Monograph of tribe Sipaneeae (Rubiaceae, Ixoroideae). A Neotropical group with its center of diversity on the Guiana Shield

The tribe Sipaneeae (subfamily Ixoroideae) includes the genera Chalepophyllum (3 spp.), Dendrosipanea (3 spp.), Limnosipanea (3 spp.), Maguireothamnus (3 spp.), Neblinathamnus (2 spp.), Neobertiera (5 spp.), Pteridocalyx (1 sp.), Sipanea (17 spp.), Sipaneopsis (8 spp.), and Steyermarkia (1 sp.), for a total of 44 species. This tribe has its center of diversity on the Guiana Shield (northern South America), where 9 of the 10 genera occur. Only Sipanea and Limnosipanea are also present on the Brazilian Shield and the Orinoquía Region with a total of 4 species, while Steyermarkia, Sipanea and Limnosipanea are present in Central America with one species each. The genera Sipaneopsis, Chalepophyllum, and Dendrosipanea are endemic to white sand areas of the Amazon Basin. The habit in the tribe Sipaneeae ranges from herbs (terrestrial, amphibian or rarely aquatic), subshrubs, shrubs to treelets. Within the tribe, Chalepophyllum, Dendrosipanea, Neobertiera, Pteridocalyx and Sipanea are heterostylous, while the other genera are homostylous. Several recent articles have been dedicated to the description of new species in this tribe, and only Limnosipanea spruceana var. macrantha is newly described here. In the present work 21 names are newly typified; a list of these names is present at the end of the taxonomic treatment. For all the taxa of this tribe, full synonymy, typification, description, ecological observations, geographic distribution, illustrations, IUCN conservation assessment, and cited specimens are provided.