Trees grow up because that is where the light is, and they need the light in order to continue growing. Like most plants, trees grow from seeds and have roots, stems, and leaves.
A seed sends its roots down into the ground when it begins to grow, to take minerals and water from the earth to help it grow. The seed keeps growing deeper, sending off new root branches in all directions as it seeks the water and minerals it needs.
After the root has started, a stem begins to grow up towards the surface, towards the light. The stem carries water and minerals into the light where all three work to form branches, leaves and, eventually, fruit.
The continuous branching and then producing of leaves is necessary because it is the leaves that make food out of sunshine to nourish the tree. Without seeds, soil, water and sunshine, trees would not grow.
Sequoia trees over 4,000 years old and 300 feet high are still alive and growing in the Northwestern United States!