Sorbus gongshanensis (Rosaceae), a new species from the Hengduan Mountains, China

Abstract Sorbus gongshanensissp. nov., a new species from the Hengduan Mountains China, is described and illustrated. It is similar to S. kurzii from China (Yunnan & Xizang), Nepal, and Sikkim in the size of the leaflets, glabrous veins, persistent (sometimes) herbaceous stipules and reddish brown villous inflorescences and red fruits, but differs in its serrate leaflet margins toothed in the distal half or often almost to their base, reddish brown villous to glabrous hypanthium and reddish brown villous infructescences, among other characteristics.


Introduction
Sorbus L. sensu lato (s.l.; Rosaceae) comprises about 260 species distributed in the temperate zone of the Northern Hemisphere (Aldasoro et al. 1998;Lu and Spongberg 2003;Zika and Bailleul 2015;Sennikov and Kurtto 2017). Both molecular (Campbell et al. 2007;Lo and Donoghue 2012) and morphological evidence (Zheng and Zhang 2007) suggests that Sorbus s.l. is polyphyletic. Sorbus sensu stricto (s.s.) includes ca. 80 species and is characterized by pinnately compound leaves. Recent molecular study of Sorbus s.s. suggests that the most recent common ancestor originated in eastern Asia (Li et al. 2017). The greatest diversity of species of Sorbus s.s. (ca. 60 species) is found in the mountains of southwestern China (principally the Hengduan mountains) and adjacent areas of Myanmar, Nepal, and the eastern Himalaya (Long 1987;Lu and Spongberg 2003;McAllister 2005;Watson and Manandhar 2012). This region is one of the world's biodiversity hotspots, as it also is for the genus Sorbus (Myers et al. 2000;Li et al. 2017).
While studying Sorbus for the Flora of Pan-Himalaya Project, we found several accessions from Yunnan and Xizang, China, at the Harvard University Herbarium (GH) that were markedly different from other species of Sorbus. After detailed morphological examination, field investigation and literature study, it was concluded that these specimens represent an undescribed species, which we name S. gongshanensis. The description of S. gongshanensis is based on dried herbarium specimens stored at GH (all herbarium acronyms in this paper follow Thiers 2019).

Material and methods
Morphological study was based on specimens deposited in the following herbaria: A, BM, CAS, CDBI, G, GH, K, KUN and NF. Macroscopic descriptions were based on the specimen sheets and notes made in the field. Detailed observations were conducted using an optical microscope. For scanning electron microscopy (SEM), dried pollen grains and stomata were mounted on metal stubs and sputtered with technical gold, and then were observed under Phenom proX SEM at 10 kV accelerating voltage at the Chengdu Institute of Biology, CAS. Pollen grains come from the field collection from Motuo [China,Xizang,Meng Li 00281(NF)]. Terminology of descriptive terms followed Flora of China vol. 9 (Lu and Spongberg 2003). Conservation assessment was based on the known distribution data and followed the IUCN red list category criteria (IUCN 2017). Diagnosis. Similar to S. kurzii, but differs in its serrate leaflet margins toothed in the distal half or often almost to their base, reddish brown villous to glabrous hypanthium and reddish brown villous infructescences.
Etymology. The specific epithet refers to the type locality, Gongshan County. Phenology. Flowering May-July, fruiting September-October. Distribution and ecology. Sorbus gongshanensis is known from the Yunnan & Xizang Province, China (Fig. 3). It grows in broad-leaved forests or on rocky slopes; 2500-3000 m. Conservation status. The distribution of S. gongshanensis is based on three collections. The collection notes mention that S. gongshanensis is occasional in forests among boulders. There is no direct or indirect information about its current conservation status or possible threats. We therefore assign the conservation status of S. gongshanensis as 'Data Deficient (DD)' according to the IUCN red list criteria (IUCN 2017).

Discussion
Sorbus species show a high level of similarity in flower structure and color. The numbers of leaflets and fruit color are also fairly consistent across the group (Li et al. 2017). The number of leaflet pairs in Sorbus range between 2 and 21 pairs, and fruit color ranges between white, pink, red and orange-red (Lu and Spongberg 2003). While  useful floral morphological characteristics are limited, pairs of leaflets, leaflet size, serra position, stipule shape and fruit color can provide valuable information for the identification of Sorbus at the species level.
There are also several species sometimes with few pairs of leaflets found in other geographic regions. They are S. gracilis