Earth & Sky

Crooked Trees

DB: This is Earth and Sky with a question from a listener. “Why don’t tree branches grow perfectly straight toward the sun?”

JB: It might seem as if, for tree branches, the higher and closer to the sun, the better. Branches support a tree’s network of leaves that catch sunlight. And trees in the wild need sunlight for photosynthesis – it’s how they generate their energy. A branch will grow to give the most leaves the most light – and that might mean that some branches will grow sideways – and away from the leaf “pack”.

DB: There’re other factors that affect the way branches grow. Gravity constantly pulls them toward Earth. And branch growth is affected by 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, they are a living record of the tree’s overall survival strategy.

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

DB: Thanks today to the U.S. Forest Service and to the National Fish and Wildlife Foundation, supporting the conservation of native fish, wildlife, plants, and their habitats. We’re Block and Byrd for Earth and Sky.