Midworld

Midworld is a planet blanketed by a single vast rain forest, ecosystem within ecosystem, where Born and his people have lived for generations high in the branches of an enormous tree, bonded in near-telepathic partnership with intelligent, photosynthetic creatures called furcots. When a man and woman from an off-world commercial venture crash nearby, Born and a fellow hunter guide them to safety through the jungle's many dangers -- only to discover that what the newcomers actually want is a rare, longevity-granting substance buried with the planet's dead, whatever the cost to the forest that makes it. An early standalone entry in Alan Dean Foster's Humanx Commonwealth series.