A new species of Hibiscadelphus Rock (Malvaceae, Hibisceae) from Maui, Hawaiian Islands

Abstract Hibiscadelphus stellatus H. Oppenheimer, Bustamente, & Perlman, sp. nov., a new, narrowly endemic species from West Maui, Hawaiian Islands is described, illustrated and its affinities and conservation status are discussed. It is currently known from three populations totaling 99 plants in Kaua`ula valley on leeward western Maui. It differs from H. wilderianus, its nearest congener, in its denser white or tan stellate pubescence on most parts; larger externally purple colored corollas that are 5–6.5 cm long; linear-subulate to lanceolate, acute to acuminate involucral bracts; globose-cuboid to ovoid capsules; and endocarp with scattered hairs.


Introduction
Joseph Rock described the endemic Hawaiian genus Hibiscadelphus Rock in 1911 based on H. giff ardianus Rock (Radlkoff er and Rock 1911). Th e genus is extremely rare, with seven previously described species from the main Hawaiian Islands, four of which are now extinct, two only persisting in cultivation (including restoration plant-ings), and a single species remaining in its natural habitat. Th e genus belongs to the tribe Hibisceae (Malvaceae), and it appears to form a distinct monophyletic group based on its curved and narrowly zygomorphic corollas forming a tubular structure with the petals unequal in length (the lower two shorter than the upper three). In contrast, in Hibiscus the corollas are actinomorphic with spreading petals of equal length (Lorence and Wagner 1995). In most species of Hibiscus the calyx is not circumscissile in fruit but persists, splitting along one side.
In addition to establishing the genus, Rock described three species: Hibiscadelphus giff ardianus Rock from Mauna Kea, H. hualalaiensis Rock from Hualalai, both on Hawai`i Island, and H. wilderianus Rock from Auwahi on the island of Maui (Rock 1913). After Rock's initial treatment, Forbes (1920) described a fourth species (H. bombycinus C.N. Forbes) based on a specimen collected in the mid 1800's by Hillebrand and Lydgate at Kawaihae in the Kohala Mountains of Hawai`i Island. Over the next 75 years three additional species were subsequently discovered and described: H. distans L.E. Bishop & D. R. Herbst on Kaua`i (Bishop and Herbst 1973); H. crucibracteatus Hobdy on Lana`i (Hobdy 1984), and H. woodii Lorence & W.L. Wagner on Kaua`i (Lorence and Wagner 1995). Th e last authors published a key to the seven taxa known at that time. Presently, six species are extinct in the wild, but two of these persist in cultivation (including restoration outplantings), and two others, including this new species, occur as natural populations (Table 1). Th e eight described species are all mostly single volcano endemics. Th e two Kaua`i species are separated by a distance of 8 km. Hibiscadelphus woodii was known from Kalalau Valley on the islands northern coast and H. distans is known from Koaie Stream in Waimea Canyon, whose outlet is along the southern shore During the course of fi eld work on west Maui in 2012 the authors discovered two populations (25 and 51 plants) over 400 m apart of a previously unknown Hibiscadelphus species on the steep slopes of Kaua`ula Valley on leeward, western Maui. A year later a third colony was found between the fi rst two locations with 23 plants. Hibiscadelphus had not been observed, reported or documented previously on west Maui. Study of the collected specimens and comparison with collections of other known species at the BISH and PTBG herbaria, and images on JSTOR Global Plants revealed they represent an undescribed species. Description. Small trees 3-6 m tall, many branched, trunks to 30 cm dbh, bark smooth, light tan to gray, young branchlets densely white to tan pubescent with 8-12-rayed stellate trichomes 0.3-0.4 mm in diam., surface scurfy-waxy, glabrescent with age; petiole scars prominent, subcircular, 2.5-4 mm in diam. Leaves chartaceous, new growth densely stellate-pubescent, mature leaves with blades broadly-ovate to suborbicular or subreniform in outline, occasionally shallowly 3-lobed, 7.5-16(-18) cm long, (8)9.5-13.5(-18) cm wide, veins prominulous, primary veins 7-9 radiate from base, midvein with 3-4 pairs of secondary veins arising along midrib, light green to occasionally