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

Leaf intercellular vapor pressure ( e i ) can be unsaturated, but its effect on leaf water heavy isotope enrichment (LWE) has not yet been quantified. We evaluated the ecological relevance of unsaturated e i for LWE, i.e., for leaf water oxygen-18 and deuterium enrichment, using data from a boreal forest stand and a large-scale dataset. Unsaturated e i can firstly affect LWE by directly decreasing e i in the Craig Gordon model (Mechanism 1), which leads to an increased influence of atmospheric vapor isotopic enrichment above source water (Δ v ), and a decreased influence of kinetic fractionation by diffusion through the stomata and boundary layer (ε k ). Unsaturated e i can secondly affect LWE by changing ε k (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 (RH cellular ), or RH cellular fitted to observed LWE. To explore the effects of Mechanism 2 to LWE, we modified the calculation of ε k and observed consequences to LWE predictions. Unsaturated e i is relevant to LWE by Mechanism 1, since a lowered RH cellular noticeably 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. Unsaturated e i must 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 unsaturated e i for both oxygen-18 and deuterium enrichment using a variable RH cellular calculated from atmospheric relative humidity.