Regulate their own growth by controlling the flow of sugars and hormones. Favor current or sub branches, with a bias to the base or crown.
Trees regulate growth with hormones. Hormones are molecules that flow through the branches and have an effect on the cells they reach. They are a means of communication, giving the sensory parts of the tree a way to tell the branches what to do.
Branch ends are the senses and the brains of the tree, and they use a hormone called auxin to tell the rest of their branch what to do. Auxin travels down the branch and slows the development of side buds, while the end itself grows full power. Creating smaller side branches and directing most energy to upward growth helps the tree grow taller and win the race for light. If you cut off the end of a branch, the hormone is gone and side buds will grow freely. When pruning a hedge for example, this cuts off all the branch ends and enables the many side buds to grow, resulting in dense foliage.
The scientific name for this effect is Apical Dominance. Simulate this effect with Favor Lead. The hormones travel down the branch, but at some point the concentration will be too low to stop side branches from growing, the Favor Lead Reach.
Each branch itself competes for light with all other branches through Avoid Shade. This value is added to Favor Lead according to how much a branch is shaded. So a shaded branch will shoot out, further suppressing its child branches in order to find light. Tree species all have their own strategies for finding light, and avoiding shade can work with that. Beech trees for example do the opposite with a negative shade avoidance. A brightly lit beech twig will have a high favor lead, and a shaded twig will allow its sides to grow much quicker. They stay low and harvest as much light as possible without investing in thick, long main branches. Once it get more light, for example when a neighboring tree dies, it shoots up with a high favor lead.
Favor Lead helps the tree get a good start in getting toward light. But when the tree grows older a different effect shows up. The hormone that suppresses side branches only travels a limited distance from the branch tip that created it. After that side branches can freely grow, and the tree now needs to favor the best performing branches, those with most leaves and those harvesting most light. These are the branches that will keep the tree healthy.