In William Faulkner's "the Sound and the Fury" why did Benjy associate Caddy with trees?

Caddy smelled like trees.
-- Benjy (Section 1 paragraph 10, pg. 6)

Importance: Benjy sees Caddy as innocent and pure. As long as she smells like trees, this is his Caddy. When she attempts to wear perfume and have relationships, she no longer smells like trees and loses her innocence.