The sugar maples, for example, store nutrients in their roots for the winter, when the tree doesn't need much food, since it is "hibernating", then when the weather gets warmer the root sugar flows upwards into the tree limbs to nourish the leaf buds until they get large enough to perform photosynthesis. (This flowing sugar is what is harvested for maple sugar). A similar process occurs in most trees, the difference being the amount of nutrients stored, and whether they go to growing leaves to start the photosynthesis process or to growing flowers to get pollinated to allow the tree to reproduce that way.
(If you look up maple sugar farms on the web, there are some that give a great description of the whole process and how it works.)
