In temperate forests, determining when winter dormancy is induced and released, is highly challenging because these physiological changes are not visible and can so far only be derived statistically or experimentally. Yet, knowing these two transient phases and, more importantly, understanding how they are regulated by the climate is crucial to accurately predict how phenology will evolve in the future warmer climate as well as to improve carbon and water cycle models. Here we propose to track the progress of winter dormancy of six major European tree species using an innovative method -never applied for this purpose-, consisting in tracing the amount of isotopically labelled water that can be transferred to the buds during the whole dormancy period. The principle feasibility has been assessed with a pre-test showing promising results. If the new method proves to be reliable this innovative project will open new avenues for testing how climate warming is affecting winter dormancy.