Croton nagaoi Tagane, Soulad. & Souvann., 2022

Tagane, Shuichiro, Souvannakhoummane, Keooudone & Souldeth, Phetlasy, 2022, Croton nagaoi, a new species of Euphorbiaceae from southern Laos, Phytotaxa 570 (2), pp. 102-108 : 102-106

Croton nagaoi Tagane, Soulad. & Souvann. , sp. nov. ( Figs. 2 & 3 )

TYPE: — LAOS. Champasak Province: Ba Chieng Chlernsouk , 15°06′44.56″N, 105°58′46.8″E, 231 m., 15 December 2018, Tagane, Nagahama, Souladeth & Pisuttimarn L 2459 (holotype KAG [KAG128559!], isotype FOF [FOF0005791!]) .

Most similar to Croton thorelii Gagnepain (1921: 560) of Laos, Cambodia, Thailand and Vietnam in size and shape of lamina, short petioles less than 1 cm long, and basal glands lateral on the abaxial midrib base, but distinguished from it by its lower leaf surface densely covered with persistent yellowish-cream stellate hairs (vs. very sparsely pubescent and glabrescent in C. thorelii ) ( Fig. 4 ), and the presence of petals in pistillate flowers (vs. absence).

Shrubs to 1.5 m tall.Young branches densely covered with stellate hairs, these persistent, yellowish-cream, 0.3–0.5 mm in diam., with (14–)18–24 free or only basally fused radii. Stipules narrowly lanceolate, 1.5–2 mm long, glabrous adaxially, stellate hairy abaxially. Leaves pseudo-verticillate, seemingly 3–5 leaves at nodes; petiole 0.3–0.6 cm long, densely pubescent; blade oblanceolate to obovate-elliptic, (3.3–)5.5–17.6 × (1.1–) 2.4–6.3 cm, thinly coriaceous, apex shortly acuminate, base rounded-subcordate, margin serrate, teeth 3–9 mm apart, grayish green to light yellowish brown adaxially, glabrous except midrib adaxially, light yellowish brown, densely covered with yellowish-cream stellate hairs abaxially; basal glands 2, sessile, elevated, lateral on the abaxial midrib base, marginal glands absent; midrib prominent on both surfaces, secondary veins 9–15 pairs, not triplinerved, prominent abaxially, tertiary veins indistinct. Inflorescence (sub)terminal, solitary, 2.7–5.9 cm long, stellate-hairy throughout, with 1–5 pistillate flowers, sometimes completely staminate; bracts caducous, not seen, bracteoles lanceolate, ca. 3 mm long, densely pubescent outside, glabrous inside. Staminate flowers: pedicel ca. 1 mm long; bracteoles narrowly triangular, ca. 2.2 mm long, pubescent outside, glabrous inside, sepals ovate-triangular, ca. 2 × 2 mm, covered with yellowish-cream stellate hairs, glabrous inside, apex acute, margin ciliate; petals ovate-elliptic, ca. 2.2 × 1.2 mm, pubescent outside, glabrous inside, apex obtuse; stamens 12 in 4 whorls, ca. 2.3 mm long, filament 1.4–2 mm long, pubescent, anthers ca. 0.7 mm long. Pistillate flowers: pedicel ca. 1 mm long, densely stellate hairy; sepals ovate-triangular, ca 3.5–4 × 2 mm, dense stellate-hairy outside, glabrous inside; petals ovate-elliptic, ca. 3 × 1.5 mm, covered with simple hairs only, apex acute, margin ciliate; ovary ca. 3.5 mm in diam., with densely stellate hairs; style column absent, stigmas ca. 3.5 mm long, bifid at ca. 1 mm from base. Fruits subglobose, ca. 0.8 cm in diam., sulcate, covered with stellate hairs. Seeds 1–3, ca. 6.5 mm long, pale brown.

Distribution:— Laos (so far known only from the type locality).

Habitat:— In area of seasonal dry evergreen forest, roadside thickets; at an elevation of 231 m.

