Rhamnella brachycarpa Z. Qiang Lu & Y. Shuai Sun, a new evergreen woody species from Hainan Island, is described and illustrated. The specimens of this new species have previously been identified and placed under R. rubrinervis (H. Lév.) Rehder, with which it shares evergreen leaves, erect and climbing habits and axillary flowering branches with bracteole leaves. However, the specimens from three distinct Hainan populations significantly differ from those of R. rubrinervis from other regions with smaller length to width ratios of leaves, fruit and seeds, smaller sizes of fruit and seeds and mucronate seed apices. Principal Component Analysis of the closely related taxa, based on multiple morphological characters, further recognised two separated groups. One of them comprises R. tonkinensis and R. rubrinervis, the other merely includes all individuals from these distinct Hainan populations. Therefore, R. brachycarpa, based on these distinct Hainan populations, is here erected as a new species, distinctly different from its published relatives.

Rhamnella brachycarpa, new species, Hainan Island

The buckthorn family (Rhamnaceae) comprises 11 tribes and approximately 61 genera (Hauenschild et al. 2016). In this family, Rhamnella (Miquel, 1867) in the tribe Rhamneae is a small genus of shrubs, small trees and climbers (Chen and Carsten 2007; Hauenschild et al. 2016). In "Flora of China", Rhamnella, with eight described species, is recognised by the distinctly pedicellate flowers and fleshy fruits, 1-stoned drupes, pinnately veined leaves, serrate leaf margins, semi-inferior ovaries, stipules without thorns and flowers in axillary cymes (Chen and Carsten 2007). Most of them are deciduous broad-leaved woody species with erect habit. According to Flora of China, R. rubrinervis (H. Lév.) Rehder is the only evergreen species with both erect and climbing habits within this genus in China and is distributed in S and SE Yunnan, SW and S Guizhou, Guangxi and Hainan Island (Chen and Carsten 2007). However, four other closely related species, R. tonkinensis (Pit.) T. Yamaz., R. hainanensis Merr., R. crenulata (Hand.-Mazz.) T. Yamaz. and R. longifolia Tsai & K.M. Feng, have also been published (http://www.theplantlist.org). Only R. rubrinervis and R. tonkinensis have been accepted as valid species, while all others are treated as synonyms of R. rubrinervis (Chen and Carsten 2007). Interestingly, these evergreen species have been treated as an independent genus because of the unique morphology and R. rubrinervis has been proposed to be conspecific to R. tonkinensis (Fan and Yang 1997; Chen and Carsten 2007). After examining all specimens of R. rubrinervis preserved in the Chinese Virtual Herbarium (http://www.cvh.org.cn) and Global Biodiversity Information Facility (https://www.gbif.org/) in 2017, we found that one distinct specimen from Hainan Island placed under R. rubrinervis might represent a new species because it was clearly different from a representative specimen of this species from Guangxi with the characters of shorter fruit and smaller leaf length to width ratio (Figure 1). In order to further test this hypothesis, we re-examined specimens (including type specimens) in herbaria to illustrate the distinct differences of this special Hainan specimen. We also conducted a field survey across the total distribution range of R. rubrinervis, including this Hainan population, to collect enough specimens for the examination of their morphological variations at the population level.

Figure 1. 

The gross morphology of two specimens identified as Rhamnella rubrinervis. A R. rubrinervis from Hainan Island (Qionghai, Tayang, 27 Nov 1936, X.Q. Liu 28256, PE) B R. rubrinervis from Guangxi (Baise, Napo, 15 Oct 2014, B.Y. Huang et al. 074LY, GXMG). The leaf length to width ratio and fruit size are totally different between two specimens.

Our specific field survey on the special record on Hainan Island showed that no extant tree similar to R. rubrinervis existed in Tayang Township, Qionghai City (Figure 1A) and that the habitat was badly destroyed by human activity. However, we finally explored two new R. rubrinervis populations with smaller leaf length to width ratio and shorter fruit in Baoting County during our field surveys in 2018 (Table 1; Figures 2–4). Morphological comparison at the population level was conducted, showing three Hainan populations distinctly differed from those of R. rubrinervis from other regions with the smaller leaf length to width ratio (1.9–2.4 vs. (2.5) 2.8–3.9), shorter fruit (6.5–7.5 mm vs. 8.2–11.1 mm), smaller fruit length to width ratio (1.3–1.5 vs. 1.6–2.2), dumpier seeds (5.0–7.0 × 4.5–5.5 mm vs. 7.1–9.9 × 4.0–5.5 mm), smaller seed length to width ratio (0.9–1.5 vs. 1.6–2.1) and mucronate seed apices (Figures 3–4; Table 2). Differences were also found in leaf base shape, petiole length (3–9 mm vs. 7–12 mm) and fleshy fruit size (8.7–10.9 × 7.5–10.6 mm vs. 10.2–12.1 × 10.1–12.5 mm). No significant difference in flowers was found between the distinct Hainan populations and those of R. rubrinervis from other regions. A further PCA of all closely related specimens (including historical type specimens of four published synonyms), based on 10 morphological characters, was carried out (Table 3), distinguishing these three Hainan populations (in Baoting and Qionghai) and those of R. tonkinensis and R. rubrinervis from other regions into two major groups (Figure 5). One of the two groups merely represented these distinct Hainan populations; the remaining populations of R. tonkinensis and R. rubrinervis formed the other group. The first principal component axis (PC1; accounting for 38.99% of the variation) significantly separated these distinct Hainan populations from the other two species, while the second principal component axis (PC2; accounting for 20.87% of the variation) failed in separating both groups (Figure 5).

Figure 2. 

Distribution and locations of R. brachycarpa, R. rubrinervis and R. tonkinensis, based on field surveys, Chinese Virtual Herbarium (http://www.cvh.org.cn/) and Global Biodiversity Information Facility (https://www.gbif.org/).

Figure 3. 

Rhamnella brachycarpa Z. Qiang Lu & Y. Shuai Sun, sp. nov., drawn from the tree of Z.Q. Lu 2018HN3001.

In this study, we demonstrated that three distinct Hainan populations previously placed under R. rubrinervis should be described as a new species, based on the distinct morphology and clustering analysis at the population level. Morphological comparison showed that they differ from those of R. rubrinervis from other regions in mucronate seed apices, shorter fruit, dumpier seeds and smaller length to width ratios of leaves, fruit and seeds. PCA analysis further clustered these special Hainan populations into a separated group distinctly different from those of R. rubrinervis and R. tonkinensis from other regions (including historical type specimens of those published synonyms). Our results also showed that R. rubrinervis and R. tonkinensis had a similar morphology, indicating the obscure species boundary between them. This finding is consistent with a previous study (Fan and Yang 1997). Obviously, these special Hainan populations are distinctly different from all extant relatives from other regions (Chen and Carsten 2007). Given this, we describe them in the following as a new species.

We thank Dr. Huizhe Feng for the help in collecting samples and Mr. Xincheng Qu for drawing the picture. This work is supported by grants from “1000 Youth Talents Plan” of Yunnan Province, CAS “Light of West China” Program and start-up research fund of XTBG (No. B18114BN).