Unsaturated intercellular vapor pressure is relevant for leaf water heavy isotope enrichment

Abstract

AbstractLeaf intercellular vapor pressure (ei) can be unsaturated, but its effect on leaf water heavy isotope enrichment (LWE) has not yet been quantified. We evaluated the ecological relevance of unsaturatedeifor LWE, i.e., for leaf water oxygen-18 and deuterium enrichment, using data from a boreal forest stand and a large-scale dataset. Unsaturatedeican firstly affect LWE by directly decreasingeiin the Craig Gordon model (Mechanism 1), which leads to an increased influence of atmospheric vapor isotopic enrichment above source water (Δv), and a decreased influence of kinetic fractionation by diffusion through the stomata and boundary layer (εk). Unsaturatedeican secondly affect LWE by changing εk(Mechanism 2). To evaluate the effect of Mechanism 1 to LWE, we employed sensitivity tests on LWE model performance using varying measured intercellular relative humidity (RHcellular), or RHcellularfitted to observed LWE. To explore the effects of Mechanism 2 to LWE, we modified the calculation of εkand observed consequences to LWE predictions. Unsaturatedeiis relevant to LWE by Mechanism 1, since a lowered RHcellularnoticeably changed LWE predictions. It clearly improved deuterium predictions and conditionally improved oxygen-18 predictions. Isotope fractionation by Mechanism 2 is unlikely relevant to oxygen-18 and deuterium enrichment. Unsaturatedeimust now be recognized as a variable that introduces error to heavy isotope enrichment models and reconstructions from organic material, via Mechanism 1. We suggest a correction for unsaturatedeifor both oxygen-18 and deuterium enrichment using a variable RHcellularcalculated from atmospheric relative humidity.