Combining high-resolution in-situ water isotope measurements with 1D soil hydraulic and transport modelling to understand root water uptake dynamics in a mixed forest ecosystem

Soil-plant interactions and root water uptake are important factors of the soil-plant-atmosphere continuum. Characterizing root water uptake dynamics in different forest stands can help to predict water stress due to climate change. Governing factors that define tree root water uptake are environmental conditions, soil hydraulic properties, species-specific rooting patterns as well as intra- and interspecific competition. As spatial-temporal patterns of root water uptake cannot be measured directly, simplified assumptions are often either based upon homogenous soil conditions or on a static root water uptake profile. This study combines high-resolution in-situ measurements of water stable isotopes of xylem and soil moisture with a set of 1D soil hydrological and transport models of the vadose zone (Hydrus 1D) to investigate dynamics in root water uptake patterns of two competing tree species in three different forest stands. 

We measured in-situ water isotopic signature in different soil depths as well as in spruce and beech xylem continuously for two vegetation periods (2021-2022) including two artificial tracer experiments, a wet period in 2021 and a drought period in 2022. Three stands were compared: i) pure beech, ii) pure spruce and iii) mixed beech and spruce, measuring sap flux, leaf water potential, soil moisture, matric potential, throughfall and stemflow. In order to address different factors of soil heterogeneity, the vadose zone is represented by a set of models covering the range of measured soil conditions. Each model is calibrated against soil water content as well as isotope measurements and subsequently related to sap flux measurements. This allows for separating effects of soil heterogeneity and to analyze the interplay of a) stand specific factors, particularly different rooting distributions, interception, and throughfall patterns, and b) soil specific factors, particularly different hydraulic conductivity and plant available water fractions under changing environmental conditions. Results show that this interplay between soil and stand specific factors is crucial under dry conditions, while soil specific factors are of minor importance under wet conditions. This contribution will present and discuss results from this data-driven modelling study and provide information about how well processes are represented in these kind of models.