Taxonomic Monograph ofSaxicolella(Podostemaceae), African waterfall plants highly threatened by Hydro-Electric projects, with five new species
Authors: Cheek Martin , Molmou Denise , Magassouba Sekou , Ghogue Jean-Paul 
Published: June 2021

The genusSaxicolellaEngl. (Podostemaceae) are African rheophytes, restricted to rapids and waterfalls as are all members of the family. Previously,Saxicolella sensu latowas shown to be polyphyletic with two separate clades in the molecular phylogenetic study of Koiet al. (2012). The namePohliellaEngl. was recently resurrected for one clade that is sister to the American generaCeratolacis(Tul.)Wedd.,PodostemumMichx. and all Old World Podostemoideae (podostemoids) (Cheek 2020).Pohliellahas distichous phyllotaxy, bilocular ovaries, filiform roots with paired holdfasts, and rootcaps. The second clade,Saxicolella sensu stricto, including the type of the generic name, has spiral phyllotaxy, unilocular ovaries, ribbon-like or crustose roots that lack both holdfasts and rootcaps.Saxicolella sensu stricto, sampled from the type species,S. nanaEngl. of Cameroon, is embedded within and near the base of the major clade of African podostemoids and is sister to all other African genera apart fromInversodicraeaR.E.Fr. andMonandriellaEngl. Recently reduced to three species in Cameroon and S.E. Nigeria by the resurrection ofPohliella(3 – 4 species in Ghana and Nigeria-Cameroon),Saxicolellasensu stricto is expanded to eight species in this monograph by description of five new taxa.Saxicolella futaCheek andS. deniseaeCheek are newly described from Guinea,S. ijimCheek from Cameroon, the informally namedS. sp. A from Gabon, andS. angolaCheek from Angola. The known geographic range of the genus is thus expanded c. 2,500 km westwards to Guinea from eastern Nigeria and c.1,500 km southeastwards from near Yaoundé to Cuanza do Sul, Angola. The greatest concentration of species occurs in the Cross-Sanaga interval of western Cameroon and eastern Nigeria, with three species. Cameroon (3 species) followed by Nigeria and Guinea (2 species each) are the countries with highest species diversity. The genus can be expected to be found in Sierra Leone, Liberia, Ivory Coast and Congo Republic. A classification is proposed grouping the species into three subgenera (Saxicolella, Butumia(G.Taylor) Cheek comb. et. stat. nov. andKinkoniaCheek subgen. nov.) based on root morphology and shoot position and morphology.

The discovery, morphology, circumscription, distribution, and ecology ofSaxicolellais reviewed, an identification key to the species is presented, together with descriptions, synonymy, links to illustrations, and extinction risk assessments for each of the eight species now recognised. All of the species are provisionally assessed as either Endangered or Critically Endangered using the IUCN 2012 standard, making this genus among the most threatened of its size globally. The major threats, above all, are hydro-electric projects.Saxicolella deniseaemay already be globally extinct, and two of the four known locations ofS. angolaappear lost,S. sp. A of Gabon is threatened at at least one of its three locations, whileSaxicolella futais threatened at all three locations, all due to incipient or active hydro-electric projects. Contamination of watercourses by increased turbidity from silt-load due anthropic changes and by eutrophication from pollution are also threats for the majority of the species.