red tinged when fresh, higher order venation prominulous on both surfaces, margins irregularly broadly crenate, base cordate, with a wide to narrow but usually open sinus, apex acute to obtuse or rounded, green when fresh with scattered tan stellate pubescence on both surfaces, densely so along veins and adaxial surface, trichomes 0.2-0.4 mm in diam. with (2-)8-16 rays, abaxial surface with principal vein axils domatiate with dense tufts of tan to white trichomes 0.2-0.3 mm long; petioles 3.5-6cm long, green or sometimes red-tinged, pubescent with dense white to tan stellate trichomes as on branchlets; stipules lanceolate to subulate, 2-3.5 mm long, apex acute, green, sparsely to densely tan or white stellate pubescent, soon caducous. Flowers solitary, axillary, erect to spreading, pedicels 22-30 mm long, green or sometimes red-tinged, densely white to tan stellate pubescent as in petioles, involucral bracts 5-6 (-7), linear-subulate to lanceolate (rarely spathulate), acute to acuminate apically, connate only at base, 9-22 mm long, 1-2 mm wide at base, erect, appressed or spreading perpendicular to the fl oral axis in anthesis, green, densely tan or white stellate pubescent with trichomes 0.2-0.3 mm in diam. Calyx tubular-saccate, mostly 5-lobed, tube 22-30 mm long, 19-20 mm wide, the lobes triangular, acute to short acuminate 5-10 mm long, 7-8 mm wide, green, surface obscured by dense tan stellate pubescence as in bracts, in mature fruit splitting along one side but persistent. Corolla zygomorphic, adaxially curved, 5-6.5 cm long, lobed nearly to base, lobes coalescent, 6-6.5 cm long, 3.5-4 cm wide, obovate-spathulate, apex obtuse, tips and outer margins slightly refl exing with age, outer exposed portion purple, purple-green or purple-yellow, inner concealed portion yellow, conspicuously veined, densely covered with gray or tan stellate trichomes especially along veins, internally yellow or purple-tinged distally, purple toward base, corolla usually becoming purplish with age, staminal column and apex of the style exserted for 1.5-2.5 cm; staminal column 8-8.5 cm long, antheriferous in distal 3.5 cm, maroon-purple, antheriferous in distal 3.5 cm, stamens c. 100, anthers reniform-curved, 0.8-1.5 mm long, purple, fi laments 6-12 mm long, purple, pollen grains purple turning golden yellow after anther dehiscence; style 8.5-9 cm long, style branches 3-5 mm long, villose, stigmas rounded, c. 1 mm long, yellow, ovary dome-shaped, 8 mm long and wide. Fruit a woody capsule, globose-cuboid to -ovoid, 5-locular, 5-valved, 2.5-3.5 (-4) cm long, 2.2-3.3 cm in diameter, surface yellowish brown, rough densely covered with dense tan stellate hair clusters, appearing tuberculate, mericarps 10, mesocarp well developed, reticulate, endocarp chartaceous, loose, with scattered long hairs, testa brown. Seeds 1-2 per mericarp, reniform, 8-10 mm long, 6-8 mm wide including the dense, lanate yellowish-tan hairs 0.4-1 mm long.
Habitat and ecology. Hibiscadelphus stellatus occurs on very steep, rocky slopes between 800 and 900 m elevation. Th ese sites have a windward aspect and are situated mid-slope between the upper rim of a deep valley and a perennial stream below. Soils at these sites are of typical volcanic, basalt origin, from the Wailuku Series of original shield building fl ows. Th e vegetation where H. stellatus grows forms a mosaic of trees and shrublands with an open canopy, best characterized as Lowland Mesic Forest (Wagner et al. 1999). Rainfall averages from 12 to 1400 mm annually and the substrate is well-drained. Etymology. Stellatus -Latin, star shaped, alluding to the stellate pubescence that characterizes the Malvaceae in general, including Hibiscadelphus. Th e name also refers to the "star-shaped" pattern formed by the fi ve involucral bracts, which contrasts with the cruciform pattern formed by the four bracts in H. crucibracteatus. Additionally, stellatus acknowledges the beautiful and stellar (outstanding) fl owers of this species. Th e Hawaiian name hau kuahiwi has been applied to other species of the genus (Rock 1913). Hau (Hibiscus tiliaceus L.), a lowland tree; kuahiwi-lit. mountain or high hill (Pukui and Elbert 1986). Hawaiians recognized the similarities of the taxa while observing that Hibiscadelphus grows at higher elevations.
