Coprosma kawaikiniensis (Rubiaceae) a new species from the Dubautia-Sadleria shrubland-fernland community on Kaua‘i, Hawaiian Islands

Abstract Coprosma kawaikiniensis K.R. Wood, Lorence & Kiehn (Rubiaceae), a rare endemic tree from Kaua‘i, Hawaiian Islands, is described and illustrated along with a previously undescribed endemic plant community, the Dubautia-Sadleria shrubland-fernland (DSSF). The new species differs from Hawai‘i congeners by its combination of opposite, long, elliptic to narrowly elliptic or ovate-elliptic leaves with revolute margins; caducous stipules 7–10 mm long, externally glabrous, densely hirtellous-pilose near the margins of the inner surface; unbranched inflorescences with peduncles 20–28 mm long; flowers 6–8 per cluster; and persistent calyx tube with 4–8 irregular dentate lobes. Known only from the windward slopes and ridges of southeastern Kaua‘i below the Kawaikini summit, Coprosma kawaikiniensis falls into the IUCN Critically Endangered (CR) Red List category.


Introduction
Coprosma J.R.Forst. & G.Forst., in the family Rubiaceae, is a genus of approximately 110 species of dioecious wind-pollinated shrubs or small trees widely distributed on Pacifi c islands, with a primary center of diversity in New Zealand (ca. 50 spp.), and secondary centers of diversity in the Hawaiian Islands (13 spp.), New Guinea (11 spp.), and Australia (8 spp.) (Wagner and Lorence 2011;Cantley et al. 2014). Th e remaining species are scattered over a wide area of the Pacifi c Basin, extending from Borneo and Java to Rapa Nui in southeastern Polynesia, and the Juan Fernández Islands (Smith 1988). Th ere are six endemic Coprosma in the Marquesas Islands including three newly described by Wagner and Lorence (2011), four in the Society Islands (Welsh 1998), three in the Australs, two in Samoa, and one each in the Tuamotu Islands, Pitcairn Island, and Cook Islands. Oliver (1935) divided the genus into seven groups, most of which were subdivided into smaller groups of presumably closely related species. He placed all southeastern Polynesian species then known into his Coprosma pyrifolia (Hook. & Arn.) Skottsb. group characterized by relatively large, usually obovate to ovate leaves with fi nely reticulate venation, entire to denticulate triangular stipules, male fl owers in small clusters with a calyx present, and three female fl owers per cluster, the calyx lobes as long as or shorter than the tube, and fruit red or orange. He hypothesized that this group was related to similar species in New Zealand. Florence (1986) described two new Marquesan species and suggested they and the one other known Marquesan species were allied with the orange-fruited Hawaiian species. Although Heads (1996) included no Polynesian Coprosma species in his sparse sampling of the genus, he supported Florence's hypothesis by placing the three known Marquesan species in a group along with the Hawaiian species, rather than the C. pyrifolia group where all of the other southeastern Polynesian species were placed. A molecular study of Tribe Anthospermeae (Anderson et al. 2001), in which 6 of 16 of the taxonomic groups recognized by Heads (1996) were sampled, indicates an apparent Australian origin of Coprosma and possible independent colonization of Fiji and Hawaiian Islands from New Zealand. Based on molecular phylogenetic analyses of ITS and ETS regions Cantley et al. (2014) provided new biogeographic insights into Pacifi c Coprosma species. Th eir analyses suggest two independent colonizations of Coprosma to the Hawaiian Islands. Th e majority (12) of the 13 Hawaiian species form a monophyletic group closely related to red-and orange-fruited species from the Marquesas and Austral Islands, whereas the single black-fruited species (C. ernodeoides A.Gray) represents a separate colonization to Hawai'i from an unknown origin, perhaps New Zealand or Tasmania (Wagner and Lorence 2011). Th is view is also corroborated by the fact that C. ernodeoides is a very high polyploid (2n ≥ 220), whereas all other cytologically investigated Hawaiian taxa are tetraploids with 2n = 44 chromosomes (Kiehn 2005).Th e discovery and publication of C. kawaikiniensis, a member of the red-and orange-fruited group, now brings the number of Hawaiian Coprosma species to 14.

