Enhancing above-belowground coupling for predictive modelling of tree water stress under deep soil water depletion

An unseen threat of increasing drought stress in forests emerges from below ground: the lack of deep soil water storage refilling between seasons. However, the reliance of trees on deep soil water is not well understood. Depth-dependent root water uptake (RWU) can be estimated with stable water isotopes, though these estimates are prone to uncertainty and the measurements are typically sparse and labor-intensive to collect. Additionally, soil water potential sensors can provide estimates of tree water stress but are also limited in vertical resolution. Soil-vegetation-atmosphere-transfer (SVAT) models can fill this gap and provide a mechanistic link between soil water availability and tree stress. Currently, SVAT models are limited in their ability to describe plant-level water status through leaf water potential or stem water storage. In this project, we enhance an existing SVAT model (LWFBrook90.jl) with plant water capacity and capacitance to simulate diurnal and seasonal variation in plant water pools. These sub-daily cycles of stem shrinkage and refilling are effectively captured by high-precision, point dendrometers, which measure micrometer-scale stem radius variations, and derived through tree water deficit (TWD) which can serve as a drought stress proxy. By comparing modelled plant water storage to TWD, we benefit from a simple, integrated measure of tree water stress that allows the partitioning of root water uptake into transpiration and refilling plant stores. We apply the updated model to a field site in Valais, Switzerland, located within a dry inner-alpine valley, where a long-term irrigation experiment has been ongoing in the Pfynwald, a 100-year-old Scots pine forest. Multiple field campaigns have collected a suite of observational data for model calibration, including soil water content and potential, soil and xylem isotopes, sap flow, and tree water deficit. Model results indicate that peak transpiration is sourced from an average of 50 cm depth, while deeper water sources are unable to compensate for late-summer water demand, contributing minimally to RWU. Irrigation considerably modified the ecosystem water balance and shifted root water uptake to shallow layers in the top 20 cm, while the legacy effects of irrigation after it was stopped show an alleviation of stress that allows more efficient RWU from deep soils, consistent with sustained root investment developed under long-term irrigation.