Why do trees drop their leaves in autumn?

If you ever studied natural sciences, you sure know that farmers do not cultivate all their land. Every season, they leave some piece of land without crops and fertilize it; in order to help the soil recover the nutrients future crops will need.

Well, farmers probably learnt to do this from trees. When trees drop their leaves in fall, they do it to have the dead leaves decompose around them, restore the nutrients in the soil, and to use those nutrients for the next spring, until they grow new leaves and continue the cycle.

Nature is wise, isn't it?

Knowledge + Why?