Checklist to the Elatostema (Urticaceae) of Vietnam including 19 new records, ten new combinations, two new names and four new synonyms

Diversity & endemism

We document 77 taxa of Elatostema in Vietnam, 14 taxa (18%) of which are endemic to the country. These taxa comprise 75 species, two of which are represented each by two taxa of a lower level (varieties and subspecies). Table 1 provides a summary of the species richness, national endemism, and new national records identified here, per floristic region. Figures 3–5 provide field images of a representative sample of these species.

We document the greatest species richness in northern Vietnam (Cao Bang, Ha Giang, Lao Cai provinces) and the lowest species richness in eight scattered provinces across northern to central Vietnam and 25 southernmost provincial units where no Elatostema species have been documented: Bac Ninh, Ha Nam, Hai Duong, Hung Yen, Nam Dinh, Thai Binh, Yen Bai (North), Da Nang City (Central) and An Giang, Ba Ria-Vung Tau, Bac Lieu, Ben Tre, Binh Dinh, Binh Duong, Binh Phuoc, Binh Thuan, Ca Mau, Can Tho City, Dak Nong, Dong Nai, Dong Thap, Hau Giang, Ho Chi Minh City, Kien Giang, Long An, Ninh Thuan, Phu Yen, Quang Ngai, Soc Trang, Tay Ninh, Tien Giang, Tra Vinh, and Vinh Long (South, Fig. 6). We recovered the greatest number of species endemic to the country in northern Vietnam (particularly Bac Kan province and Ha Noi City) (Fig. 6).

Relationship among floristic regions

Figure 7 provides the proportion of diversity and endemism and the relationship of each floristic region. The order of the proportion of diversity of two floristic regions is the following: South Chinese–North Indochinese (49.2%) >Sikang-Yunnan–North Indochinese (46.9%) >Sikang-Yunnan–South Chinese (35.8%) >Central Annamese–South Annamese (35.3%) >North Indochinese–Central Annamese (30.0%) >South Chinese–Central Annamese (20.7%) >North Indochinese–South Annamese (19.4%) >Sikang-Yunnan–South Annamese (13.2%) >South Chinese–South Annamese (13.0%) >Sikang-Yunnan–Central Annamese (10.6%).

New records, new combinations, new names & synonymy

We found 19 new records of Elatostema for Vietnam, i.e., E. attenuatoides W.T.Wang, E. austrosinense Y.H.Tseng & A.K.Monro nom. nov., E. backeri H.Schroet., E. brunneinerve W.T.Wang, E. crassiusculum W.T.Wang, E. crenatum W.T.Wang, E. fengshanense W.T.Wang & Y.G.Wei, E. glochidioides W.T.Wang, E. malacotrichum W.T.Wang & Y.G.Wei, E. nanchuanense W.T.Wang, E. oblongifolium Fu, E. obtusum Wedd., E. oppositum Q.Lin & Y.M.Shui, E. pergameneum W.T.Wang, E. prunifolium W.T.Wang, E. pseudolongipes W.T.Wang & Y.G.Wei, E. pycnodontum W.T.Wang, E. salvinioides W.T.Wang, and E. xichouense W.T.Wang. All new records came from the northern provinces: Cao Bang, Ha Giang, Lao Cai, Dien Bien, Lang Son, Phu Tho, Bac Kan, Hoa Binh and Ninh Binh (Table 1); all of them are also known from the neighbouring regions of China, i.e., Guangxi, Guizhou and Yunnan. Of the Vietnamese provinces, Cao Bang and Ha Giang had the greatest number of new records.

