Arran whitebeams complex fashion, which involved the common whitebeam ("Sorbus aria") giving rise to the tetraploid rock whitebeam ("Sorbus rupicola") which is still found on Holy Isle. This species is able to survive at higher altitudes and therefore occupies a less competitive niche with fewer tree species able to tolerate the harsher conditions. The rock whitebeam interbred with the rowan / mountain ash ("Sorbus aucuparia") to produce the hybrid, a fertile separate species the Scottish whitebeam ("Sorbus arranesis") which grows well in this zone of reduced competitive growth at higher altitudes. The bastard mountain ash ("Sorbus pseudofennica") arose from a further cross
North Fork Mountain pine barren in the Central Appalachians. Other rocky summits, including the summit of Pike Knob, are blanketed by red pine ("Pinus resinosa") forests, here at their southernmost natural occurrences. The Monongahela National Forest has designated of old growth red pine forest on the mountain as the North Fork Mountain Red Pine Botanical Area. Another unusual vegetation type on North Fork Mountain is an elfin forest dominated by yellow birch ("Betula alleghaniensis"), mountain ash ("Sorbus americana"), and mountain holly. The oak-and-pine vegetation of North Fork Mountain contrasts strongly with the more diverse vegetation of the nearby mountains just west of the