Using natural abundances of stable water isotopes to constrain vertically distributed root water uptake of forest trees

The water balance in forest soils is strongly affected by vertical distribution of root water uptake. Our objective is to constrain the parametrization of root water uptake in the field by using the naturally occurring, seasonal variability in stable isotope signatures in precipitation to trace water fluxes through the soil and into the trees.

The 1D soil hydrologic model LWFBrook90.jl contains the necessary processes to accurately reproduce hydrometric observations of volumetric soil moisture content and soil matric potential at forest sites in Switzerland. Root water uptake is described with a gradient-driven model using vertically varying root density and moisture-dependent rhizosphere resistivities. The hydrologic model will be extended with transport and fractionation processes to enable the modeling of isotopic signatures in soil and tree water.

We present a planned field sampling campaign over two subsequent vegetation seasons at 10 long-term monitoring forest sites. Soil water is sampled with lysimeters at four soil depths, and tree water is sampled from the xylem with increment corers. Both types of samples are taken bi-weekly. First results from an ongoing multi-year soil water sampling campaign show that the signal can be traced along the soil profile and are presented to illustrate the approach.