Unless we're not

[frindle2]

Hello. I don't understand exactly the last sentene in the context below. Would you help me? Thank you.

The speaker is a tree(wishtree) and Bongo is a crow, the speaker's best friend.
"I" want to help a lonely girl who wishes for a friend. "I" want to grant her wish.
Bongo doesn't want "me" to be a busybody. Bongo doesn't think they can make it happen.


-------from the children's book, <Wishtree> by Katherine Applegate
Bongo had been right. I was hatching an idea.
She'd always said I was a busybody, not to mention an optimist.
An optimistic buttinsky.
Well, there were worse things.
Trees are the strong, silent type.
Unless we're not.

 

[Tarheel]

It is not exactly a sentence. In any case, it means this tree is not the strong, silent type. Because it's a talking tree.