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

Abstract

Leaf 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 unsaturated ei 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 ei can firstly affect LWE by directly decreasing ei in the Craig Gordon model (Mechanism 1), which leads to an increased influence of atmospheric vapor isotopic enrichment above source water ({Delta}v), and a decreased influence of kinetic fractionation by diffusion through the stomata and boundary layer ({varepsilon}k). Unsaturated ei can secondly affect LWE by changing {varepsilon}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 (RHcellular), or RHcellular fitted to observed LWE. To explore the effects of Mechanism 2 to LWE, we modified the calculation of {varepsilon}k and observed consequences to LWE predictions. Unsaturated ei is relevant to LWE by Mechanism 1, since a lowered RHcellular 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 ei 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 ei for both oxygen-18 and deuterium enrichment using a variable RHcellular calculated from atmospheric relative humidity.