Question

why trees are almost cylindrical?

Answer 1

because they must with stand air drag

Answer 2

the scientific idea here, IMHO, is evolution by natural selection

and the circular **cross-section** is just part of it.

growth in the vertical y direction creates a shadow that guarantees success vs other plant life forms.

and i'd argue from logic that it is really conical but curtailed.

but it is the cross-section of the trunk that presumably creates a symmetry that is equally effective against winds coming from all directions. otherwise, were the wind direction guaranteed, the genetic mutation that created an aerofoil that pointed into the wind direction would prevail.

random genetic mutation creates all the possibilities.

nature, ie the prevailing physical conditions, once again prefers circular shapes. so maybe the fact that the circle is the outcome when stuff happens randomly, here cell expansion, is also relevant, or even the key driver. stuff that has to expand, in a completely random process, has no sentience nor a preferred direction. spheres and circles result.

within itself, the tree needs to transpire to function; but all of that can happen within any cross-section. i think.

Answer 3

I wonder what the calculus of variations would have to say about the optimal shape for a tree over it's (long) life time and the long time span of evo .... ? But I won't spend long wondering about it ...

Answer 4

pretty clear to me that a cylinder is the strongest formation of tensile fibers that would support the greatest side load coming from any direction, trees are also somewhat harmonic so they oscillate in somewhat random directions as well.