Trees

Why do trees have leaves and branches only on the top of there trunk? My dad said so the leaves can get the nutrients (food) from the sun. Then use photosynthesis, and use the nutrients. Maybe its so the trunk stays together and doesn't fall apart. I agree with my dad even thou some trees have leaves on the bottom, but they are cone shaped so the snow/rain easily falls off, but those trees are for more harsh weather. do bushes have a trunk? I still agree with my dad thou.