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 (Delta C-13) and oxygen isotope enrichment (Delta O-18) of newly assimilated organic matter. For more than 1 year, we quantified delta H-2 and delta O-18 of potential water sources and xylem water as well as Delta C-13 and Delta O-18 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, delta O-18 and delta H-2 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. Delta O-18 but not Delta C-13 in phloem organic matter was directly indicative of recent environmental conditions during the whole year. Delta O-18 could be described applying a model that included O-18 fractionation associated with water exchange between leaf and atmosphere, and with the production of organic matter as well as the influence of transpiration. Phloem Delta C-13 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.