Begonia fimbristipula subsp. siamensis (sect. Diploclinium, Begoniaceae), a new taxon of the megadiverse genus endemic to Thailand

The genus Begonia has not only been recognised to be one of the mega-diverse plant genera but also as one found to comprise many undiscovered species. In particular, the increase of extensive field surveys in tropical regions of Southeast Asia has added to the discovery of many new species that are often found only in a few localities. In this study, the new taxon Begonia fimbristipula subsp. siamensis subsp. nov. from Thailand is described. The Thailand accessions are highly similar in their morphology to accessions of B. fimbristipula from southern China but differ in their tuber shape, peduncle trichomes, petiole trichomes and number of female tepals. The new taxon has been found only in the northern parts of Thailand occurring at elevations above 1,300 meters. The new findings not only contribute to our knowledge of the plant diversity of Thailand but provide also critical information contributing to the protection of this species. In China, this species is endangered which is of special concern given its utilisation as a medical herb in traditional Chinese medicine. Considering IUCN Red List Categories, the new subspecies is considered to be Vulnerable. The disjunct distribution of the two subspecies of B. fimbristipula encourages urgently needed comparative taxonomic studies across the Indo-Burma biodiversity hotspot.