When a leafless deciduous tree 'comes to life' in the spring and puts forth new leaves, where does the mass for these leaves come from? The tree has no leaves to make use of photosynthesis to get the carbon it needs to grow these leaves, so where does the carbon come from: has it been stored in the root system over winter just for this purpose? And if so, has enough been stored for the tree to produce most if not all of its new leaf growth for this season, or only enough to get a few leaves out that will 'take over' the task of providing carbon fixation (making sugars) via photosynthesis to grow the remaining new leaves?
It has been suggested in the comments that the new leaf buds were grown at the end of the previous season. It would seem to me that this strategy for new growth in the spring would be fraught with all sort of dangers from 'winter kill', so these buds would not remain viable come the next spring. So, are the sugars needed for new spring growth coming from the roots or last seasons leaf buds?