Conservation eff orts. Th e conservation status of Hibiscadelphus is precarious at best. Th ree species (H. crucibracteatus, H. giff ardianus, and H. wilderianus) were each only known from a single naturally occurring tree (Hobdy 1984;Rock 1913). However, H. giff ardianus survives in cultivation and is planted within the type locality at Kipuka Puaulu in what is now Hawai`i Volcanoes National Park. Hillebrand provided no information on the abundance or scarcity of H. bombycinus when he fi rst collected it but the species is presumed extinct. Hibiscadelphus crucibracteatus is presumed extinct in the wild since the single known tree died a few years after its discovery from damage by introduced axis deer (Axis axis) despite it being fenced; there is no ex situ material although there were several attempts at propagation (R. Hobdy, pers. comm.). Hibiscadelphus woodii was known from four individuals, but evidently has recently gone extinct (Wood 2012). Th ere are no plants in cultivation despite attempts to propagate it. Hibiscadelphus hualalaiensis is considered extinct in the wild as of 1992 but is in cultivation. Hibiscadelphus wilderianus is also presumed extinct. Although Rock mentioned that Wilder (who discovered the species with Rock, later returning and making several additional collections from the only known tree) had succeeded in raising a single seedling (Rock 1913) no surviving material is known. Hibiscadelphus distans is known from two wild populations of approximately 15-20 individuals total on Kaua`i, and over 100 ex situ collections at the McBryde and Limahuli gardens of the National Tropical Botanical Garden (NTBG). With 99 known plants, H. stellatus has the largest known wild populations plus the only known naturally occurring seedlings of any species in the genus.
Seeds were collected from 12 individuals of H. stellatus representing the three known subpopulations. Th e subpopulations were mapped with GPS and each individual plant numbered and tagged. Cuttings from three plants were also made although these failed to take root. Material is being propagated at the Olinda Rare Plant Facility on Maui, NTBG on Kaua`i and the Lyon Arboretum on O`ahu. Th e fi rst seeds germinated in conventional propagation approximately 50 days after sowing and under three weeks in tissue culture. As of May 2013 four parent trees from two sites are represented ex situ, with seeds from four additional trees in the third site now in propagation at Olinda and Lyon.
Th reats to the existence of Hibiscadelphus stellatus include habitat erosion, fi re, weeds, drought, probably rats (Rattus rattus, R. exulans) (Baker and Allen 1978) and mice, (Mus domesticus), slugs such as Derocerus and Limax or other invertebrates such as seed weevils (Giff ard 1920) and caterpillars (Lorence and Wagner 1995), and potentially feral goats (Capra hirca) and/or pigs (Sus scrofa). Small populations of feral goats and pigs are encroaching in surrounding areas, although the West Maui Moun-tains Watershed Partnership is constructing strategic fencing. In 2007 a large wild fi re burned within 180 m of the plants; succession of its habitat presently includes non-native fi re-adapted grasses that were absent before the fi re. Erosion is a natural process but is exacerbated by invasion by weeds and ungulates and the destruction of vegetation by fi re. Woody non-native plants are currently low in diversity and number, but are represented by known aggressive, habitat -modifying species such as Grevillea robusta A. Cunn. ex R. Br., Lantana camara L., Psidium guajava L., and Schinus terebinthifolius Raddi. Herbaceous understory weeds are similarly low in number of taxa but include serious habitat modifi ers such as Adiantum hispidulum Sw., Beauv., all of which may hinder establishment of seedlings.
Conservation status. When evaluated using the IUCN Red List criteria (IUCN 2013) Hibiscadelphus stellatus falls into the Endangered (EN) category, a designation for taxa facing a very high risk for extinction in the wild. Th e species merits this designation by meeting the following criteria: B2(a)(biii, v) + D, where the area of occupancy (AOO) is less than 500km² (B2), with severely fragmented or number of locations <5 (a), and a continuing decline observed, estimated, inferred or projected in (biii) quality of habitat and (bv) number of mature individuals; and D: <250 mature individuals. Although there is some reproduction observed, there is not a suffi cient population structure that will allow enough immature plants to replace mature individuals as they perish, therefore a decline is almost a certainty under current conditions. Th e AOO is 2.28 hectares (5.63 acres) much less than the threshold. Th e habitat is inferred to be in decline due to the eff ects of introduced taxa such as invasive plants and rats, as well as the eff ects of introduced rats and diseases on pollinators. Continued monitoring over the next fi ve years will possibly lead to an updated assessment to CR. Discussion. Th is new species clearly belongs to Hibiscadelphus based on its fl owers that have their corolla lobes coalescent into a curved, tubular zygomorphic structure. Hibiscadelphus stellatus diff ers from its congeners in the following combination of characters: moderate to dense stellate pubescence on all parts; involucral bracts 5 (-7) in number that are linear-subulate to lanceolate, 9-22 mm long, and acute to acuminate apically; 5-lobed calyx with tube 22-25 mm long and lobes 5-8 mm x 7-8 mm; externally purplish-colored corolla 5-6.5 cm long; and globose-cuboid to ovoid capsules with scattered hairs on the endocarp. Th e species of Hibiscadelphus can be separated by the following key.