Evaporative enrichment and time lags between δ¹⁸O of leaf water and organic pools in a pine stand

Understanding ecosystem water fluxes has gained increasing attention, as climate scenarios predict a drier environment for many parts of the world. Evaporative enrichment of ¹⁸O (Δ¹⁸O) of leaf water and subsequent enrichment of plant organic matter can be used to characterize environmental and physiological factors that control evaporation, based on a recently established mechanistic model. In a Pinus sylvestris forest, we measured the dynamics of oxygen isotopic composition (δ¹⁸O) every 6 h for 4 d in atmospheric water vapour, xylem sap, leaf water and water-soluble organic matter in current (N) and previous year (N-1) needles, phloem sap, together with leaf gas exchange for pooled N and N-1 needles, and relevant micrometeorological variables. Leaf water δ¹⁸O showed strong diel periodicity, while δ¹⁸O in atmospheric water vapour and in xylem sap showed little variation. The Δ¹⁸O was consistently lower for N than for N-1 needles, possibly related to phenological stage. Modelled leaf water Δ¹⁸O showed good agreement with measured values when applying a non-steady state evaporative enrichment model including a Péclet effect. We determined the time lags between δ¹⁸O signals from leaf water to water-soluble foliar organic matter and to phloem sap at different locations down the trunk, which clearly demonstrated the relevance of considering these time-lag effects for carbon transport, source-sink and carbon flux partitioning studies.