From what I understand about trees, they should be spread apart far enough that their canopy does not overlap the tree next to it. This would be the ideal spacing. When they are closer together, they have to compete for the same nutrients. When they are super close together, they choke each other out and become very weak. Once the tree is in a weakened state, it becomes easy for bugs and bacteria to attack it.
This is very similar to planting a vegetable garden too close together and not removing the plants as they fill out.