Hydrangea xinfeniae W.B.Ju & J.Ru, a new species of Hydrangeaceae from Sichuan Province, China, is described and illustrated. The new species belongs to Hydrangea sect. Dichroa (Lour.) Y.De Smet & Samain, with its distinctive characteristic being the nearly superior ovary. It shares morphological similarities with H. yaoshanensis (Y.C.Wu) Y.De Smet & C.Granados, but can be distinguished by its hirsute trichomes densely covered on the branchlets, leaves, peduncles and pedicels, broadly elliptic to rectangular-elliptic leaf blade with nearly rounded base, coarse teeth leaf margins, 3–4 pairs of lateral veins, corymbose cyme with few and loose branches, lanceolate bract, the calyx tube and lobes margin with sparsely hirsute trichomes, adaxially glabrous and abaxially sparsely hirsute petal, outer whorl filaments are linear, inner ones are awl-shaped, glabrous styles, and the nearly superior ovary. H. xinfeniae sp. nov. currently known from only three relatively small populations of the type locality, and its conservation status is assessed as Data Deficient (DD).

Hydrangea sect. Dichroa, morphology, taxonomy

Hydrangea L. (Linnaeus 1753) had been treated to be a member of the tribe Hydrangeeae in Hydrangeaceae, comprising approximately 73 species distributed from Eastern to Southeastern Asia, as well as from Southeastern North America to Central America and Western South America (Huang et al. 1995; Wei and Bartholomew 2001). However, morphological and phylogenetic studies have indicated that Hydrangea is not monophyletic (Hufford 1995, 1997; Soltis et al. 1995; Ge 2003; Jacobs 2010; Samain et al. 2010; Zhang et al. 2021). Based on molecular phylogenetic analyses, De Smet et al. (2015) proposed the broad concept of Hydrangea, which encompasses Hydrangea s. str. and the remaining eight satellite genera within Hydrangeeae, including Broussaisia Gaudich., Cardiandra Siebold & Zucc., Decumaria L., Deinanthe Maxim., Dichroa Lour., Pileostegia Hook.f. & Thomson, Platycrater Siebold & Zucc., Schizophragma Siebold & Zucc., grouped as a section within Hydrangea s. l. Furthermore, Platycrater is merged into Hydrangea sect. Asperae (Rehder) Y.De Smet & Samain. The broad concept of Hydrangea has been supported by Yang (2022). In this study, we also adopt the broad concept of Hydrangea.

Hydrangea sect. Dichroa (De Smet et al. 2015) comprises 12 species, widely distributed in the tropical and subtropical regions of Southeast Asia, with only a few species extending to Pacific islands. In China, there are six species of this section, distributed from the southwestern to eastern regions (Huang 1987; Huang et al. 1995; Wei and Bartholomew 2001). In recent years, new species from this section have been discovered in China (Huang et al. 2018; Deshmukh and Shende 2021).

During field investigations in Huagaoxi National Nature Reserve of Shuwei Town, Xuyong County, Sichuan Province, an unknown population of Hydrangea was discovered. After conducting a comprehensive review of relevant taxonomic literature (Chun 1954; Huang 1987; Huang et al. 1995; Wei and Bartholomew 2001; Jacobs 2010; Huang et al. 2018) and meticulously examining voucher specimens from various herbaria (A, B, C, CAS, CDBI, E, IBK, IBSC, P, PE, K, KUN, L, NYBG, US), we have identified that it represents a new species within Hydrangea sect. Dichroa, exhibiting morphological similarity with H. yaoshanensis (De Smet et al. 2015). In this study, we provide a detailed morphological characterization of this species based on our field observations and a thorough examination of the holotype specimen, and describe it as a new species.

The voucher specimens of the new species in this study were collected from the type locality, Huagaoxi National Nature Reserve, and are deposited in CDBI and KUN. The morphological description of characteristics of the new species was conducted through both living plants in the field and voucher specimens. The morphological measurements of the new species were based on living plants. We examined available online specimen images of Hydrangea sect. Dichroa species stored in A, B, C, CAS, CDBI, E, IBK, IBSC, P, PE, K, KUN, L, NYBG, and the US through the Chinese Virtual Herbarium (https://www.cvh.ac.cn/) and JSTOR Global Plants (https://plants.jstor.org/). Additionally, we compared the morphological characteristics of the new species with those of similar species, relying on online voucher specimen images and published literature (Huang 1987; Huang et al. 1995; Huang et al. 2018; Deshmukh and Shende 2021).

Hydrangea sect. Dichroa is distinguished from related sections within Hydrangea by characteristics such as being an erect shrub, inflorescences with all fertile flowers, calyx lobes never petaloid, and fruit as berry. Based on a series of morphological characteristics, including shrub, opposite leaves, bisexual and isomorphic flowers, an inverted conical calyx tube attached to the ovary, petals with slightly inwardly curled tips forming hooks, outer whorl filaments are linear, inner whorl filaments awl-shaped, and separated styles, Hydrangea xinfeniae sp. nov. is classified within the Hydrangea sect. Dichroa (Huang 1987; Huang et al. 1995; Wei and Bartholomew 2001; De Smet et al. 2015). Species within Hydrangea sect. Dichroa were previously treated as members of the genus Dichroa Lour. (Loureiro 1790) in Hydrangeeae. In China, Hydrangea sect. Dichroa comprises six known species, including H. daimingshanensis (Y.C.Wu) Y.De Smet & C.Granados, H. febrifuga (Lour.) Y.De Smet & C.Granados, H. hirsuta (Gagnep.) Y.De Smet & C.Granados, H. mollissima (Merr.) Y.De Smet & C.Granados, H. yaoshanensis, H. yunnanensis (S.M.Hwang) Y.De Smet & C.Granados, as well as one new species, H. fistulosa (G.H.Huang & G.Hao) U.B.Deshmukh & M.B.Shende (Huang et al. 1995; Huang et al. 2018; Deshmukh and Shende 2021). Among the aforementioned species distributed in China, H. xinfeniae sp. nov. exhibits morphological resemblances to H. yaoshanensis in having the shape of leaf blade, corymbose cymose, inverted conical calyx tube and spherical-shaped berry. However, H. xinfeniae sp. nov. can be distinguished by its dense hirsute trichomes on branchlets, leaves, peduncles, and pedicels, as well as its broadly elliptic to rectangular-elliptic leaf blades, nearly entire circular leaf basis, coarse teeth on the leaf margin, 3–4 lateral veins on each side, corymbose cyme with sparsely and loosely branched, calyx tube with sparse hirsute trichomes, lobes with sparse hirsute trichomes only on the margins, glabrous on the inner surface of the petals, scattered hirsute trichomes on the outer surface, outer whorl filaments are linear, inner ones are awl-shaped, styles are glabrous, and nearly superior ovary. Notably, the nearly superior ovary is a unique characteristic of H. xinfeniae sp. nov. compared to other species within Hydrangea sect. Dichroa. For a detailed comparison of features, please refer to Table 1.

We are grateful to Zhenlong Liang for providing hand-drawn illustrations. We appreciate the staff at the herbarium for supplying online specimen images. We would like to express our appreciation for the support received from the Huagaoxi National Nature Reserve during our field investigations.