system
Given the following post, respond as if you're commenting on Reddit:
human
What causes a tree to split from its main trunk into multiple branches?

At what point does it do this and what causes it? I noticed that some same species trees were about the same size but their trunks split at different levels (I don't know the correct term for this). I assume it's something to do with spreading itself out but one tree splits twice as high up than the other.
gpt
Most trees that naturally have a single trunk display "apical dominance" where all lateral growth only sprouts from a central shoot (what we think of as a tree's trunk). This phenomenon occurs because the single bud at the very top of the tree or "apex" produces the plant hormone auxin (IAA) that supresses the growth of the lateral buds or "branches." 

As the tree grows and gets taller the apical bud gets further away from the branches below, this results in a decreased concentration of auxin which allows those buds to break dormancy and continue to grow out from the central stem. The easiest way to visualize this is to think of a spruce tree or christmas tree and how it naturally forms a sort of triange shape with one shoot sticking out the top and the branches growing out wider as you go down the trunk.

Now, what happens if the apical bud is destroyed? Maybe the wind broke it off, or it was eaten by insects, or it froze during the winter time. What happens then?

Well, if the apical bud is no longer there then it's not producing that hormone anymore. So how all those lateral buds that were supressed now break dormancy and have free reign. They grow very quickly, and many of them even begin to grow upwards. For a time, these branches compete for dominance and eventually one takes over or none of them take over and now you have a tree with two or more apical buds and therefore two or more trunks. Since trees cant control the weather or bugs or the wind, these forces affect the growth of the tree at different stages of growth and fairly randomly. That's why you will see trees splitting at different heights.

It's also important to remember there are other factors that affect a tree's growth- genetics, minor changes in soil, minor changes in nutrient availability, shade relative to nearby buildings or trees... these influences mean that two trees of the same species that are right next to each other are often very different in branching structure.