its all in the genetics ... in nature ...

but once you add the horticultural industry ... you add all kinds of variables.. for trees when young ...

think about every pine tree at bigboxstore.... shaped like an xmas tree ...when in fact ... in nature ... most dont have that shape in any sense at all ... surely not at 6 or 8 feet ...

but sooner or later.. genetics .. given time... they all revert to natural form ...

and of course... all us gardeners.. who like to prune things.. into bizarre shapes ... lol... especially .... if its damage repair ...

i am surprised i bothered to answer.. considering you mentioned that other plant...

ken

the environment shapes a tree.....how the tree responds is a matter of genetics however.....an oak that grows in an open field will be short and stocky...to keep stable in strong winds....an oak in a forest will be tall with a thinner trunk....to complete with neighboring trees and also because it does not need the wide thick trunk since wind is not a major factor in the middle of a forest.

I do know that in many species the "terminal bud" produces hormones that discourage other buds from becoming "terminal buds" and encourages branching rather then massive vertical growth in other buds.

I know in many species when the tree isn't getting enough light the tree branches less and grows upward more.

I know buds produce hormones that modulate root growth.

I know that in a forming human embryo, certain cell produce hormones that attract certain other kinds of cells or tell neighboring stem cells to become certain other kinds of cells.

I suspect plants have a complex system of hormones released by buds,some in response to sunlight, that determine the tree's shape. They probably "know" if they have a second trunk based on shade and the hormones released by buds on that trunk.

I wonder if you could test this? Find three identical double trunked trees. Take one tree, use mirrors to direct sunlight to simulate the removal of a trunk. Take another, and remove all the buds on the second trunk. See if either grow differently then your control.

tlbean2004, I totally know what you mean, I make some strange posts out of boredom, or wanting a totally new point of view. I agree with the genetics answer, but, I have seen more American hollies with 3-4 trunks than single trunks, but the shape is the same, anyhow. I will mention though, that sometimes trees do seem to take on a different growth pattern, or at least seem to in youth. I have 2 Quercus Robur (English oak) trees I started from seed (acorn) from the same tree. One is short and has wildly growing branches and a "general dissarray" of growth. The other acorn brought forth a tree that is more upright, and nearly 3X the height of the "general dissarray" seedling. This taller tree is very balanced and near perfectly formed oak, shaped tree. In the distant future, maybe they will look more the same, but now there is a real noticable difference. On a funnier comparison, I don't watch the Kardashians, but anyone could notice, nobody but Kim has that kinda tushie. It was a random genetic thing, it coulda came from her great grama, who knows. I don't see any other girls in the family sportin that much junk in the trunk. poaky1

I have read that some trees are "site plastic" also. I THINK that means that besides the branch structure, sun exposure, full sun vs a woodlot, that leaves vary by whether the tree is in full sun, part shade or lots of shade.