The woods near the spring do not change.
In this story, we learn the Tucks are immortal because they drank from a spring in the woods near Winnie’s house. We also find out that not only have the Tucks not changed, but neither have the woods.
"It hadn't changed, no more'n we had," said Miles. "And that was how we found out. Pa'd carved a T on the tree, remember, twenty years before, but the T was just where it'd been when he done it. That tree hadn't grown one whit in all that time. It was exactly the same. And the T he'd carved was as fresh as if it'd just been put there." (Ch. 7)
The Tuck family had no idea they would become immortal when they drank from the spring. They were just thirsty. The Tucks later realized they could not die or appear to grow older. They were stuck. This meant they had to live an isolated life because anyone who spent too much time around them would realize they were different.
The Tucks try to keep the spring in the woods quiet so no one else has to go through what they do. The fact that the woods never change is interesting; it helps explain how the spring affects the area around it. The woods are not immortal, though; when the Tucks return later, the woods are gone.
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. (Epilogue)
This seems to be nature’s way of correcting for the anomaly. The Tucks may be immortal, but no one else ever will be. When the woods were destroyed, the spring was destroyed along with them. This should give the Tucks a measure of peace, since they no longer have to protect everyone from the spring.