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

Elatostema (Urticaceae) comprises several hundred herbaceous species distributed in tropical and subtropical Africa, Asia, Australia and Oceania. The greatest species richness occurs on limestone karst in Southeast Asia. Taxonomic revisions of Elatostema are largely out of date and contradict each other with respect to the delimitation of Elatostema and Pellionia. Most herbaria in SE Asia and worldwide contain significant amounts of unidentified material. As part of a broader revision of Elatostema in SE Asia, we present an updated checklist for Vietnam based on field visits, a review of specimens in herbaria worldwide, a review of type material and nomenclature. We recognize 77 taxa (75 species and two infraspecific taxa) of Elatostema in Vietnam, 23 of which were previously ascribed to Pellionia. Nineteen of these are new records for the country, i.e., E. attenuatoides, E. austrosinense, E. backeri, E. brunneinerve, E. crassiusculum, E. crenatum, E. fengshanense, E. glochidioides, E. malacotrichum, E. nanchuanense, E. oblongifolium, E. obtusum, E. oppositum, E. pergameneum, E. prunifolium, E. pseudolongipes, E. pycnodontum, E. salvinioides and E. xichouense. We place E. baviensis in synonymy of E. platyphyllum, E. colaniae in synonymy of E. myrtillus, P. macroceras in synonymy of E. hookerianum, and P. tetramera in synonymy of E. dissectum for the first time. Fourteen taxa (18% of all the recognized taxa) are endemic to Vietnam, which makes Elatostema one of the richest genera for endemic species in this country; this level of endemism is comparable to levels observed in Orchidaceae. Our checklist suggests that the highest diversity and endemism of Elatostema occurs in northern Vietnam, and that there is the greatest floristic similarity of northern Vietnam to SW China. The relationship among floristic regions is also investigated. We could find no records of Elatostema for 33 out of 63 provincial units of Vietnam, including all the southernmost provinces. We propose that further studies on the diversity of Elatostema in central and southern Vietnam are severely needed.

Introduction

Elatostema JR Forst. & G Forst. (Urticaceae) comprises several hundred species of succulent herbs and subshrubs that grow in shade in forests, gorges, stream sides and caves (Fu et al., 2017; Monro et al., 2018). There are currently 626 accepted names within the genus Elatostema (The Plant List, 2018). Elatostema is most diverse in subtropical and tropical climates and is characterized by apparently alternate (opposite but frequently with ephemeral and occasionally absent nanophylls), strongly asymmetrical leaves arranged distichously, small staminate flowers borne in dense cymes with receptacle-like involucres or paniculate cymes and minute pistillate flowers borne in dense capitate cymes, the bracts of which form a receptacle-like involucre (Tseng et al., 2019). As in most species in the tribe Elatostemateae (Conn & Hadiah, 2009), the stamens open explosively and the seeds are ejected by reflexive staminodes (Friis, 1989). Elatostema is distributed throughout tropical and subtropical Africa, Madagascar, Asia, Australia and Oceania, with highest diversity on limestone karst in China and Southeast Asia (Yahara, 1984; Lahav-Ginott & Cronk, 1993; Wang & Chen, 1995; Lin, Friis & Wilmot-Dear, 2003; Wang, 2014). The previous studies suggest that this diversity has been driven by the challenges of colonising karst substrates, which once overcome leads to species radiations (Chung et al., 2014; Fu et al., 2017), and that past temperature fluctuations and East Asian monsoons have driven the rates of plant diversification in karst ecosystems by accelerating the rate of karstification (Kong et al., 2017).

Vietnam comprises 331,000 km2 of the eastern Indochinese Peninsula between latitudes 8° and 24°N, and longitudes 102° and 110°E. Mountains account for 40% of the surface area, and ca. 42% of the land is forested (De Queiroz et al., 2013). Vietnam’s climate divides broadly into humid subtropical in the north, monsoon in the centre and tropical savannah in the south (Peel, Finlayson & McMahon, 2007). However, on the basis of more detailed and accurate climate classification by Nguyen et al. (2000), seven types of climate are defined in Vietnam (Fig. 1) which are largely congruent with the floristic region classification (Takhtajan, 1978; Takhtajan, 1986; Averyanov et al., 2003a). Within Vietnam, six floristic regions have been delimited: the Sikang-Yunnan, South Chinese, North Indochinese, Central Annamese, South Annamese and South Indochinese Floristic Regions (Fig. 2). Vietnam also forms part of the Indo-Burma biodiversity hotspot and hosts 110 Key Biodiversity Areas (Mittermeier et al., 2004), and 65 Important Bird Areas (BirdLife International, 2002). Its flora comprises ca. 12,000 vascular plant species (Averyanov et al., 2003a), of which ca. 10% are endemic to the country (Pilgrim & Tu, 2007). Despite Vietnam’s rich plant diversity, it is still poorly sampled, with only ca. 14 specimen records per 100 km2 (Newman et al., 2007). A comparison of the 20 most species-rich families for neighbouring southern China places Urticaceae in the top ten most species-rich families (Zhu, 2017), yet it is absent from the top 20 families for Vietnam (Zhu, Yan & Qin, 2003). This suggests that Urticaceae are undersampled in Vietnam.

