We actually have never had a problem with Jager against a tree. That first year we just worked at making sure he understood that the tree was off limits, and if we couldn't keep an eye on him then he was in his crate or another room away from the tree. Now he seems to understand that trees in the house are not his and he genenrally doesn't pay any attention to them.
Now my cat...she's another story!!!
Leah and the critters
Jager & Kat