Nocturnal transpiration causing disequilibrium between soil and stem predawn water potential in mixed conifer forests of Idaho

摘要

Soil water potential (Ψ_s) is often estimated by measuring leaf water potential before dawn (Ψ_pd), based on the assumption that the plant water status has come into equilibrium with that of the soil. However, it has been documented for a number of plant species that stomata do not close completely at night, allowing for nocturnal transpiration and thus preventing nocturnal soil?plant water potential equilibration. The potential for nighttime transpiration necessitates testing the assumption of nocturnal equilibration before accepting Ψ_pd as a valid estimate of Ψ_s. We determined the magnitude of disequilibrium between Ψ_pd and Ψ_s in four temperate conifer species across three height classes through a replicated study in northern Idaho. Based on both stomatal conductance and sap flux measurements, we confirmed that the combination of open stomata and high nocturnal atmospheric vapor pressure deficit (D) resulted in nocturnal transpiration in all four species. Nocturnal stomatal conductance (g_s-noc) averaged about 33% of mid-morning conductance values. We used species-specific estimates of g_s-noc and leaf specific conductance to correct Ψ_pd values for nocturnal transpiration at the time the samples were collected. Compared with the unadjusted values, corrected values reflected a significantly higher Ψ_pd (when D > 0.12 kPa). These results demonstrate that comparisons of Ψ_pd among species, canopy height classes and sites, and across growing seasons can be influenced by differential amounts of nocturnal transpiration, leading to flawed results. Consequently, it is important to account for the presence of nocturnal transpiration, either through a properly parameterized model or by making Ψ_pd measurements when D is sufficiently low that it cannot drive nocturnal transpiration. Violating these conditions will likely result in underestimation of Ψ_s.