I have a book (Photo Art, by Worobiec and Spence), which mentions sealing tree leaves in plastic, with a fiber print. In humid conditions, the leaves bacteria and fungi will begin to eat the gelatin, beginning the decay. According to the book, it will take a few weeks, and different leaves, different papers, and differing times will produce different effects. I don't know if this is what you have in mind, but it's worth a thought.