While it is true that 'Arborescent' is an adjective, nothing stops a person from using a word newly, especially in terms of fantasy. We are talking about
Perky's use of Tolkien's imagined species of walking, talking trees, after all. In the primary world, we might not have humans in tree form, which the noun form would suggest, but we're talking subcreation here.
I see no reason why
Perky couldn't use Arborescent as a noun, especially since it implies a word play upon "Adolescent" which might seem suitable for
Perky. Well, I don't know really, but I like the idea of thinking that our
The Perky Ent is a teenager in perpetuum.
Animated Arborescent. It even includes the idea of perkiness!