We place Elatostema baviensis Gagnep. in synonymy of E. platyphyllum Wedd. based on the characters of large lamina (10–25 × 4–7.5 cm) with auriculate base, large stipule (15–25 × 3–5.5 mm) and pistillate inflorescence with short peduncle and obvious receptacle. Then, we place E. colaniae Gagnep. in synonymy of E. myrtillus Hand.-Mazz. based on the characters of small lamina (13–28 × 6–10 mm) with auriculate base and furfuraceous stem base. We also place Pellionia macroceras Gagnep. in synonymy of E. hookerianum Wedd. based on the characters of falcate lamina with cordate base. Finally, we place P. tetramera Gagnep. in synonymy of E. dissectum Wedd. based on the characters of oblong-lanceolate lamina and staminate inflorescence with long peduncle. In addition and in line with the study by Tseng et al. (2019), we transfer all previously known Pellionia species found in Vietnam (except for P. repens) to Elatostema including ten new combinations and two new names proposed accordingly.

Diversity & endemism

Our checklist of Elatostema in Vietnam doubles the number of species cited in the most recent checklist (Hiep, 2005) bringing the total diversity to 77 taxa (75 species). This is the product of both recent checklists for the region, field collecting and herbarium specimen examination, aided by the availability of protologues, type images and other collections online (e.g., The JSTOR Global Plants portal, 2017; Seregin, 2018).

We also document that species richness and endemism increase with latitude, away from the equator. Whilst counter-intuitive, a similar pattern has been documented for the diversity of Begonia L. (Averyanov & Nguyen, 2012) and Orchidaceae (Averyanov et al., 2003a). This is likely a reflection of the distribution of karst in Vietnam with which Elatostema (Wang, 2014), Primulina Hance (Kong et al., 2017) and Begonia (Chung et al., 2014) species richness in Southeast Asia is strongly associated rather than Elatostema’s preference for a more seasonal or cooler climate. Given the total absence of Elatostema records from Vietnam’s southernmost provinces this pattern may also have been over-emphasised by unequal sampling effort across the country as well as by unequal distribution of intact vegetation. Indeed, the southernmost provinces are largely plain and therefore occupied by agricultural landscapes.

The documented 18% level of endemism of the Elatostema taxa in Vietnam is nearly twice the average for the country’s flora (10%, Pilgrim & Tu, 2007) and comparable to that in the most species-rich family Orchidaceae (19%, Averyanov et al., 2003a). However, this level is still lower than the corresponding level in Guizhou (39%), Guangxi (64%) and Yunnan (71%) of China, which are well documented (Wang, 2014). As in China, an association with karst is likely the driver for much of this diversity indicating Vietnam’s capacity of higher endemism which is likely to be revealed when Elatostema is comprehensively documented. Current researches (Chung et al., 2014; Kong et al., 2017) suggest that rates of karstification over time, and likely in the Miocene, associated with seasonal climate and temperature, may have driven speciation on karst. Species that were able to adapt to what is a relatively inhospitable substrate speciating as karst habitat was formed. It is notable that of the documented genera that have ‘radiations’ associated with karst, i.e., Begonia, Impatiens L., Primulina and Elatostema, all are mostly herbaceous, three of the four are succulent, three of the four are insect-pollinated and all have seeds that are abiotically rather than animal dispersed.

Comparison of diversity among floristic regions, and reasons of its differences

Among the six floristic regions delimited in Vietnam (see Fig. 2; Takhtajan, 1978; Takhtajan, 1986; Averyanov et al., 2003a), we found the greatest species richness and endemism in the South Chinese Floristic Region (Figs. 6 and 7; see Table 1). We attribute this to widespread presence of karst, monsoon tropical climate with cold winter and summer rains and tropical forests (Fig. 2) with which Elatostema species richness and endemism is known to be associated (Wang, 2014). For example, the checklist of Elatostema in China documented 184 out of the 280 species recorded as restricted to karst, many of which are range-restricted endemics (Wang, 2014). A similar pattern in species richness in Vietnam has been documented for Begonia by Averyanov & Nguyen (2012) who also ascribe part of this richness to the presence of karst in this floristic region.

