The role of rock fractures on tree water use of water stored in bedrock: Mixing and residence times

Abstract. The processes of tree water uptake in karst environments are poorly understood. One of the main challenges to improved understanding is the complex interaction between soil water and bedrock water, especially in systems characterized by rock fractures. While some studies have highlighted the potential importance of fractured bedrock as a water source for plants, few have quantitatively assessed the sources, residence times, and seasonal dynamics of tree water uptake from both soil and rock fracture compartments. Here, we combine stable isotope tracing, a Bayesian mixing model (MixSIAR), and hydrometric monitoring to quantify the contributions and mean residence times (MRT) of soil and rock water accessed by trees across seasons. We use a four-compartment sampling framework that distinguishes between soil water (mobile and bulk) and rock water (fracture and infilled fracture). Our results reveal clear seasonal shifts in plant water sourcing: during the peak rainy season, mobile soil water (mean MRT = 88 days) dominates uptake (mean contribution 41 %), whereas in late growing season, trees increasingly rely on bulk soil water (mean MRT = 95 days, mean contribution 55 %). Strikingly, in early spring, trees in fracture-rich areas exhibit the highest reliance on rock water (mean MRT = 113 days, mean contribution 69 %). During the subsequent early growing season, large trees derive up to 85 % of their water from rock, particularly from soil-filled fractures with apertures >10 mm, which act as transitional reservoirs capable of retaining precipitation for extended periods (MRT = 84–303 days). Trees preferentially access short-MRT sources under wet conditions and shift to longer-MRT pools during dry periods, reinforcing the concept of ecohydrological separation between tightly bound and dynamically recharged water pools. However, this separation is attenuated during periods of high precipitation due to increased hydraulic connectivity and water mixing. This work advances our understanding of vegetation resilience in structurally complex and hydrologically dynamic karst landscapes with important insights for sustainable water resource management under changing climatic conditions.