Tropism

Just like giraffes adapted to reach their food source, trees are tall to reach the sunlight. Tropism is defined as the way a plant moves or grows based on a stimulus. And although most people don't think of trees as "moving" upward, they are growing that way for a reason. There's a reason the tallest trees (like Secoyas) live the longest. There's a reason the leaves are at the top. And there's a reason trees tower over other plants. That reason is sunlight.