The study aims to assess variability in leaf water isotopic enrichment occurring in the field under natural conditions. We focused on seasonal variation and difference between sun-exposed and shaded leaves. Isotopic composition (δ18O, δ2H) of leaf water was monitored in a beech tree (Fagus sylvatica L.) growing in the forest-meadow ecotone together with δ18O (2H) of water compartments which are in close relation to this signal, namely twig and soil water. The sampling was carried out in approximately two-week intervals during five consecutive vegetation seasons. The δ18O (2H) data showed a distinct seasonal pattern and a consistency in relative differences between the seasons and sample categories. Leaf water was the most isotopically enriched water compartment. The leaf water enrichment decreased toward the autumn reflecting the change in δ18O (2H) of source water and evaporative demands. The soil and twig water isotopic signal was depleted against current precipitation as it partly retained the isotopic signature from winter precipitation however the seasonal pattern of soil and twig water followed that of precipitation. No significant differences between sun-exposed and shaded samples were detected. Nevertheless, the observed strong seasonal pattern of isotope composition of leaf, twig and soil water should be taken into account when using leaf water enrichment for further calculations or modeling.

Institute of Plant Molecular Biology Academy of Sciences of the Czech Republic Branišovská 31 CZ 37005 České Budějovice Czech Republic

University of Hradec Králové Faculty of Science Rokitanského 62 CZ 50003 Hradec Králové​ Czech Republic

University of South Bohemia Faculty of Science Branišovská 31 CZ 37005 České Budějovice Czech Republic

VSB Technical University of Ostrava Institute of Environmental Technology 17 listopadu 15 CZ 70833 Ostrava Czech Republic

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