The second highest species diversity and endemism occurred in the Sikang-Yunnan Floristic Region. Notably, this region occupies one of the smallest areas in Vietnam, comparable only with the area of the South Annamese Region; this indicates extraordinarily high average Elatostema species density in the Sikang-Yunnan Floristic Region. This region lies within the SSE extension of the Himalayan Highlands. It can be compared with another representative Himalayan area, i.e., Xizang province, which is the fourth most species-rich province for Elatostema in China (Wang, 2014). Within the Sikang-Yunnan Floristic Region, 17 species of Elatostema (22.7% of the species that we document for Vietnam) occur in Hoang Lien Son Range of Lao Cai province. It is notable that the Sikang-Yunnan Floristic Region is the only one within the territory of Vietnam which represents the Holarctic floristic kingdom, while the other five regions belong to the Paleotropical floristic kingdom (Takhtajan, 1978; Takhtajan, 1986; Averyanov et al., 2003a; Averyanov et al., 2003b). This explains the high diversity of Elatostema in this region, as this genus is also known to be mostly associated with the southern part of the Holarctic kingdom (Wang, 2014).

The next most important floristic region for species richness and endemism of Elatostema in Vietnam is the North Indochinese Floristic Region (Fig. 7; Table 1). It possesses the same species number as the previous region and a single national endemic species instead of two. This region covers a chain of continuous limestone plateaux (Averyanov et al., 2003a). Monsoon tropical climate with cold winter and summer rains is typical of the largest part of the northern portion (Fig. 2) which is similar to those found in the South Chinese Region. Therefore, the diversity and endemism of this floristic region share similar pattern to the South Chinese Region which is congruent with our result that nearly half of species (49.2%) co-existed. This region also includes the southern extension of the Himalayan Highlands and our result revealed 46.9% co-existed species (Fig. 7) between it and Sikang-Yunnan Region, which also suggests their similarity. It should also be taken into account that the North Indochinese Floristic Region occupies almost the largest area within Vietnam, comparable only with the area of South Indochinese Region. Therefore the high species diversity in this region is also an effect of its large territory.

We found low species richness and endemism in the Central Annamese Floristic Region (Fig. 6; see Table 1). The representative provinces of this floristic region are Quang Tri and Thua Thien-Hue which contain karst mountains and a climate very similar to that of the North Indochinese Floristic Region (Averyanov et al., 2003a); besides, 30.0% of species co-occurred in these two floristic regions (Fig. 7). We would therefore expect species richness and endemism in the Central Annamese Floristic Region to be similar to those in the North Indochinese Floristic Region rather than lower (Table 1). We would also expect the diversity to be higher (rather than the same) in the Central Annamese Floristic Region than in the South Annamese Floristic Region, as the latter lacks karst and experiences drier climate. We also found 31.3% species shared between the Central Annamese and South Annamese Floristic Regions. Therefore, we propose that both of these phenomena reflect a lack of collecting effort.

The South Indochinese Floristic Region, the largest in Vietnam (Fig. 2), comprises lowland area of 23 provincial units and yet we document no species records from all of them (Fig. 6). This is unlikely to be due to the absence of karst, edaphic factors or climate alone. Whilst this floristic region has little karst (Kien Giang province only), it does have multiple climate types (Fig. 1) and significant forest cover (Global Forest Watch, 2014) which elsewhere in SE Asia would support a diversity of Elatostema species (Robinson, 1910; Conn, 2008). Again, we propose that this absence of records is the result of a lack of collecting effort rather than a consequence of total absence of this genus. This inference is also supported by the absence of known Elatostema collections from several Vietnamese provinces surrounded by areas with rather high diversity of this genus: Yen Bai, Thai Binh, Nam Dinh, Bac Ninh, Hai Duong, Hung Yen, Ha Nam, Da Nang, Quang Ngai and Binh Dinh. We therefore propose that South Indochinese Floristic Region should be of a high priority for subsequent studies of Elatostema in Vietnam.