Viburnum furcatum

Family: Adoxaceae
English: Scarlet-Leaved Viburnum
Japanese: オオカメノキ

These look a bit like hydrangea trees blooming at high altitudes. Later in the year, look for the berries, and in the autumn, look for the blazing pink leaves of it's relative, V.　urceolatum, often perched high up in larger trees.

References:
屋久島の植物　 川原勝征,初島住彦(監修),1995：八重岳書房
「BG Plants 和名－学名インデックス」（YList） 米倉浩司・梶田忠 (2003-)　，http://bean.bio.chiba-u.jp/bgplants/ylist_main.html（2014年5月9日）.
Tropicos.org. Missouri Botanical Garden. http://www.tropicos.org/Name/6000267 (08 May 2014)