There are many reasons why tree shapes differ. A tree growing in poor soil may be stunted due to lack of nutrients, and a tree growing right next to an apartment building may have more leaves on the side facing the sun. Different kinds of trees have their own unique form, but the form that any tree has is also affected by the environment where it grows. Some tree species grow quite tall and receive much sunlight.
Many trees collect sunlight that is filtered through the leaves of taller trees.
These shaded understory trees survive by gathering indirect sunlight or sun flecks that break through openings in the canopy. A rounded crown seems to work best for gathering filtered, understory sunlight, which comes from many different directions. The shape of the tree's crown also has a lot to do with where it lives. Nearer to the equator, the noontime sun is almost directly overhead all year. Up nearer to the Arctic circle, the sun is never directly overhead and is usually quite low in the sky.