The following is only my opinion based on nothing more than........my reading of the book so there!
The Worm is alive, Earthpower derives from the Worm, like sweat or even the salt found in sweat.
The "One Tree" is the Worms antler, much like a deer's.
Berek, used Earthpower to peacefully take a piece off of the antler to make the Staff of Law and the Worm didn't have a problem with that.
Breaking the "antler" like Sea Dreamer was trying to do was an act of violence. The Worm responded in kind.
The Guardian and Brinn are one in the same and have always been.
"Thus I am renewed"
I'm betting/hoping this will be explained in future books.
Berek knew what would happen at the One Tree, he was an oracle.
He knew that Covenant needed to see that self sacrifice was the answer.
He was an oracle because of some twisted Ceasure thing that Linden did, trust me.
Why did Vain's arm turn to wood when a Worm "star" struck it?
Foreshadowing .....and more.
The "star" was a power beyond Earthpower, yet not so beyond like Wild Magic, and when it hit Vain it transcended any restrictions or rules the ur-Viles and created within Vain. But because it was purposeless power it couldn't do any more than that.
According to SRD, that was the whole hidden yet crucial point of the whole Quest!
If you didn't know that don't be surprised, I didn't either.
SRD said so in an interview (GI) and also said it was a concept that he was surprised many readers didn't get! It was so clear to him that he didn't focus in on it more apparently.