Differs from Hawaiian congeners by its combination of distinct carpels and ramiflorous inflorescence; plants monoecious; leaf blades (5–)8–30 × (4–)6–11 cm, with abaxial surface tomentose, especially along midribs; and very long petioles of up to 9 cm.

United States of America. Hawaiian Islands, Kaua‘i: Waimea District, Mākaha Valley, Metrosideros-Acacia montane mesic forest, 22°7'1.8258"N; 159°40'45.534"W, 997 m elev., 24 Jan 2016, K. R. Wood & Kahekili Lee 16727 (holotype: PTBG-073080!; isotypes: BISH!, K!, MO!, NY!, UC!, US!).

Trees (3–)5–12 m tall, trunks up to 25 cm diameter, bark smooth, mottled gray to light brown, new growth and young branchlets tomentose, yellow-tan, glabrate in age. Leaves opposite, unifoliolate, coriaceous, the blade ovate, oblong-ovate, oblong-elliptic, or oblong-lanceolate, (5–)8–30 × (4–)6–11 cm, margin entire, base rounded, apex obtuse to acute, occasionally rounded, secondary veins 7–15 pairs, connected by an arched vein 5–20 mm from margin, higher order venation reticulate, adaxial surface glabrous, abaxial surface densely tomentose, yellow-tan, rarely glabrate, with midrib and secondary veins densely tomentose, interspersed villous, the midrib sometimes also long-villous or woolly along the sides; petiole shallowly canaliculate, (2.5–)3.5–9.0 cm long, puberulent to tomentose. Inflorescences in axillary and ramiflorous, fasciculate cymes to 22 mm long, peduncles 1–5(–10) mm long, short-villous, pedicles 1–4 mm, short-villous, bracteoles deltoid-ovate, 1–2 mm long, densely short-villous. Flowers male or female, plants monoecious, 3–5(–7), sepals deltoid-ovate, naviculate, 2.5–3.0 × 1.5–2.0 mm, densely short-villous externally, glabrous internally, tip acute, persistent in fruit; petals whitish green, ovate, tip acute, slightly thickened and hooked inward, 3.5 × 2.5 mm, densely short-villous externally, glabrous internally; stamens 8, filaments glabrous, papillose on distal half, antisepalous ones ca. 4 mm long in male flowers (1.5 mm in female flowers), antipetalous ones 3 mm long (1.5 mm in female flowers), anthers ca. 1 mm long, all with pollen in male flowers, (0.6–0.7 mm long in female, apparently sterile); nectary disk glabrous; ovary glabrous; gynoecium 1 × 2 mm, style including stigma 1 mm long in male flowers (1.5 mm in female), glabrous, stigmas capitate, 4-lobed, papillose, 0.5 mm wide. Capsules medium to dark green when fresh, irregularly pusticulate, 5–9 × 15–21 mm, of 4 distinct follicles, slightly ascending, occasionally 1 or more abortive, exocarp glabrous, glandular punctate, endocarp glabrous. Seeds 1–2 per carpel, ovoid, 6–8 mm long.

Melicope stonei has been observed with flower buds in January, May, and September, and with both flower and fruit during January, February, and July.

We are pleased to name Melicope stonei in honor of Benjamin Clemens Masterman Stone, British-American botanist, born in Shanghai, China in1933 and passed in 1994 while working at the Philippine National Museum on the Flora of the Philippines Project. He contributed over 300 publications to science during his career, including taxonomic monographs of Hawaiian Pelea (Stone 1969) and Platydesma (Stone 1962). For all his contributions, especially his keen insights into Hawaiian Melicope, we gratefully extend him due recognition.

