Even if they were made out of all wood, a walking tree isn't considered a "beast"
Beasts are soft bodied creatures that deal piercing and are weak to piercing.

Now, as far as a tree being weak to piercing goes:
Go outside with a sharp pointy object, like a fork, and stab that tree that you see outside.
How'd that work out for yah?
Fork break? Probably. How much force did it take? My guess is alot.

So now just imagine you're a little knight, that fork is a Flourish you brought as your only weapon, which is now bent in half, and that tree is very, very angry.

Bad situation. You've broken alot of bones, your skull is now a fine powder and there are bandages on your ears to keep brain matter from leaking out.

But Dibsville... the lumbers are weak to fire, volcanic pepperbox is amazing for them..

I'm still kinda "What was OOO thinking we only have 2 types of beasts (dust bunnies witheld), and like 7 different kinds of everything else".

Everytime I go to CW tunnels and it's bast themed, when turrets come up it's always slimes. SLIMES!
The pierce weapons I want to use against mindless woofies are worthless against the SLIMES that show up in the same place.
Who thought this was a good idea? Just because they both do pierce?

I think it would make more sense to have beasts + undead/fiends together to make use of gear like Radiant Silvermail (which still needs a helm counterpart).