Coprosma kawaikiniensis
Phenology. To date, Coprosma kawaikiniensis has been observed in fl ower during early April, and with fruit from late August to mid-September.
Etymology. Th e new species is named after the holotype locale, Kawaikini, the highest peak on Kaua'i and one of the rainiest places on earth (Juvik and Juvik 1999). Literally, Kawaikini means "the multitudinous waters" in Hawaiian (Pukui et al. 1974).
Distribution and ecology. Th e volcanic island of Kaua'i is the oldest of the main high Hawaiian Islands (ca. 5 Ma) featuring a physical geography that is quite variable with deeply eroded drainages, well-defi ned canyons, and tall coastal seacliff s. It is also the most fl oristically rich Hawaiian Island (Wagner et al. 1990, Imada 2012) exemplifi ed by high levels of habitat diversity and endemism, which includes ca. 244 single island vascular plant endemics (Wood 2015). Careful botanical research conducted over the last few decades by staff of the National Tropical Botanical Garden (NTBG), especially around cliff s and remote regions, has contributed 32 new published plant taxa from Kaua'i (Imada 2012).
Th e recent discovery of Coprosma kawaikiniensis was made around extremely steep, narrow wind swept ridges, slopes, and boulder strewn stream banks below Kawaikini, the highest peak on Kaua'i which summits at 1598 m elevation (Figures 2-4). Th is particular habitat is the remotest of Kaua'i's eco-regions and can be further characterized by its mist-shrouded, dark, narrow basalt canyon walls seeping with springs, and having the distinction of being one of the rainiest places in the world (Juvik and Juvik 1998). Th e holotype region represents a previously undescribed plant community dominated by two Hawaiian endemic genera, namely Dubautia Gaudich.    (Wood 2013). Occasional stands of Metrosideros-Cheirodendron forest with low-statured canopies of 5-7 m also occur in a mosaic of random patches surrounding the DSSF community, and Coprosma kawaikiniensis has also been observed ranging into this habitat. Th ese forests are usually dominated by a signifi cant diversity of mixed understory associates very similar to that of the DSSF. Th e ripar-ian regions that dissect these relic Metrosideros-Cheirodendron forests and form the headwater drainages that feed the respective lower elevation valleys retain a fl ourishing treasure trove of rare plant taxa along nearby stream banks, including endangered species of Cyanea, Cyrtandra, Hesperomannia A.Gray, Isodendrion A.Gray, Labordia, Melicope, Phyllostegia Benth., Platydesma, and Polyscias.
Vegetation cover on several nearby steep slopes is composed of just a few species of matting ferns (i.e., Dicranopteris linearis, Diplopterygium pinnatum, and Sticherus owhyhensis (Hook.) Ching) and may be the resulting succession of past landslides. Vertical wet cliff communities of sedges, herbs, and ferns, accentuated with numerous waterfalls form the prevalent back-drop that tower over and surround these habitats below Kawaikini (Wood 2014) (Figure 2).
Conservation status. IUCN Red List Category. When evaluated using the World Conservation Union (IUCN) criteria for endangerment (IUCN 2001), Coprosma kawaikiniensis falls into the Critically Endangered (CR) category, which designates this species as facing a very high risk of extinction in the wild. Our formal evaluation can be summarized by the following IUCN hierarchical alphanumeric numbering system of criteria and subcriteria: CR B1ab(i,ii,iii,v)+2ab(i,ii,iii,v); C2a(ii); D; which refl ects a severely limited Extent of Occurrence (EOO) and Area of Occupancy (AOO) of less than 3 km 2 and a population of less than 50 individuals.
Discussion. All species of Coprosma from Kaua'i have stipules shorter than 4.5 mm except for C. kawaikiniensis and C. kauensis. Th e features distinguishing those two species are the number of the fl owers per partial infl orescence [6-8, sometimes with an additional pair of 3-5 fl owered cymules in C. kawaikiniensis vs. 3(-5) fl owered in C. kauensis], the length of the calyx in staminate fl owers (1.8-3.2 mm in C. kawaikiniensis vs. 0.4-0.6 mm in C. kauensis), the length of the staminate peduncle (20-28 mm in C. kawaikiniensis vs. 4-8 mm in C. kauensis), the pubescence of the stipules (externally glabrous in C. kawaikiniensis vs. densely strigose on both surfaces in C. kauensis), and the number and form of the stipular appendage(s) [only 1 terminal claw-like appendage and occasionally a few short, rounded appendages or callous protuberances 0.1-0.2 mm long c. equaling the ciliate marginal hairs in C. kawaikiniensis, vs. (3-)5-7(-8) pairs of thickly ovoid to digitate, shiny dark brown-black marginal appendages 0.3-0.5 mm long in C. kauensis] ( Figure 5).
Species of Coprosma that have stipules of 4.5 mm or greater from the remaining Hawaiian Islands include: C. longifolia A.Gray, C. ternata W.R.B.Oliv., C. rhynchocarpa A. Gray, C. montana Hillebr., and C. ochracea W.R.B.Oliv. Coprosma kawaikiniensis diff ers from C. longifolia and C. ternata in having opposite vs. usually ternate leaves (with only occasionally opposite leaves on some stems). Coprosma kawaikiniensis has peduncles 20-28 mm long vs. 2.5-16 mm long in C. rhynchocarpa and only 0-1(-4) mm long in C. ochracea. Th e new species diff ers from C. montana in having 6-8 fl owers per cluster, sometimes with an additional pair of 3-5 fl ower cymules vs. 1-2 per cluster. In terms of morphology, C. kawaikiniensis most closely resembles C. longifolia (Oahu), but diff ers from the latter in having opposite vs. usually ternate leaves, stipules with a shorter sheath (1-)2-4 mm long, and free apical portion (3-)4-6 mm long terminated by a thickly carinate-subulate apex with a claw-like appendage, vs. sheath 7-9 mm long and free apical part 2-3 mm long with short-attenuate, acute tip.
Th e following couplets can be inserted into the existing key to Hawaiian Coprosma by Wagner et al. (1990Wagner et al. ( : 1123