I admit I have a fairly hard time imagining the Calabim as a Leaves worshipping civ, but the Barbarian ones? Seems to fit like a glove. Remember, Fellowship of Leaves does not change alignment. It is no pansy peace-mongering hippie approach to life, it is, as I understand/see it, simply a reverence for nature. Picture the Doviello with Leaves as a more simple nature-spirit worshipping tribal society, and then remember they are still Evil, so they probably emphasize the harsher truths of nature, survival of the fittest, the right of the strong and such. Seems to fit neatly into Charadon's "fluff" text, for example.
Just my two cents of course.