Rock fracture characteristics regulate water storage and seasonal tree water uptake in karst

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 structurally heterogeneous rock fractures. While some studies have highlighted the potential importance of fractured bedrock as a water source for plants, few have quantitatively assessed how fracture characteristics regulate water storage, residence times, and plant water uptake across seasons. 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 as a function of fracture properties. We use a four-compartment sampling framework that distinguishes between soil water (mobile and bulk) and rock water (fracture and infilled fracture). Our results show that fracture characteristics exert a primary control on seasonal tree water uptake patterns. During the peak growing season, mobile soil water (mean MRT =88 d) dominates uptake (mean contribution 41 %), whereas in late growing season, trees increasingly rely on bulk soil water (mean MRT =95 d, mean contribution 55 %). During the transition from the dry to the wet season, reliance on rock water increased in fracture-rich areas. In the reactivation stage, trees exhibited a mean rock-water contribution of 69 % (mean MRT =117 d), and in the subsequent early growing season, large trees derived up to 85 % of their water from rock, primarily from soil-filled fractures with apertures >10 mm that act as seasonal storage reservoirs with prolonged residence times (MRT =84–303 d). Trees preferentially access short-MRT sources under wet conditions and shift to longer-MRT pools during dry periods, demonstrating that seasonal water-use strategies are strongly regulated by fracture-controlled storage and connectivity. This study demonstrates that fracture characteristics play a central role in regulating rock water storage and tree water uptake in karst systems, providing new insights into vegetation resilience in structurally complex landscapes and implications for water resource management under changing climatic conditions.