Begonia catbensis (sect. Coelocentrum, Begoniaceae), a new species from northern Vietnam

Abstract Begonia catbensis, a new species in Begonia sect. Coelocentrum is described and illustrated. The new species was discovered in lowland limestone hills at Cat Ba National Park and can be easily distinguished from all its congeners by having dendritic hairs on the petiole, adaxial veins and stipules, fimbriate bracts and bracteoles, dense conical bullae on the upper surface of the leaf blade, two tepals in the pistillate flowers and a glabrescent ovary with verrucose wings. Based on IUCN Criteria, the species is currently assessed as “Endangered” (D).

In Vietnam, there were only a few publications on Begonia before 2000. After describing eight new begonias from Vietnam (Gagnepain 1919), Gagnepain went on to record a total of 18 taxa of the genus for Vietnam in 1921 (Gagnepain 1921). In the late 20 th Century, Pham (1991Pham ( , 1999 made short descriptions in Vietnamese including sketches of 35 species and varieties of native Begonia in Vietnam. However, this publication has raised questions for taxonomists because the account does not cite specimens and has some misidentifications. For example, Kiew (2007) excluded four species of Begonia out of Pham's account, renamed one and described three new species for sciences which were misidentified by Gagnepain (1921) and Pham (1991Pham ( , 1999. The number of species in Begonia sect. Coelocentrum recorded for Vietnam has rapidly increased in recent years, from four species in 2007 (Gagnepain 1921;Nguyen 2004;Shui and Chen 2005;Kiew 2007;Peng et al. 2007) to 21 up to now (Averyanov and Nguyen 2012;Chung et al. 2014;Peng et al. 2014Peng et al. , 2015Chen et al. 2018;Radbouchoom et al. 2019).
During our field surveys of northern Vietnam in 2019, we found an interesting species of Begonia in lowland limestone hills at Cat Ba National Park. This begonia represents characteristics of B. sect. Coelocentrum (Shui et al. 2002;Chung et al. 2014), such as perennial habit, rhizomatous stems, staminate flower with 4 tepals, ovary 1-locular with parietal placentation and 3 unequally winged capsules. In having conical bullae on the upper leaf surface, it is similar to B. ferox C.I.Peng & Yan Liu (Peng et al. 2013 (Shui and Chen 2005). This plant, however, is obviously differentiated from them by having dendritic trichomes on the petioles, abaxial veins and on the keeled mid-rib of the stipules, 2-tepalled pistillate flowers and glabrescent ovaries with verrucose wings. It is clear that our plant represents a new taxon, therefore we describe and illustrate it here. Furthermore, a key to identify species of B. sect. Coelocentrum with conical bullae on the upper surface of the leaves is provided.
Etymology. The species is named after the type locality, Cat Ba National Park. Phenology. Flowering in August; fruiting in August -reported earlier.
Distribution and habitat. Cat Ba National Park is composed by many islets. All islets here are limestone mountains. We have had several days to survey in Cat Ba National Park and asked guides working in this National Park for the existence of this species, but we only found and recorded a single population here. Perhaps more populations will be found if more fieldwork is done in the future in this National Park. So far, Begonia catbensis is only known from the Cat Ba National Park, Cat Hai District, Hai Phong City, in northern Vietnam. Within its distribution area, the new species grows on semi-shady tops of small rocks and on steep slopes under the broad-leaved evergreen forest in lowlands of limestone hills.
Conservation status. Begonia catbensis is considered as a rare species because a single population with less than 200 mature individuals clustered into 10 clumps within an area of ca. 1 km 2 has been recorded in Cat Ba National Park. Field observation shows that the single population is strictly managed and protected by the staff of the Cat Ba National Park and the number of individuals has been stable for at least two decades. Following the guidelines of the IUCN Red List Categories and Criteria version 14 (IUCN 2019), its conservation status is currently assessed as "Endangered" (D).
Identification key to Begonia species with conical bullae on leaf surface within sect. Coelocentrum