About 10 years ago I left my wife's book on a plane. The book was probably published many years ago. It was about a world with nine vegetation levels above the ground. Each level is very different, and most are very dangerous. The residents of the planet, adapted humans, live in a symbiotic relationship with the trees. A human spaceship crashes at the top level of the world. One of the inhabitants of the planet aids the human in surviving and they travel through the vegetative levels together. I promised my wife I would replace the book, but never did since we don't know the author or title! Any help would be greatly appreciated.