From Source to Sink: Integration and Alteration of Oxygen Isotope Signals during the Transfer from Precipitation to Leaf Water, Leaf Sugars, Twig Phloem Sugars into the Stem Phloem Sugars of Four Mature European Tree Species

Abstract:

Stable oxygen isotope ratios (δ¹⁸O) of stem cellulose record physiological and ecohydrological information and are increasingly being used for the reconstruction of past environments. Studies that have investigated the environmental and physiological drivers of δ¹⁸O values in tree ring cellulose have typically focused either on the source of the signal, e.g. the leaf and the water therein, or on the sink, e.g. the cellulose in the stem. In contrast, hardly any research has investigated the transfer of the δ¹⁸O signal from precipitation, to soil water, xylem water, leaf water, leaf sugars, phloem sugars all the way to cellulose in the tree ring. As such, critical uncertainties remain regarding the seasonal integration and precision by which precipitation and leaf water δ¹⁸O signals are recorded in the tree ring cellulose δ¹⁸O values. In our talk, we will present a unique three year dataset that shows the seasonal variation of δ¹⁸O values in precipitation, soil water, xylem water, leaf water, leaf sugars, twig and stem phloem sugars for four common European tree species, which are growing in a mature temperature Swiss mixed broadleaf/evergreen forest. This dataset allows us to assess, (i) to what degree the substantial seasonal variation in precipitation δ¹⁸O values influences the δ¹⁸O values of tree ring cellulose and (ii) if physiological and environmental δ¹⁸O signals imprinted on the tree’s leaf water δ¹⁸O values and the assimilates formed therein are altered on their way downstream to the tree stem. The new insight that we provide into the integration and possible alteration of δ¹⁸O signals along the leaf-stem pathway will contribute significantly to a better understanding of the environmental and physiological signals that can be obtained from tree ring δ¹⁸O chronologies. In addition it will be relevant for the incorporation and parameterization of tree ring isotope models into dynamic global vegetation models.