Figure 1

Climate map of Vietnam.

The data based on Nguyen et al. (2000) showing seven types of climate in Vietnam.

Figure 2

Map of floristic regions in Vietnam.

The data modified from Averyanov et al. (2003c) showing six floristic regions in Vietnam.

The delimitation of Elatostema has been controversial with respect to Elatostematoides C.B.Rob., Pellionia Gaudich. and Procris Comm. ex Juss. Using reconstruction of molecular phylogeny and re-evaluation of morphological characters of leaves, inflorescences and flowers, the latest study (Tseng et al., 2019) demonstrates that Elatostema is a monophyletic group which includes Pellionia but excludes Procris, Elatostematoides and Pellionia repens (Lour.) Merr.

Despite the large number of species descriptions and numerous revisions of Elatostema in China (Wang, 1980; Wang & Chen, 1995; Lin, Friis & Wilmot-Dear, 2003; Wei, Monro & Wang, 2011; Wang, 2014; Wang, 2016), there has been little work on the Southeast Asian taxa (Robinson, 1910; Gagnepain, 1930; Yahara, 1984; Beaman, 2001; Ho, 2003; Hiep, 2005). In addition, the most recent world-scale revision of this genus was undertaken in the 1930s before much of the region had been extensively covered by collections (Schroeter & Winkler, 1935; Schroeter & Winkler, 1936). Lack of taxonomic effort combined with high species diversity of Elatostema, high rate of species discovery and the frequency of point endemism has resulted in many Southeast Asian and world herbaria having significant numbers of undetermined Elatostema collections. The lack of a comprehensive species and distribution list for Elatostema in Vietnam therefore represents a barrier not only to its study but also to the assessment of extinction threats, conservation and sustainable use.

The first checklist of Elatostema in Vietnam (Gagnepain, 1930) documented 34 species (together with Pellionia). This was followed by two studies (Ho, 2003; Hiep, 2005) which documented 35 species and 32 species (together with Pellionia) respectively. Since that time further 27 species have been recorded from Vietnam in regional floras and checklists and other works (Wang, 1983; Wang & Chen, 1995; Lin, Friis & Wilmot-Dear, 2003; Lin, 2008; Lin et al., 2011; Fu et al., 2013; Fu et al., 2014a; Fu et al., 2014b; Lin, Duan & Bi, 2014a; Lin, Duan & Bi, 2014b; Wang, 2014; Do et al., 2017) and several species have had their circumscription changed (Shao, Lin & Duan, 2004; Wang, 2016). The generic placement of some species was also reevaluated (see above). Considering these numerous additions, our ongoing research in Southeast Asian Elatostema, the recently revised genus circumscription (Tseng et al., 2019) and the need to critically assess the extinction threat of the species we decided to produce an updated checklist to support further taxonomic and broader scientific studies.

