Alien flora of D.R. Congo: improving the checklist with digitised herbarium collections

The Democratic Republic of the Congo (D.R. Congo) represents a striking gap of knowledge on alien plant species. In this paper, we use digitised herbarium collections to assemble a new checklist of alien plant species in D.R. Congo and to examine patterns in the alien flora. The new checklist comprises 436 alien species i.e., 189 (43%) casuals, 247 (57%) naturalised of which 80 (18% of aliens) are invasive. Discrepancies with previous databases are discussed. For many species in previous databases, all herbarium specimens come from cultivated specimens (e.g. botanic gardens) and we failed to find evidence for occurrence outside of cultivation. A total of 166 taxa were not included in previous lists, 41 of which are new records to the flora of D.R. Congo. Considering the size of the country and its rich native flora, the alien flora of D.R. Congo does not appear to be species-rich. The alien flora is particularly rich in Fabaceae (16%) and in annual species (37%). The Americas are by far the most important source continent (65%) and the proportion of annuals of American origin is particularly large among the most widespread species. 90% of invasive species are from the Americas. Invasive success is discussed in terms of residence time. The very low number of new species records after 1960 is similar to other African countries and could be due to decreasing sampling effort. The results illustrate how herbarium collections can be used to critically revise existing checklists of alien species in tropical Africa. Field work is urgently needed to improve coverage of recent introductions and to monitor the status of alien species, especially in protected areas and around botanic gardens.