If you go into a deciduous forest and look up you will see what appears at first glance to be a complex tangle of of branches. This is the forest canopy, where the trees compete for light.
On closer inspection, however, one finds a remarkable feature. This is that very few of the branches from any given tree rub on the branches from its neighbors.
In fact it appears to me that much of the complexity in the way the branches have grown is specifically to avoid rubbing on a neighbor. Rubbing is dangerous for a tree, because it creates the equivalent of an open wound. in many cases a branch will actually change direction, in a way that seems designed specifically to avoid a neighboring branch. Or so it seems to me.
Of course there are exception and some rubbing does occur. I think this is analogous to people accidentally bumping into things or other people. But if you took two open grown trees and moved them together there would be a tremendous amount of contact. That this does not occur when the trees grow close together is thus quite remarkable. If they grew as though their neighbors were not there, there would be a lot of rubbing.
If the trees actually grow so as to avoid rubbing on their neighbor's branches, then they must know where those branches are, without touching them. In the last post I discussed the idea that trees know where their parts are and grow them so as to maintain their balance. Now it seems that the deciduous also know where their neighbor's parts are, and they grow so as to avoid rubbing on them.
This all sounds rather farfetched, but I have difficulty coming up with any other explanation for this apparent growth behavior. Perhaps it is a function of the way the neighboring trees affect the light. In any case it is certainly a challenging research question.