Self-pruning

Trees that grow in isolation take on a different growth habit than those that grow in a forest: when trees are isolated, all branches (even those at the base of the trunk) remain alive and grow in length and diameter, whereas in a forest, the lower branches tend to dry out and fall off more or less quickly. This is determined by a matter of energy efficiency: trees grow through leaf photosynthesis, which allows them to produce nutrients from solar radiation; however, part of these resources is also needed to maintain the leaves themselves. If the leaves receive little solar radiation, the cost of maintaining them will exceed the resources they themselves can produce through photosynthesis; thus, through self-pruning, trees eliminate the lower branches—that is, the least efficient ones that receive the least solar radiation.