Four new deciduous species of Hyperacanthus (Rubiaceae: Gardenieae) from western Madagascar: the sofikomba alliance

The genus Hyperacanthus comprises 11 species, nine in Madagascar and two in Africa, although many Madagascan species remain undescribed. All Madagascan species of Hyperacanthus are evergreen trees occurring in humid and semi-humid forest. Four deciduous species occur in the dry forests of northern, western, and southern Madagascar but are unknown to science. These four species, collectively and informally referred to as the sofikomba alliance, are described here: H. decaryi, H. piliformis, H. septentrionale and H. sofikomba. Three of the four species are illustrated and all four are mapped. A key to species, species descriptions, other key taxonomic information, and preliminary conservation assessments are also provided.