New Caledonia is an archipelago in the Southwest Pacific. Its flora is rich, unique, and disharmonic. The spatio-temporal origin of this flora is one of the major topics of botanists. A systematic study was conducted on the genus Psychotria (Rubiaceae), and its allied genera (tribes Psychotrieae-Palicoureeae) to understand the patterns of establishment and diversification of plants in the archipelago. Both tribes have 86 New Caledonian species distributed between the genera Geophila (onespecies), Margaritopsis (four species) and Psychotria (81 species). The taxonomic study determined all valid species names for each of these latter genera, and identified 26 new species of Psychotria and Margaritopsis. Nine new specific combinations and names are proposed. New Caledonia was colonized at least four times by Psychotrieae-Palicoureeae. Although their arrivals were simultaneous during the Neogene, these four lineages have different evolutionary histories,linked with their own patterns of establishment and diversification. The genus Geophila has not diversified. Margaritopsis had a moderate diversification, as the other species of Pacific islands. Psychotria clade NC1is a relictual lineage, which probably underwent an extinction, and would have only been preserved inultramafic shrubby vegetations. Psychotria clade NC2 is the largest, youngest, and fastest plants radiation of New Caledonia, probably originated from Australian rainforests. The clade NC2 is expanding evolutionary. It consists of 12 internal lineages. Its ability to tolerate ultramafic substrates, acquired before its arrival in the archipelago, has promoted its local establishment. Its large diversity has probably different origins: a lability relative to the nature of geological substrates, likely changes of pollinators, an on going change of ecological niche, related to the acquisition of drought adaptations, and the onset of a more arid climate in New Caledonia during the Pliocene.
