The complexity of forest ecosystems poses challenges in using tracers to study water resource(s) fluxes, especially when biological entities like trees can significantly alter their isotopic and chemical composition. Understanding these modifications is crucial for accurately tracing water fluxes through
tree water uptake and help competent authorities in the decision-making processes related to water allocation and reforestation strategies. This thesis proposes new methods for tracing water resources absorbed by trees, revolutionizing the common eco-hydrological approach based on comparing the
water isotopic composition of tree sap with that of potential water sources. For this purpose, new tracers, namely rare earth elements, have been employed to achieve a higher precision in identifying the absorbed water resources, and new methods for sampling tree sap have been developed. This multi-tracer approach allows for a more in-depth study of the isotopic and chemical composition of sap,
thereby enhancing our understanding of water dynamics in forests.