Carpinus gigabracteatus Z. Qiang Lu, a new hornbeam species from southeast Yunnan of China, is described and illustrated in this study. It possesses extremely large bracts and is closely related to C. tsaiana Hu and C. tschonoskii Maxim., based on the characters of large bract size and bracts without lobes at the base of inner margins. Furthermore, morphological comparison suggested it was distinctly different from C. tschonoskii by a series of characters from leaf, infructescence, bract and nutlet and from C. tsaiana by its leaf length to width ratio (1.4–2.0 vs. 2.0–2.4), lateral veins significantly impressed adaxially, number of lateral veins on each side of midvein (9–14 vs. 14–17), bract length (3.9–4.8 vs. 2.5–3.2 cm) and bract length to width ratio (2.3–3.1 vs. 1.5–2.1). Therefore, this hornbeam, based on only one population from southeast Yunnan, is here erected as a new species, named as C. gigabracteatus.

Carpinus gigabracteatus, large bract, new species

The hornbeam genus Carpinus L. is the largest genus in the subfamily Coryloideae of Betulaceae (Holstein and Weigend 2017; Li et al. 2018). To the present time, more than 40 species have been published (Hu 1964; Qi 1981; Liang and Zhao1991; Li and Skvortsov 1999; Tong et al. 2014; Holstein and Weigend 2017; Lu et al. 2017, 2018). Due to their peculiar and beautiful fruit cluster, some hornbeams are used as important ornamental plants (Fini and Ferrini 2011; Li et al. 2018). The bract characters of fruit clusters are also important evidence for species identification (Hu 1964; Li and Skvortsov 1999; Lu et al. 2017). According to the bract characters, three morphological groups are separated by bracts completely covering the nutlet, all bracts with conspicuous lobes at the base of inner margins and bracts without lobes or rarely with inconspicuous lobes at the base of inner margins, respectively (Li and Skvortsov 1999; Lu et al. 2017). In China, the last is the largest group, including about 26 species (Holstein and Weigend 2017; Lu et al. 2018), most of them being narrow endemics within China (Li and Skvortsov 1999). Bract size is the critical trait for distinguishing these species between each other (Hu 1964; Li and Skvortsov 1999). Almost all species in this group have bracts less than 3.2 × 1.3 cm. However, the present author found a hornbeam population during field surveys in southeast Yunnan with bracts without lobes at the base of inner margins, but with large bracts (3.9–4.8 × 1.4–2.0 cm) and these could not be ascribed to any described species. In addition, those hornbeams distributed in other regions, including Carpinus betulus L., C. caroliniana Walter, C. faginea Lindl., C. laxiflora (Siebold & Zucc.) Blume, C. orientalis Mill. and C. tropicalis (Donn.Sm.) Lundell, all have smaller bract size than this Yunnan population, which also distinctly differs in bract lobes at the base of inner margins and leaf characters (Hu 1964; Furlow 1987; Holstein and Weigend 2017). However, in China, within the morphological group possessing bracts without lobes at the base of inner margins, only C. tsaiana Hu has the same bract width but differs from the Yunnan population by bract length. C. tschonoskii Maxim. has similar bract length but with different bract width (Li and Skvortsov 1999). The present author, therefore, hypothesised that this morphologically different population from southeast Yunnan may represent a potential new hornbeam. In order to test this hypothesis, the present author carried out morphological comparisons with representatives of all hornbeams in China.