Materials & Methods

Fieldtrips were undertaken by two groups, a Sino-Vietnamese Group (SVG) and a Russian-Vietnamese Group (RVG). Between 2012 and 2017, SVG undertook five fieldtrips to the provinces or cities of Bac Kan, Cao Bang, Ha Giang, Ha Noi, Hai Phong, Hoa Binh, Nghe An, Lang Son, Lao Cai, Ninh Binh, Quang Binh, Thanh Hoa, Tuyen Quang and Vinh Phuc in northern Vietnam. During the course of these trips SVG made ca. 100 Elatostema collections. Over the last 30 years RVG has undertaken dozens of fieldtrips throughout Vietnam, which have resulted in ca. 200 Elatostema collections. In addition to field collecting we reviewed ca. 1,000 specimens of Elatostema at A, BM, GXMI, HN, IBK, IBSC, K, KUN, LE, MHA, MO, MPU, MW, P, PE, SING, TAI and VNMN. This was accompanied by a literature review which encompassed species protologues, the most recent monographs (Schroeter & Winkler, 1935; Schroeter & Winkler, 1936), and regional checklists (Robinson, 1910; Gagnepain, 1930; Backer, 1965; Yahara, 1984; Ho, 2003; Hiep, 2005; Wang, 2014; Wang, 2016). Using these sources of information, we have generated a preliminary species list. We then confirmed or refuted each name on the list against type material, either the original specimens or high resolution images of them, using the subscription-based full version of The JSTOR Global Plants portal (2017) and online databases of some well-digitized herbaria (e.g., E, K, P, PE) and consulting original description of each species so that only names for which a herbarium specimen or field image could be confidently associated with a type image or specimen or original description were included in the final checklist.

We used the morphological species concepts (Schroeter & Winkler, 1935; Schroeter & Winkler, 1936; Wang, 1980; Wei, Monro & Wang, 2011; Wang, 2014) to delimit species. These concepts place emphasis on peduncle bract shape and length, the number, morphology and arrangement of the bracts comprising the receptacle-like involucre, the number of bracteoles per flower and leaf lamina length/width ratios. Material was examined under a Changfang XTL-240 binocular microscope and Planapo lens at ×14 to ×90 magnifications.

For counting the diversity and endemism of Elatostema across floristic regions, we followed the concepts of Takhtajan (1978), Takhtajan (1986) and Averyanov et al. (2003a) to assign the location of each specimen to particular floristic region. The specimens without detailed localities that could not be assigned with confidence to a floristic region were not used in this analysis.

Results

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.

Table 1

The species richness, endemism, and new records identified per Floristic Region.

Floristic region	Species no.	Endemic species no.	New records no.
Sikang-Yunnan Floristic Region	36	2	9
South Chinese Floristic Region	54	5	12
North Indochinese Floristic Region	36	1	4
Central Annamese Floristic Region	16	2	0
South Annamese Floristic Region	7	0	0
South Indochinese Floristic Region	0	0	0

Figure 3

Plate I of representative Elatostema species in Vietnam: (A) E. crassiusculum; (B) E. glochidioides; (C) E. hookerianum; (D) E. prunifolium; (E), E. arcuatobracteatum; (F), E. retrohirtum; (G), E. obtusum; (H) E. ramosum; (I) E. integrifolium; (J) E. fengshanense; (K) E. austrosinense; (L) E. malacotrichum.

Figure 5

Plate III of representative Elatostema species in Vietnam: (A), E. caulialatum; (B), E. radicans; (C), E. tsoongii; (D), E. sinense; (E), E. brevifolium; (F), E. macintyrei; (G), E. balansae; (H), E. myrtillus; (I) E. platyphyllum; (J) E. petelotii; (K) E. longistipulum; (L) E. pseudolongipes.

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).

Figure 6

Variation in Elatostema species richness and endemism across provinces of Vietnam.

The tones of green indicate number of species per province. The red figures in brackets indicate number of national endemic species recorded for a province where it is non-zero.

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%).

Figure 7

Proportion of diversity and endemism and the relationship of the floristic regions.

Large circles designate floristic regions. Percentage numbers above the line in the large circles indicate the relative proportion of diversity occurring in a certain region (within Vietnam); percentage numbers below the line indicate the relative proportion of national endemics occurring in a certain region; percentage numbers in the small circles indicate the relative proportion of co-exsisted species occurring in the pairs of region connected by the line and show their relative floristic relationships.

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.

Discussion

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.

Conclusions

This study combines taxonomic and field expertise from China, Russia, the United Kingdom and Vietnam. It has strongly benefited from the availability of type images online which has accelerated the process of identification and the evaluation of taxon names. Ongoing research to document the floras of Cambodia, Laos, Thailand and Vietnam provides a huge opportunity for the taxonomy of Elatostema. Once combined with the completed Flora of China and Flora Malesiana accounts our updated checklist for Vietnam will fill a significant knowledge gap for this species-rich genus and lay the foundations for a global checklist. This fourth checklist for Vietnam not only doubles estimates of the diversity of the genus, it also identifies major knowledge gaps for the country, i.e., central Vietnam and, most notably, southern Vietnam. We propose that greater sampling effort of the flora of central Vietnam and southern Vietnam will result in a number of new additions to the flora of the country.