Sapindus thurstonii Rock 1911
Creators: name: Franck, Alan R.
Publisher: name: Zenodo
Year: 2024
Subjects: Biodiversity
Taxonomy
Plantae
Tracheophyta
Magnoliopsida
Sapindales
Sapindaceae
Sapindus
Sapindus thurstonii

description: 10. Sapindus thurstonii Rock (1911: 6) (Figures 1 and 16).
≡ Sapindus saponaria var. thurstonii (Rock) Skottsberg (1926: 244).
Lectotype (designated here):— HAWAII. Hawaii: Mauna Loa, Kipuka Ki, April 1911, J.F. Rock 8772 (lectotype, BISH1014749!; isotypes, BISH1014750!, BISH1014751!, IBSC 0417433, L 0014584!, M-0224782!, M-0224787!, NY00338022!, US00428753!, US00428754!).
Etymology:—Named for Lorrin Andrews Thurston, a politician and businessman who sought annexation of Hawaii by the USA. Thurston had notified Rock about the species of Sapindus on Mauna Loa.
Description:—Tree to 26 m tall, bark smooth to finely roughened, developing few irregular longitudinal fissures breaking up the bark into plates that often peel away from the sides. Petiole 2–5 cm long, adaxially densely villosulous, abaxially sparsely pubescent, brown-speckled especially towards the base, often winged with wing to 1 mm wide (rarely to 3.5 mm, in seedlings) or unwinged; rachis adaxially densely villosulous, abaxially sparsely pubescent, often winged with wing to 1 mm wide or unwinged; leaflets 4-8; petiolule 0–3 mm long, adaxially densely villosulous, abaxially sparsely pubescent; leaflet blade ovate to ovate-lanceolate, asymmetric to nearly symmetric, falcate to nearly straight, the apex acuminate, acute, to obtuse, 4-15 cm long, 1.5–5 cm wide, 2.5–3.5 times longer than wide, adaxially drying dark brown-green, purplish or reddish brown, to pale brown, midrib pale green, sparsely pubescent, at mid-blade the ridge 0.1–0.3 mm wide, 0.2–0.3 mm high, the blade often sunken along the midrib, secondary veins pale yellow, tertiary venation prominent to prominulous, pale yellowish green, discolorous from the blade surface, quaternary venation prominulous to obscure and nearly concolorous with the blade surface, abaxially drying pale green, moderately to sparsely pilose, midrib pale yellow to brownish yellow, secondary veins prominent, pale yellow, tertiary venation prominent to prominulous, pale yellowish green, discolorous from the blade surface, quaternary venation prominulous to obscure and nearly concolorous with the blade surface; foveolae conspicuous on the abaxial leaflet blade surface, exudate clear. Petal lacking appendages, but often adaxially with a tuft of hair on the inner surface above the claw. Mature mericarp 17–20 mm wide, pericarp 0.4–0.9 mm wide, seed 10–14 mm wide.
Distribution, habitat, and phenology:— Hawaii [Big Island] (Figs. 2 & 3); 900–1400 m. Mesic, transition forests (Robyns & Lamb 1939). Flowering August–October.
Notes:— Sapindus thurstonii is characterized by its brown-speckled petiole base, pubescent and dark leaflet blades, and petals lacking appendages. Rock (1911) described the leaflets as “dark green and somewhat shining above” and noted the exceptionally flaky bark (Little, Jr. & Skolmen 1989, Hodel 2012). Young, immature plants have somewhat lighter leaflets that are more narrowly lanceolate (Forbes 395H) or have a winged rachis (Stone 3087).
Sapindus thurstonii was first named by Rock in 1911. Soon thereafter Rock (1913) synonymized it with S. saponaria, but still felt that the Hawaiian trees were exceptionally larger than those of S. saponaria s.str. Skottsberg (1926) thought Hawaiian material was nearer to plants of the Philippines (= S. tricarpus). St. John (1977) reinstated S. thurstonii as a distinct species, but this was not followed by later authors.
Morphologically, S. thurstonii seems nearest to S. saponaria subsp. saponaria. Compared to S. saponaria, S. thurstonii has much darker leaflets and adaxially the blade is sunken along the midrib (vs. lighter leaflets and adaxially the blade not sunken or only scarcely so along the midrib in S. saponaria). As Hawaii has experienced numerous ancient dispersal events from the Americas (Baldwin & Wagner 2010), the origin of S. thurstonii could owe to a dispersal from the Americas, given the morphological similarity to S. saponaria subsp. saponaria. Plants of true S. saponaria subsp. saponaria are cultivated in Hawaii, but these flower December–March and do not overlap with the flowering time of S. thurstonii.
Sapindus thurstonii is briefly deciduous, “as the young leaves come out before all the old ones drop, it is hardly bare for any length of time” (Rock 1913). Rock (1911) mentioned an unidentified caterpillar that fed heavily on the developing inflorescence. The species was illustrated in Wagner et al. (1999). A tree of S. thurstonii in Hawaii was registered as the USA National Champion Tree (as S. saponaria), measuring 21.6 m tall with a circumference of 4.1 m (American Forests 2023).
In the protologue for S. thurstonii, the herbarium or holotype were not explicitly indicated, and here a specimen at BISH is designated the lectotype. In their treatment of the Hawaiian flora, both Rock (1913) and Skottsberg (1926) cited the specimen Remy 566, but that specimen clearly belongs to S. saponaria subsp. jardinianus.
Conservation Status:—Being confined to only a handful of locations with a small area of occupancy (<500 km 2) and having a small population (<2,500 mature individuals) on the island of Hawaii, S. thurstonii should be categorized as Endangered (IUCN 2012) given the likelihood for further decline in the population.
Specimens examined:— HAWAII. N. Kona, Puuwaawaa, 15 June 1909, Rock 3826 (BISH, BO, US); Hualalai, April 1911, Rock 3827 (M); Mauna Loa, Jul-August 1911, Rock 10102 (BISH, L, P, US); Kapapala, August 1911, Forbes 395 H (BISH, IBSC, L, MICH, P, SFV, US); Mauna Loa, 13 September 1916, Hitchcock 14637 (US); Puuwaawaa, 7 June 1923, Munro 524 (BISH, US); Huehue, 24 August 1926, Degener 30241 [cultivated] (BISH); Hawaii National Park, Kipuka Ki, 4000 ft., 8 August 1959, Stone 3087 (BISH); North Kona District, west slope of Puu Waa Waa near summit, 23 September 1975, Herbst & Spence 5475 (L, US); Kipuka Puaulu, Hawaii Volcano National Forest, 18 September 1977, Degener & Degener 34434 (FLAS, IND, MICH); Hawaii Volcanoes National Park, Mauna Loa strip road, Kipuka Puaulu, just NE of Bird Park, 3 November 1987, Lorence & Plews 5682 (BISH).
description: Published as part of Franck, Alan R., 2024, Revision of Sapindus sect. Sapindus (Sapindeae, Sapindoideae, Sapindaceae), including the description of three new species, pp. 1-71 in Phytotaxa 648 (1) on pages 50-51, DOI: 10.11646/phytotaxa.648.1.1, http://zenodo.org/record/13215835