, and ecosystem responses can be advanced, aiding in more accurate predictions of future water resources and climate dynamics