Novelties in Erythroxylum P.Browne (Erythroxylaceae) from the Comoros Archipelago: two new, range-restricted and threatened species, and notes on the Mount Choungi biogeographical singularity

A study of the genus Erythroxylum P.Browne from the Comoros archipelago reveals two new species, E. choungiense E.Bidault, Traclet & M.Pignal, sp. nov. from Mayotte and E. labatii E.Bidault & M.Pignal, sp. nov. from Mwali and Ngazidja, both of which are described and illustrated. The distribution of a third species, E. elegans Baill., is now only known with certainty to include Mayotte and Nzwani, and possibly Ngazidja, following the exclusion of material now assigned to the new species. A risk of extinction assessment using the IUCN Red List criteria is provided for the new taxa and the morphological features that distinguish them from their apparently close relative are summarized. A key to the species of Erythroxylum in the Comoros is provided in both English and French. Erythroxylum choungiense sp. nov. is endemic to Mount Choungi, and its conservation status is assessed as Critically Endangered. Mount Choungi harbors the unique patch of hygrophilous submontane shrubland in Mayotte, to which at least five species are now known to be strictly endemic: Cassipourea ovata Tul., Eugenia choungiensis Byng & N.Snow, Psiadia pascalii Labat & Beentje, Syzygium labatii Byng & N.Snow, and Erythroxylum choungiense sp. nov.. Erythroxylum labatii sp. nov. is only known from the submontane forests on ridges and steep slopes on Moheli and Ngazidja, at an elevation of 600 to 1300 m a.s.l., and is assessed as Endangered.