Here's why:

* Biotic means something is living or was once living. Bark is produced by the tree, which is a living organism.

* Abiotic means something is non-living. While bark is a dead layer of the tree, it was produced by a living organism, making it biotic.

Think of it this way: while your skin is dead, it's still part of your living body. Bark is similar - a dead layer of the tree, but a crucial part of its living structure.