This hornbeam population from southeast Yunnan possesses extremely large bracts (3.9–4.8 × 1.4–2.0 cm) (Figures 1, 2). Phenotypic differentiation of bract length and width for hornbeams in China showed it was closely related to C. langaoensis, C. tsaiana and C. tschonoskii (Figure 3). Bracts, with and without lobes at the base of inner margins, corresponded to C. langaoensis and the Yunnan population, respectively (Figure 3). Morphological comparison with C. tsaiana and C. tschonoskii showed the Yunnan population distinctly differed from C. tschonoskii by leaf length to width ratio (1.4–2.0 vs. 2.0–2.3), lateral veins significantly impressed adaxially, infructescence size (8.0–12.0 × 5.0–5.5 cm vs. 6.0–10.0 × 3.0–4.0 cm), bract width (1.4–1.8 vs. 0.6–1.2 cm), nutlet shape (ovoid-ellipsoid vs. broadly ovoid), nutlet size (5.3–7.0 × 4.0–5.5 mm vs. 4.0–5.0 × 3.0–4.0 mm) and densely pubescent or villous and resinous glandular on nutlet (Table 2) and from C. tsaiana by leaf length to width ratio (1.4–2.0 vs. 2.0–2.4), lateral veins significantly impressed adaxially, number of lateral veins on each side of midvein (9–14 vs. 14–17), bract length (3.9–4.8 vs. 2.5–3.2 cm) and bract length to width ratio (2.3–3.1 vs. 1.5–2.1).

Figure 1. 

Carpinus gigabracteatus Z. Qiang Lu A the whole plant, habitat and location B branches with infructescences and leaves C infructescences D leaf E bracts F nutlet.

Figure 2. 

Carpinus gigabracteatus Z. Qiang Lu was drawn from Z.Q. Lu 2019GY0801 (HITBC).

Taxonomic treatment

Carpinus gigabracteatus Z. Qiang Lu, sp. nov.

urn:lsid:ipni.org:names:77209333-1

Figures 1, 2 大苞鹅耳枥

Diagnosis

Carpinus gigabracteatus differs from C. tsaiana by leaf length to width ratio 1.4–2.0 (compared to 2.0–2.4), lateral veins impressed adaxially, 9–14 lateral veins on each side of the midvein (compared to 14–18), bract length 3.9–4.8 cm (compared to 2.5–3.0 cm) and bract length to width ratio 2.3–3.1 (compared to 1.5–2.1).

Type

China. Yunnan: Wenshan Prefecture, 23°09'35"N, 104°05'53"E, 1591 m alt., karst limestone hill, 23 Sep 2019, Z.Q. Lu 2019GY0801 (holotype, HITBC; isotypes, HITBC and LZU).

Description

Tree to 8 m tall, deciduous; bark grey, smooth. Branchlets black-brown, glabrescent. Petiole 7–14 mm, densely yellow pubescent when young, glabrescent in the following few months; leaves alternate, leaf blade elliptic, ovate-elliptic or ovate, usually 7–12 × 4–7 cm, length to width ratio 1.4–2.0, leathery, abaxially sericeous-villous or sparsely villous along veins, bearded in axils of lateral veins, adaxially densely villous when young, base rounded, rounded-cuneate or cordate, margin regularly or irregularly and doubly minutely serrate, apex acuminate; lateral veins 9–14 on each side of midvein, raised abaxially, significantly impressed adaxially. Male inflorescence pendulous, spicate-cymose, cylindrical, enclosed by buds during winter, with many overlapping bracts, 1.0–3.0 × 0.4–0.6 cm when mature; flowers without bracteoles, inserted at base of bracts. Female inflorescence terminal or axillary on dwarf shoots, racemose; flowers paired; bracts leaf-like, complanate, overlapping. Mature infructescence 8.0–12.0 × 5.0–5.5 cm; peduncle 1.5–2.5 cm, densely yellow hirsute; giant bracts loosely overlapping, 3.9–4.8 × 1.4–1.8 cm, abaxially densely yellow hirsute along reticulate veins, outer margin coarsely dentate and rarely entire, commonly without but sometimes with basal lobe, inner margin entire, with inflexed basal auricle, apex acuminate; veins 5–6. Nutlet ovoid-ellipsoid, 5.3–7.0 × 4.0–5.5 mm, densely pubescent, densely villous at apex, densely brown resinous glandular, prominently 9 or 11-ribbed.

Etymology

This hornbeam from southeast Yunnan has extremely large bracts, which are distinctly different from other closely related hornbeams, and therefore is given the epithet gigabracteatus.

