Gastrochilus yehii S. I. Hsieh, C. T. Lee & J. H. Wu sp. nov. (Fig. 1 & 2)

Type: TAIWAN, Nantou County, Rwn’ai Township, 24° 04’ 20.4” S, 121° 13’ 59.1” E, alt. 2000-2500 m elevation, 20 Sep. 2021, J. H. Wu 538 (holotype: PPI!; isotype: TCF!)

Epiphytic pendulous herb. Stems clustered, rarely branched, 4–12 cm long, 1.5–2.5 cm wide, with purple spots, usually rooting. Leaves distichous, blade narrowly elliptic or falcate-lanceolate, 18–25 × 5–7 mm, base cuneate, apex acute, surface glossy, with purple dots. Inflorescence corymbose, 15–20 mm long, 4–6 flowered; peduncle 3–8 mm long, with a tiny bract; bracts deltoid, 0.6–0.7× 0.3–0.4 mm; pedicle and ovary yellow-green with purple spots, 5–10 mm long. Flowers yellow-green, with purple blotches, 6–8 mm tall, 5–8 mm wide, lips reddish purple with yellow-whiteness. Dorsal sepal oblong-elliptic, concave, 4.0– 5.5 mm × 2.5–3.0 mm, apex obtuse; lateral sepal similar to dorsal sepal; petals sub-obovate, 4.0– 5.5 mm × 2.0– 2.5 mm, apex acute. Lip with distinct partition between a flat epichile and a saccate hypochile; epichile semiorbicular, 2.0–2.5 × 4.0– 4.5 mm, apex obtuse, whitish with purple dots, densely ciliate on upper surface, central cushion greenish, adaxially covered with dense white long hair; hypochile subconical, 2.5– 3.0 mm tall, 2.0– 2.5 mm in diam., purple with reddish speckles, acute at bottom, glabrous on both surfaces. Column stout, ca. 1.7 mm long, 1.2 mm wide; rostellum forked at apex; anther cap galeate with curved emarginate apex, ca. 1.0 × 1.2 mm; pollinarium ca. 1.5 mm long; pollinia 2, ca. 0.5 mm in diam.; stipe elongate, ca. 1.2 mm.

Etymology: The specific epithet is derived from Dr. Chuan-Rong Yeh, who is an eminent plant taxonomist from Taiwan.

Distribution and ecology: Gastrochilus yehii is currently only found in the mixed broadleaf-conifer forest between 1500–2000 m elevation in Ren’ai Township of Nantou County (Fig. 3). The forest is dominated by Quercus spp., Castanopsis cuspidata var. carlesii (Hemsl.) T. Yamaz., Trochodendron aralioides Siebold & Zucc., Machilus pseudolongifolia Hayata, Litsea morrisonensis Hayata, Acer rubescens Hayata, Fatsia polycarpa Hayata, Daphniphyllum himalense Blume subsp. macropodum (Miq.) Huang, Pinus taiwanensis Hayata and planted Cunninghamia konishii Hayata. The new species is epiphytic on the large trunks and branches of forest trees. Flowering occurs from August to October.

Conservation status: The new species Gastrochilus yehii is known from type locality, Ren’ai Township, Nantou County, Taiwan. So far only 50 mature individuals have been recorded. Extent of occurrence is about 5 km 2 and area of occupancy is about 1 km 2. In Taiwan, the most wildlife orchids are still under threat of illegal wild collection. Based on its limited range of distribution and number of mature individuals, it is provisionally assessed as critically endangered (CR) according to the criteria of the IUCN guidelines (Editorial Committee of the Red List of Taiwan Plants 2017; Guidelines for Using the IUCN Red List Categories and Criteria 2022).

Similar species discussion: Despite its similarity to other members of the Gastrochilus alliance in terms of gross floral and leaf morphology, Gastrochilus yehii is readily distinguished by the features of its lip. Morphologically, it shows a close affinity to G. ciliaris F. Maek., G. deltoglossus T.C. Hsu, S.I. Hsieh, J.H. Wu & H.C. Hung and G. raraensis Fukuy (Fig. 4). Based on the essential features, G. raraensis, an endemic species in Taiwan, is the most similar species to G. yehii, but the former can be differentiated by its pale green to yellowish lip, and longer (ca. 3.5– 4.5 mm) distinctly-curved-forward conic-cylindrical hypochile with blue spots (Fukuyama 1934; Lin 1987; Chung & Leou 1993; Su 2000; Chen et al. 2009; Hsu et al. 2018). Furthermore, G. deltoglossus, another endemic species in Taiwan, is resemblant to G. yehii in plant habit and hypochile shape, but differs in flower color and has a narrower triangular epichile (2.0–2.5 × 3.0– 3.5 mm) adaxially covered with short hair (Hsu et al. 2018). In addition, G. ciliaris, recorded from Taiwan and Japan, can be clearly distinguished by its yellow-green lip, shorter (2 mm tall) semispherical hypochile and narrower (3 mm in diam.) semiorbicular epichile (Chung and Leou 1993; Su 2000; Chen et al. 2009; Hsu et al. 2018). The morphological characters between the four species listed above are compared in Table 1.