The Lucky Tree of Chapel Rock

There is a very famous tree at Pictured Rocks National Lakeshore in Northern Michigan that has continued growing after a sandstone archway fell away and left it stranded on the "Chapel Rock" formation in the 1940's. Nature can take hold in precarious places. Chapel Rock on Lake Superior has a single tree perched atop its column. By rights the tree should not be there: the small surface area of land on the top of the rock is insufficient to sustain a tree of this size.

There is hardly any topsoil, certainly not enough for an obviously thriving tree. How then does it flourish?