The answer to this question can be found in the epilogue of this great novel. As Mae and Tuck return to Treegap once more after a gap of about seventy years, they are shocked to see that the wood has vanished and that the whole area has been bulldozed. They decide to stop in a diner to see if they can gain some news or information about what has happened during the interim and why this transformation has occurred. They speak to the counterman in the diner who gives them the following information:
Had a big electrical storm, though, about three years ago now or thereabouts. Big tree got hit by lightning, split right down the middle. Caught fire and everything. Tore up the ground, too. Had to bulldoze her all out.
Thus it is that Mae and Tuck find out about what happened to the tree and to the spring that was so important to them and gave them such longevity. The identity of the person who tells them this information is therefore an anonymous counterman who works in the diner.