I would speculate that [reflected] light also has a unique color (wavelength) distribution that a plant could sense and respond to

It seems as far as we can tell, trees can detect “far red” spectrum light, suspected to be done via phytochromes, and that spectrum of light is in higher quantities when closer to other tree leaves because it gets reflected off.

They detect that, and don’t grow as much in that direction since it would cause diminishing returns.