Polystichum hagiangense (subg. Haplopolystichum; Dryopteridaceae), a new fern species from Ha Giang Province, northern Vietnam

A new fern species, Polystichum hagiangense belonging to P. subg. Haplopolystichum (Dryopteridaceae), is described and illustrated from Ha Giang Province in northern Vietnam. Polystichum hagiangense is most similar to P. minutissimum by having a small habit (lamina 3.6-5.8 x 1.0-1.6 cm), but the former has the widest part of the lamina close to the base and ovate and larger pinnae (5.3-8.2 x 3.2-5.8 mm). In contrast, the latter has the widest part of the lamina above the middle and nearly rectangular and smaller pinnae (2.9-5.8 x 2.8-4.5 mm). Also, the two species have different ecological requirements. Polystichum hagiangense was found on limestone cliffs in forest at an elevation of 1290 m with dry and light conditions, while P. minutissimum grows on dripping limestone walls and stalactites inside a karst cave at an elevation of 750 m with humid and shady conditions. Polystichum hagiangense is currently known only from a single population occurring at Bat Dai Son community in Ha Giang Province, northern Vietnam, and would be classified as Critically Endangered (CR), following IUCN Red List criteria