Almost all of a big tree is dead, and the dead part is doing the work. The pipes that carry water from the roots to the leaves are built from cells that finish growing, take apart their own insides, and die, leaving hollow tubes laid end to end. The leaves draw the water up through them as one unbroken thread, and a living cell, full of its own machinery, would only be in the way. Further in, the older rings stop carrying water at all and become heartwood, there just to hold the thing up, which is why an oak can be hollow from root to crown and still come into leaf every spring. What is properly alive and growing is a sheath a few cells thick just under the bark, spending its whole life laying down the next layer of the dead.