How beech trees use isotopically heavier precipitation because of seasonality

Soil-vegetation systems partition incoming precipitation into either the freshwater system ("blue water") or back to the atmosphere as evapotranspiration ("green water"). The isotope signatures of these fluxes are observed to be distinct. We investigate the partitioning of precipitation and illustrate one potential mechanism for this "apparent" isotopic fractionation of root water uptake by means of an isotope-enabled mechanistic water balance model [1].

Stable water isotope signatures were collected at a Swiss forest site dominated by beech trees (>60 cm DBH, 37m stand height) with a mean annual precipitation of ~1050 mm/year, at 800 m.a.s.l throughout two vegetation seasons. Up to bi-weekly samples of xylem and mobile soil water and six bulk soil water campaigns (down to 150 cm) were combined with continuous hydrometric measurements (down to 200 cm) to constrain modelled water fluxes such as preferential infiltration patterns and seasonal patterns of root water uptake.

During the model validation period in 2022, the model faithfully reproduced seasonal and vertical patterns in isotope signatures. The goodness of fits of time series showed δ¹⁸O RMSE smaller than 0.4‰ for mobile soil water at 50cm or 80cm or smaller than 1.0‰ for stem xylem water at breast height, while the goodness of fits of vertical profiles had δ¹⁸O RMSE smaller than 1.7‰ for bulk soil water profiles down to 150cm. Reduced soil water availability in the topsoil during the summer of 2021 led to a downward shift of the flux-weighted average water uptake depths of beech trees. However, while the relative contribution to water uptake of soil layers below 80 cm increased during the summer of 2021, their absolute contribution did not increase sufficiently to compensate the water missing in the topsoil layers where most roots are located. 

Modelled infiltration pathways and root water uptake illustrate how seasonal and vertical selectivity of root water uptake leads to distinct isotope signatures in the modelled green and blue water fluxes. This behaviour is obtained at this site without a two-domain representation of the soil domain nor preferential flow to deeper layers. Further, simulations with synthetic seasonal isotope patterns in precipitation demonstrate how this "apparent" fractionation factor depends on the timing of transpiration together with the seasonality of the precipitation isotope signature. In conclusion, this study highlights that the soil-vegetation system may fractionate heavier precipitation for the green water fluxes mainly because of the seasonal patterns in precipitation isotope signatures and transpiration rates.

[1] Fabian Bernhard. (2024). fabern/LWFBrook90.jl: v0.9.8 (v0.9.8). Zenodo. https://doi.org/10.5281/zenodo.10463109