From Lord of the Rings Spoof Page
Peregrin-Pip looks over there.
And at the water he does stare.
He sees a thing. The round thing glows.
He tip-toes over on his toes.
Itís a spying stone. Itís big and round.
He picks it up from off the ground.
TREEBEARD: Bless my bark! Oh dear! Oh dear!
I think I need to drink a beer!
It's merely an exclamation - a variant of "Bless my soul!" or "Bless my cotton socks". The author is thinking in terms of what a tree might say.