Phenology:— Flowering and fruiting in December.

Etymology:— The species epithet is dedicated to the excellent Nagao Natural Environment Foundation (http:// www.nagaofoundation.or.jp/e/) that has promoted nature conservation in Asia-Pacific countries since its establishment in 1989. Thanks to their support, we could have intensive field surveys on the Bolaven Plateau, which resulted in a better understanding of the flora of Bolaven and the discovery of more than 18 new species including the new species of Croton described in this study and 65 new country records (e.g. Souladeth et al. 2020, Tagane et al. 2020, Souvannakhoummane et al. 2021, Vongthavone et al. 2021).

Vernacular name:— Pao Dong (suggested here; ‘Pao’ refers to the genus Croton and ‘Dong’ refers to the forest area).

Preliminary conservation assessment:— Critically Endangered (CR). During our five field surveys in Bolaven Plateau in 2018–2020, we found only a small population with ca. 10 individuals of Croton nagaoi in the type locality. Therefore, the most appropriate initial assessment of the IUCN conservation status for this species is Critically Endangered (CR) under criteria D ( IUCN 2012). We collected this species in the roadside thickets beside a rubber plantation field, and this species might be locally common in such disturbed areas. However, surrounding areas of the type locality have been converted to rubber plantation at large scale, implying that much of the habitat was already lost. We need to collect further information on its distribution and number of individuals/populations to accurately assess the conservation status.

Note:— Croton nagaoi is characterized by the indumentum of dense, persistent yellowish-cream stellate hairs on the young twigs, lower leaf surface, inflorescences and fruits, pseudo-verticillate leaves in seemingly 3–5 at nodes, short petioles 0.3–0.6 cm long, oblanceolate to obovate-elliptic lamina (3.3–)5.5–17.6 × (1.1–) 2.4–6.3 cm, and pinnate secondary veins of 9–15 on each side, by which combination it is clearly distinguished from the other species of Croton in Laos and its surrounding countries. Croton nagaoi is most similar to C. thorelii , and the two species might be close relatives. However, in addition to the diagnosis mentioned above, C. nagaoi is distinguishable from C. thorelii in its larger stellate hairs (0.3–0.5 mm in diam. in C. nagaoi vs. 0.2–0.3 mm in diam. in C. thorelii ), longer stipules (1.5–2 mm vs. 1 mm long), smaller sepals and petals of staminate flowers (sepals and petals ca. 2.2 mm long vs. 3 mm long and 2.5 mm long, respectively), and longer bifid styles ca. 3.5 mm long (vs. 1.5–2 mm long and often quadrifid; Esser 2005). Further phylogenetic work will be necessary to understand their relationship and evolutionary history. The specimens of C. thorelii we examined in this study are shown in the Appendix.

Figures

FIGURE 2. Croton nagaoi Tagane, Soulad.& Souvann.A. branches; B. staminate flower with a bracteole; C. petals (outside); D. stamens, back view; E. stamen, front view; F. stamen, lateral view; G. fruit; H. seed.All from Tagane et al. L2459 (KAG [KAG128559]). Scale bars: A = 3 cm, B–F = 1 mm, G = 8 mm, H = 5 mm. Line drawing by K. Souvannakhoummane.

FIGURE 3. Croton nagaoi Tagane, Soulad. & Souvann.A. Flowering branch; B. portion of abaxial leaf surface; C. base of leaf showing basal gland lateral on base of midrib; D. stellate hairs on lower leaf surface; E. inflorescence consisting of staminate flowers; F. staminate flower; G. petal of staminate flower; H. stamen; J. fruit; K. pistillate flower, lateral view, showing sepals and petals; L. petals of pistillate flower; M. stigma; N. seed. All from Tagane et al. L2459. Scale bars: D = 0.5 mm, G, H, L, M = 1 mm, N = 5 mm. Photographs by S. Tagane.

FIGURE 4. Croton thorelii Gagnep. A & C from Tagane et al. 6796 (FU), B from Tagane et al. 6798 (FU). Photographs by S. Tagane.