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Why do open-grown trees have branches lower on their trunks than forest trees?

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Tall giant sequoia trees with reddish bark and thick trunks rise closely together in a dense forest at Sequoia National Park, California. — Tree A stand of giant sequoia trees (*Sequoiadendron giganteum*) in Sequoia National Park, California.

Open-grown trees and forest trees exhibit different growth patterns primarily due to the intensity of competition for resources, particularly light and space.1

Open-grown trees: In environments such as parks or gardens where competition is minimal, trees have access to light along the entire length of their trunks. Consequently, they typically retain foliage and branches extending down the length of the bole for a considerable distance.1
Forest trees: In a forest, trees must compete intensely for growing space and light. As the trees grow, the lower branches are often shaded by neighboring trees. This lack of light leads to the death or suppression of lower branches, a process known as natural pruning. As a result, forest trees generally feature a foliage-free trunk below a more limited, elevated crown.21

Essentially, the shape of a tree is an ecological construct; while open-grown trees utilize available space to maximize their foliage distribution, forest trees adapt to the crowded conditions by concentrating their growth in the canopy where light is more accessible.1