This study investigated the influence of meteorological, pedospheric and physiological factors on the water relations of Scots pine, as characterized by the origin of water taken up, by xylem transport as well as by carbon isotope
discrimination (~D13C) and oxygen isotope enrichment (~D18O) of newly assimilated organic matter. For more than 1 year, we quantified ~d2H and ~d18O of potential water sources and xylem water as well as ~D13C and ~D18O in twig and trunk phloem organic matter biweekly, and related these values to continuously measured or modelled meteorological parameters, soil water content, stand transpiration (ST) and canopy stomatal conductance (Gs). During the growing season,~ d18O and ~d2H of xylem water were generally in a
range comparable to soil water from a depth of 2–20 cm. Long residence time of water in the tracheids uncoupled the isotopic signals of xylem and soil water in winter. ~D18O but not ~D13C in phloem organic matter was directly indicative of recent environmental conditions during the whole year. ~D18O could be described applying a model that included 18O fractionation associated with water exchange between leaf and atmosphere, and with the production of organic matter as well as the influence of transpiration. Phloem ~D13C was assumed to be concertedly influenced by Gs and photosynthetically active radiation (PAR) (as a proxy for photosynthetic
capacity).We conclude that isotope signatures can be used as effective tools (1) to characterize the seasonal dynamics in source and xylem water, and (2) to assess environmental effects on transpiration and Gs of Scots pine, thus helping to understand and predict potential impacts of climate change on trees and forest ecosystems.
