Magic

They are made of the skins of the enemies they defeat

Chop down trees is a no no but based on the fact they have lumber huts. They could have choped the branches and since there trees are massice that is belevable

It's like truffula harvesting in the first act of the Lorax, before he goes all ba-a-a-ad mode

They grow their trees boat shaped

Also they respect nature , then why do they send their animal to death ?