Thalictrum yambaruense Michim., Yokota & S.Fujii, 2025

Michimoto, Kanae, Yokota, Masatsugu, Abe, Atsushi, Fujii, Shinji, Takahashi, Daiki, Kakishima, Satoshi, Yu, Chih Chieh, Ito, Takuro & Maki, Masayuki, 2025, New Species of Thalictrum (Ranunculaceae) from Okinawa Island in Japan and Its Phylogenetic Implications, Phytotaxa 696 (1), pp. 55-70 : 64-65

Thalictrum yambaruense Michim., Yokota & S.Fujii , sp. nov.

Diagnosis: Similar to Thalictrum watanabei , this species differs in having pedicels shorter than 5 mm (vs. 10–15 mm in T. watanabei ). Sepals are approximately 2 × 1 mm (vs. 3–4 × 1.5–2 mm). Pistils are not bent or are slightly bent (vs. strongly bent ( Table 1; Figs. 2 & 3 ).

Holotype: JAPAN, Okinawa Prefecture: Okinawa Island, Kunigami-gun, Kunigami-son , Aha. 16 April 2022, A. Abe, M. Yokota, T. Ito, K. Michimoto, T. Nakasone, S. Higa & J. Shinjo 2095 ( OCF).

Description: Herbs, perennial, glabrous, up to 20 cm tall. Roots tuberous. Stems simple or few branched. Basal leaves 3–5; leaf blade 1- or 2-ternate; leaflets broadly ovate or orbicular, herbaceous, abaxially greenish white, 1–1.8 × 1–1.5 cm, apex 3-lobed; petiole 2–5cm. Inflorescences cymes with 2–15 flowers. Flowers 0.3–0.5 cm in diam.; pedicels less than 5 mm. Sepals 3–4, persistent, white, elliptic, ca. 2 × 1mm. Stamens white, 2–4mm; filament base filiform, apex oblanceolate, same as broad as anther. Carpels 5–10; style not bend or bend slightly, stigmas lanceolate. Achenes stipe ca. 0.5 mm, body fusiform, compressed, ca. 2 mm.

Distribution and habitat: Known only from the type locality in the Yambaru region. The upper reaches of the river basin consist of well-preserved mountain streams in natural forest stands ( Kyushu Regional Forest Office, 2000). The species grows on dripping cliff faces.

Etymology: The specific epithet “yambaruense ” refers to the type locality, the Yambaru region.

Japanese common name : Yambaru-karamatsu (nov.).

Paratypes: JAPAN, Okinawa Prefecture: Okinawa Island, Kunigami-gun, Kunigami-son, Aha. 16 April 2022, Takuro Ito 7857 (TUS). 29 August 1994, M. Yokota 10369 (RYU).

Conservation status: Thalictrum yambaruense is known from a single population, exclusively located in the upper reaches of the Yambaru region in northern Okinawa Island ( Yokota et al. 1997). It is classified as a critically endangered species (Category IA, CR), similar to Thalictrum minus ( Okinawa Prefectural Department of Environmental Affairs 2018). The total number of mature plants or ramets is estimated to be fewer than 50, classifying it as a critically endangered species IA (CR) according to the IUCN Red List Categories and Criteria “D” (IUCN 2000). A water storage dam built in the lower reaches may have eradicated unknown populations downstream.

Figures

FIGURE 2. Thalictrum yambaruense nov. from Okinawa Island. A, Habit; B, Leaves; C, Flowers; D, Tuberous roots. B & D: scale bar = 1 cm; C: scale bar = 1 mm. [A. Wild individuals on Okinawa Island. B–D from Takuro Ito 7857. All photos from Takuro Ito].

FIGURE 3. Morphological comparisons between T. yambaruense nov. from Okinawa Island (A, C, E, G, I, & K) and T. urbaini (B, D, F, H, J, & L). A & B: Leaflets; C–F: Inflorescence; G & H: Sepals; I & J: Stamens; K & L: Carpels.A& B: scale bar = 1 cm; C–L: scale bar = 1 mm. [A, C, E, G, I & K from Takuro Ito 7857; B, D, F, H, J & L from Takuro Ito 8947. All photos from Takuro Ito].