Where does the carbon in new spring leaves of a deciduous tree come from?
While I can't provide references to actual studies, simple observation suggests that it must be stored in the roots, at least in part.  That observation is the existence of maple syrup (and other tree syrups, such as birch: https://practicalselfreliance.com/trees-species-tap-syrup/ In the spring, before leaves start to grow, sugar-rich sap rises from the roots, and can be collected by tapping the tree.