Why don’t tree branches grow straight up?

A tree branch’s job is to provide a way for tree leaves to act as a net for sunlight.

Tree branches will grow to give the most leaves the most light, even if that means growing sideways. Trees need light for photosynthesis, which is how green plants generate their energy.

There are other factors that affect the way branches grow as well. Gravity pulls the branches downward. And branch growth is affected by the wind. Part of the trade-off any tree has to make is between gathering light, staying stable in the wind, and succeeding against nearby competitors. So when branches grow crookedly, that’s part of a tree’s overall survival strategy.

Trees have sensors that detect light and gravity. From the moment a tree begins its life, it knows which end is up. Trees will generally attempt to grow toward the light and away from gravitational pull. But, as a tree gets older, its branches tend to grow more outwards than upwards. That’s so the tree can cast a wider net to catch the light of the sun.

Our thanks to:
Dr. Robert B. Jackson
Assistant Professor
Department of Botany
Nicholas School of the Environment
Duke University
Durham, NC