"Rivers of sky"

Why don't trees' branches crowd into each other, forming an aerial thicket? The background to the answer (which I haven't found) is that trees grow at the tips of branches, at the apical meristems (zones of cell division). Trees also expand in diameter (via the vascular cambium), but a point on the trunk will stay at the same height. Plant hormones in the meristem inhibit branching, so perhaps when the branch tips wave about in the wind and encounter other branches, the hormone is activated? A process called lateral inhibition controls the spacing of leaves - perhaps it's at work between trees as well as within them?

What this little exercise has shown me is that you need to know a lot of specialised vocabulary to read a scientific paper, even if the sentences are clearly written!