If you look at any natural forest, you will find that most trees grow to the same height then spread out their branches until they are more or less touching. They do this to soak up as much of the sunlight as possible. The result is to make a more or less closed leaf layer at the top of the forest. It is like a natural canopy and so these trees make the forest canopy.

The canopy varies in height between different parts of the world. The highest canopy is in the tropical rainforest, where the canopy is 20-30m from the ground.