Comment by culi

It's basically a decentralized algorithm involving sugar defecits, auxin signalling, calcium flooding whenever cells stretch (due to gravity or wind), and ethylene release.

Trees can use all these signals to know which branches are being shaded or dealing with a lot of wind, which side it's being pulled towards more, etc.

Trees also optimize for their overall wind profile. Dropping branches that cause it more drag than they are worth in the sugar they make. The lower their wind profile, the lower the risk of the whole tree being uprooted during a storm.

When humans cut the wrong branches, it throws everything off balance.