Phenology

Flowering from April to May and fruiting from July to September.

Habitat, distribution and conservation

Up to now, only one C. gigabracteatus population has been collected from southeast Yunnan. For its population census, only six mature trees (6–8 m in height) and 13 seedlings grow on a steep karst limestone hill. To the present author’s knowledge, the bract size of this species is now the largest across the whole hornbeam genus in China. Hence, it has great horticultural and ornamental value and some people like to dig them up to grow them as ornamental trees. Manual digging involves removing lots of large rocks on the limestone hill where this new species grows, resulting in significant damage to the habitat. So far, no other population has been found, even though multiple rounds of field surveys in Wenshan Prefecture and adjacent regions have been carried out in the years from 2013 to 2019. Therefore, this hornbeam is exposed to significant threats from human activity due to its rarity and horticultural and ornamental value. According to the IUCN Categories and Criteria (IUCN 2016), the present author here classifies this species as “Critically Endangered” (CR). Fortunately, these mature trees can provide the possibility to expand population based on seeds.

Additional specimens examined

China. Yunnan: Wenshan Prefecture, 23°09'35"N, 104°05'53"E, 1591 m alt., karst limestone hill, 23 Sep 2019, Z.Q. Lu 2019GY0802; the same locality, 10 July 2019, Z.Q. Lu 20189801–Z.Q. Lu 20189804.

Figure 3. 

Phenotypic differentiation of bract length and width, across all Chinese hornbeam species according to Holstein and Weigend (2017). Data from all examined specimens in Table 1 and descriptions by Hu (1964), Qi (1981), Liang and Zhao (1991), Li and Skvortsov (1999), Tong et al. (2014) and Lu et al. (2017, 2018). Those hornbeams, whose bracts are without lobes or rarely with inconspicuous lobes at the base of inner margins, are in bold.

Bract morphology in the Carpinus genus provides important traits for species identification (Hu 1964; Li and Skvortsov 1999; Lu et al. 2017, 2018). In this study, the present author demonstrated a hornbeam population from southeast Yunnan as a new species, based on the following evidence. First, its large bract size, including the characters of bract length and width, showed it to be closely related to C. langaoensis, C. tsaiana and C. tschonoskii (Figure 3). However, this Yunnan population, with its bract without lobes at the base of inner margins, can be easily distinguished from C. langaoensis, whose bracts have conspicuous lobes at the base of inner margins (Li and Skvortsov 1999; Lu et al. 2017, 2018). In addition, more characters, based on leaf and nutlet, can also distinguish both of them (Lu et al. 2017). Furthermore, other hornbeams distributed outside of China, including C. betulus, C. caroliniana, C. faginea, C. laxiflora, C. orientalis and C. tropicalis, are all different from this hornbeam population from southeast Yunnan, by the smaller bract size and other characters of bract and leaf (Hu 1964; Furlow 1987; Holstein and Weigend 2017). Finally, morphological comparison suggested it differed from C. tschonoskii by a series of characters from leaf, infruGctescence, bract and nutlet (Table 2), which was consistent with the description by Li and Skvortsov (1999). Therefore, the most similar species to the Yunnan population is C. tsaiana, based on similar morphology and distribution (Li and Skvortsov 1999; Holstein and Weigend 2017). However, all eight typical specimens of C. tsaiana (including seven type specimens) from three populations were distinctly different from this Yunnan population by leaf length to width ratio (1.4–2.0 vs. 2.0–2.4), lateral veins significantly impressed adaxially, number of lateral veins on each side of midvein (9–14 vs. 14–17), bract length (3.9–4.8 vs. 2.5–3.2 cm) and bract length to width ratio (2.3–3.1 vs. 1.5–2.1) (Table 2; Figures 1–3). Hence, the present author proposes to recognise this hornbeam population from Yunnan as a new species.

This work is supported by grants from the National Natural Science Foundation of China (31901326) and CAS “Light of West China”.