Question
We have two red sunset maple trees that are in their second year of planting.

They are approximately 16 feet tall.

They lose leaves at the ends of the top branches during the summer.  The top branches are then bare. They are watered and fertilized properly.  Do they require iron?

What could be causing this leaf loss?

Answer
The root system has not caught up with the foliage growth. The roots can just support so much foliage and if the foliage grows faster than the root system the tree will drop some of its foliage to compensate. This happen often with large trees that are planted usually in the first couple of years. It should out grow this. Iron is needed IF the color of the overall leaves are yellowing or turning pale. I do not think this tree needs iron. If you have not done so mulch around the tree with not more than 3 inches deep of organic mulch not piled up on the trunk. This will help hold moisture around the roots.