The vascular plant diversity of Burundi

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The vascular plant diversity of Burundi is still insufficiently explored, described, and understood. The goal of this paper is to show the degree of botanical exploration and the spatial patterns of botanical diversity in Burundi to date.  The study is based on a dataset containing virtually all plant collections, observed in herbaria, recorded in databases, or cited in literature, made in Burundi. All data were compiled, cleaned, and each record georeferenced. Various distribution analyses were carried out, some of which were based on a grid of 199 hexagonal cells. The dataset comprises 37,200 herbarium collections representing 3,860 species grouped in 1,290 genera and 216 families. The expected species richness is estimated at 4,869. The average number of collections per species is 8.8, but 1,149 species (27%) are sampled only once. The seven most species-rich families are Fabaceae (539 spp.), Poaceae (387), Asteraceae (298), Orchidaceae (286), Cyperaceae (272), Rubiaceae (227), and Acanthaceae (128), which together account for over 50% of the vascular plant flora of Burundi. The seven largest genera are Cyperus (90 spp.), Crotalaria (60), Indigofera (50), Polystachya (48), Habenaria (47), Vernonia (45), and Eragrostis (41). In terms of number of herbarium collections, the six most important families are Poaceae (4,754 collections), Fabaceae (4,300), Asteraceae (2,226), Rubiaceae (2,191), Cyperaceae (1,730), and Lamiaceae (1,275). The four areas most intensively explored and with the highest known species diversity are the Rusizi plain, the Kibira rain forest belonging to the Albertine Rift, the Bururi and Rumonge areas in the west, and the Mosso depression in the east.  With a collecting index of 133 collections per 100 km2 , the botanical exploration of Burundi can be considered as relatively good. However, 28% of the species are only represented by a single record and some 1,000 species are potentially present but have remained uncollected to date. For every 100 new collections, there are on average 6 new species records, indicating that Burundi’s inventory is still not complete.