The Key card represents “The Answer” – which also includes any Ultimate Answer. It is for this reason that the Key is often said to represent Fate in readings, something fixed, but an answer to be discovered and unlocked. The key has a sort of finality to it; the idea that there is one Truth out there for us, our Destiny, our personal Answer.
The Tree, on the other hand, represents energy and life force in itself. As such, it involves the universal and natural forces and energies that govern and underpin our lives—including karmic forces. The Tree is less about fate, destiny and a single answer like the Key is, and more about reaping what you sow in terms of energy.