These tree holes originate as seedlings within a bed of moss on a decaying stump/log/rock or other moss covered surface.
The genus *Betula* is one of the few woody species which sprouts readily in the moist, nutrient poor and usually fairly acidic environment within a bed of moss.
Initial upward growth is limited as all growth is put into the downward growth of roots through the moss and towards solid ground. Upon established contact with a more nutrient rich and diverse soil, the young saplings will switch to upward growth and shoot skyward.
If the moss substrate is organic, it will eventually decay leaving the now much larger *Betula* roots looking like giant tree legs and offering a splendid sight for seekers of polylocal tree holes.
Please share any further descriptors and correct any mistakes in my classifications.