Assessing environmental and physiological controls over water relations in a Scots pine (Pinus sylvestris L.) stand through analyses of stable isotope composition of water and organic matter

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 (Δ¹³C) and oxygen isotope enrichment (Δ¹⁸O) of newly assimilated organic matter. For more than 1 year, we quantified δ²H and δ¹⁸O of potential water sources and xylem water as well as Δ¹³C and Δ¹⁸O 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 (G_s). During the growing season, δ¹⁸O and δ²H 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. Δ¹⁸O but not Δ¹³C in phloem organic matter was directly indicative of recent environmental conditions during the whole year. Δ¹⁸O 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 Δ¹³C was assumed to be concertedly influenced by G_s 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 G _s of Scots pine, thus helping to understand and predict potential impacts of climate change on trees and forest ecosystems.