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

﻿Abstract 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 Begoniafimbristipulasubsp.siamensissubsp. 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.


Introduction
With about 2,000 accepted species, the pantropical, herbaceous flowering plant genus Begonia L. (Linnaeus 1753) (Begoniaceae, Cucurbitales) is an outstanding example of a mega-diverse plant genus (Stevens 2001;Hughes et al. 2015;Li et al. 2022). In recent years, the number of known species has rapidly expanded as a consequence of a combination of factors including extensive field surveys in tropical regions (Hughes et al. 2015;Moonlight et al. 2018). Species surveys are especially needed to document the diversity in subtropical to tropical Asia because this region, with 1,159 recorded species, is a diversity hotspot of Begonia (Hughes et al. 2015). Taxonomic studies of this mega-diverse genus are challenged by several aspects including (1) the vast number of species required to be compared, (2) the high frequency of disjunct local occurrences, and (3) phenotypic plasticity observed in many species (McLellan 2000;Wang et al. 2016;Wahlsteen 2021). Until now, 58 species of Begonia have been recorded as occurring in Thailand of which about 40 percent are endemic (Phutthai et al. 2019). Here, we report results of our ongoing efforts to improve the documentation of this mega-diverse genus in Thailand. These research activities have been motivated by two key-assumptions. Firstly, the diversity of Begonia in Thailand is incompletely known and thus several species await to be found. Secondly, at least some, if not many, species of Thailand's Begonia are threatened as a consequence of the ongoing biodiversity crisis caused by anthropogenic activities.
In our studies, we focused specifically on occurrences of Begonia section Diploclinium (Lindl.) A.DC. (Lindley 1847;de Candolle 1859). This section requires a re-definition because the currently used circumscription has been considered to be a taxonomic dustbin (Shui et al. 2002;Rubite et al. 2013). This expectation has been confirmed in phylogenetic studies recovering the section to be polyphyletic (Thomas et al. 2011;Rubite et al. 2013). As a consequence, all species of this section occurring in the Philippines were reassigned to Begonia sect. Baryandra (Rubite et al. 2013). 19 out of the 58 Begonia species occurring in Thailand have been assigned to Begonia sect. Diploclinium (Phutthai et al. 2019). These placements have not yet been confirmed using phylogenetic studies and the current definition of the section is somewhat ambiguous, in that it is a combination of two key characters for the section, namely tuberous habit and bifid placentae (Doorenbos et al. 1998). Besides the challenges of classification of recognised species, many species of this section are expected to still await their discovery. The main aim of our study is to estimate the real number of species belonging to Begonia sect. Diploclinium that occur in Thailand. As a first step toward achieving this goal, field surveys were carried out in remote parts of Thailand to collect accessions of putative unknown taxa. These accessions were then carefully compared to previously described species by considering not only species known to occur in Thailand but also species occurring in other parts of continental Asia including southern China.

Methods
New accessions were examined and photographed in their natural habitats during field surveys in northern and north-eastern Thailand. They were consequently carefully compared to previously described species and as far as possible voucher specimens either by access to the voucher itself or to images. Special attention was given to type vouchers. To avoid describing an already published species, extensive comparisons were carried out with known species that showed high similarity in our first round of examination. The newly obtained voucher specimens were deposited at the two key herbaria in Thailand namely Forest Herbarium at Bangkok (BKF) and Queen Sirikit Botanic Garden Herbarium at Chiang Mai (QBG) besides several prominent international herbaria (K, E, P, HAST). Distribution maps were generated using QGIS software (QGIS 2021) by combining the records obtained during our field surveys with records from specimens deposited in herbaria. The assessment of the International Union for Conservation of Nature (IUCN) Red List status was evaluated using information from field observation and the available specimen records.
Habitat. The new taxon is currently known from seven collections obtained from three provinces located in northern and north-eastern Thailand (Fig. 2), where it grows on both limestone and sandstone outcrops in evergreen forest at high elevation (1300-1900 m). Individuals growing on limestone rocks are distinct from individuals growing on sandstone rocks by the presence of a dense indumentum formed by red trichomes. The indumentum of individuals growing on sandstone is less dense and the trichomes have a brighter colour. Accessions of Begonia fimbristipula subsp. siamensis show high phenotypic variation among individuals of the same population especially in leaf size and abaxial leaf surface colour (Fig. 1H, H'). The notable phenotypic disparity resembles reports from occurrences of B. fimbristipula subsp. fimbristipula in southern China (Wang et al. 2016;Tian et al. 2018). Phenotypic plasticity has been considered to result from adaption to the harsh environments occupied by these plants (Wang et al. 2016).
Phenology. Flowering Time: June-September; Fruiting Time: August-December. Etymology. The subspecific epithet siamensis alludes to the nativity of this taxon to "Siam", the exonym that was used for Thailand before 1949.
Conservation assessment. The recorded occurrences of the new subspecies were mostly obtained in protected areas, e.g. National Parks. The high protection is arguably reduced by threats caused by tourism activities. In Phisanulok province, this species creates a beautiful landscape by growing on the sandstone cliff along the trekking trail, which is a famous spot among tourists. Some of its populations are outside the protected area where the new taxon has been threatened by illegal trade in wild ornamental plants, and habitat infringement. Until now, there is still no conservation plan for the new subspecies at any of these localities. Further surveys are arguably required to obtain a more accurate description of the distribution of this species in Thailand but also in Laos, Myanmar and Vietnam. The currently known area of occupancy (AOO) has been estimated as 32 km 2 (Bachman et al. 2011; http://geocat.kew.org/). Based on IUCN criteria, the status assigned is "Vulnerable" (VU: B2 a, c (iii, iv); C2 (ai, ii)) (IUCN 2019).
Additional   (Peng et al. 2017;Hughes 2017a, 2017b;Phutthai et al. 2019Phutthai et al. , 2021. In this study, we report a new subspecies of Begonia sect. Diploclinium from Thailand, resulting in the number of Begonia sect. Diploclinium species known from Thailand increasing to 20 species (of which one is an endemic subspecies). We treat the newly found taxon as subspecies B. fimbristipula subsp. siamensis because of the distinct morphological affinity to B. fimbristipula subsp. fimbristipula. A comparison between these two taxa and a third similar species, namely B. poilanei Kiew (Kiew 2007;Shui and Chen 2017), was assembled (Table 1). Our taxonomic conclusions consider also the geographic isolation of the new taxon occurring only in northern and north-eastern Thailand (Chiangmai, Nan, Phitsanulok,) away from typical B. fimbristipula occurring exclusively in southern China (Fujian, Guangdong, Guangxi, Hainan, Hunan, Jiangxi, Zhejiang). Thus, they are geographically disjunct taxa but future studies need to confirm the absence of occurrences bridging the gap in eastern Myanmar, northern Laos and northern Vietnam. In this context, it is worth noting that several putative relatives based on shared morphological similarities are known to occur in this region including B. poilanei from China and Vietnam (Kiew 2007) and Begonia mengdongensis H.H.Xi from southeastern Yunnan, China (Xi et al. 2020). Currently, it is also not known if the Thailand accession may also be harvested as a natural resource similar to the typical B. fimbristipula, whose plants are used to brew herbaceous teas, and as herbal medicine (Zhao et al. 2016).