I found a young elder tree (Sambucus nigra) today growing in the fork of the trunk of the wild service tree (Sorbus torminalis) just outside our house.
The elder has found sufficient nourishment and water trapped in the trifurcation of the trunk to grow well in it its first season and it may persist for some years.
The fork in the wild service is at chest height and came into being when the single stem of the young tree (which I grew from seed sown about 1974) was toppled by a bright blue weevil called Rhynchites caerulea. These cut right round the young green shoot which topples over and usually falls to the ground. The adult weevils lay their eggs in these fallen shoots which subsequently feed their larvae.
The following year two or three stems arise from the damaged maiden in the absence of a leading bud and this creates the fork. As leaves and water, not to mention baby elders, gather in the fork it makes a weak point from which decay can easily get into the centre of the trunk and ultimately bring about the death of the tree.
So it goes (as Kurt Vonnegut would say).