It is perhaps strange that bark could be referred to as hollow. It’s generally extracted in strips, or patches, but nothing that could act as a container. Indeed there would need to be an entire tree without a centre to make it hollow, and that’s exactly the case here.
When the withering began it was souls that suffered first, torn away and dragged into the underworld to be consumed by the cursed dragon. Yet, as time drew on, Imaru’s hunger did not diminish, only grow, and so the world itself began to suffer.
Things taste worse in Fellhollow. Dreams are vapid and empty, if you have them at all. A campire feels colder, even if it warms you just as well as before. Because the substance, the soul of that thing if you will, is being consumed.
Hollow bark comes from a tree that has been hollowed out from within. Its insides less than they should be, weaker and wrong. As awful as this is, it makes the bark develop in strange ways desirable by skilled mages with a poor sense of fashion.