United States. Hawaiian Islands, Kaua‘i: Waimea District, Ku‘ia, upper headwater gulch, 1027 m elev., 03 Sep 2015, K. R. Wood, S. Perlman, S. Walsh, M. Query 16579 (PTBG); Miloli‘i ridge road, Mahanaloa, 933 m elev., 06 Nov 2008, C. Trauernicht & N. Tangalin 617 (PTBG); Mahanaloa, 1055 m elev., 21 Oct 2015, S. Walsh & A. Williams 136 (PTBG); Pa‘aiki-Mahanaloa flats, 1036 m elev., 07 Oct 2001, K. R. Wood 9128 (BISH, PTBG); Pa‘aiki-Mahanaloa flats, 991 m elev., 04 Mar 2001, K. R. Wood & M. LeGrande 8879 (PTBG); Pa‘aiki-Mahanaloa flats, 1000 m elev., 14 Dec 1994, K. R. Wood 3840 (PTBG, US); Pa‘aiki-Mahanaloa flats, 1000 m elev., 24 Feb 1999, K. R. Wood 7696 (PTBG); Pa‘aiki-Mahanaloa flats, 1000 m elev., 24 Feb 1999, K. R. Wood & R. Aguraiuja 7697 (BISH, CANB, MO, NY, PTBG, US); Pa‘aiki-Mahanaloa flats, 1015 m elev., 06 May 2000, K. R. Wood 8432 (BISH, NY, PTBG, US); Pa‘aiki-Mahanaloa flats, 1015 m elev., 06 May 2000, K. R. Wood 8433 (PTBG, US); Mākaha Ridge Road, down .8 mi to 988 m elev., north side of road, just inside forest, 970 m elev., 7 Feb 1988, D. Lorence, G. Lorence & E. Crum 5779 (PTBG); loc. cit., 970 m elev., 2 Jan 1989; D. Lorence, T. Flynn & Smith 6316 (PTBG); loc. cit., 970 m elev., 1 Mar 1989, T. Flynn & Decker 3280 (AD, BISH, F, MO, MU, PTBG, US); loc. cit., 970 m elev., 31 May 1990, D. Lorence et al. 6449 (PTBG); loc. cit., 920 m elev., 25 Feb 2000, W. Wagner & K. R. Wood 6891 (PTBG); south side of Mākaha road, 988 m elev., 06 May 2000, K. R. Wood 8430 (PTBG); loc. cit., 988 m elev., 06 May 2000, K. R. Wood 8431 (PTBG, US); loc. cit., 991 m elev., 21 Jul 2012, K. R. Wood & T. Bierly 15104.02 (PTBG); loc. cit., 997 m elev., 24 Jan 2016, K. R. Wood & K. Lee 16726 (BISH, PTBG); loc. cit., 997 m elev., 24 Jan 2016, K. R. Wood, K. Lee 16727 (PTBG); loc. cit., 997 m elev., Jan 2016, K. R. Wood, S. Perlman & R. Aguraiuja 16729 (BISH, PTBG, US); loc. cit., 997 m elev., K. R. Wood & K. Lee 16728 (BISH, K, MO, NY, PTBG, UC, US); loc. cit., 997 m elev., 28 Jan 2016, K. R. Wood, S. Perlman & R. Aguraiuja 16730 (PTBG, US); loc. cit., 997 m elev., 28 Jan 2016, K. R. Wood, S. Perlman & R. Aguraiuja 16731 (BISH, PTBG, US); loc. cit., 997 m elev., 28 Jan 2016, K. R. Wood, S. Perlman & R. Aguraiuja 16732 (BISH, PTBG, UC, US); Mākaha, forests around dividing ridge between upper north and south fork, 1037 m elev., 18 Feb 2016, K. R. Wood & S. Perlman 16741 (PTBG); Nu‘ololo, north facing slopes above northern branch, 1097 m elev., 18 Jul 2012, K. R. Wood, M. Query & M. Kirkpatrick 15101 (BISH, K, MBK, MO, NY, P, PTBG, UC, US); Nu‘ololo, north of trail, headwaters of central Nu‘ololo stream, 1061 m, 3 Oct 2012, K. R. Wood, M. Kirkpatrick & M. Query 15267 (BISH, PTBG, US); Nu‘ololo, 1052 m elev., 02 Jan 2013, K. R. Wood & M. Kirkpatrick 15319 (BISH, PTBG); Nu‘ololo, 1049 m elev., 02 Jan 2013, K. R. Wood & M. Kirkpatrick 15320 (BISH, NY, PTBG, UC, US); Nu‘ololo, 1085 m elev., K. R. Wood, M. Kirkpatrick & S. Perlman 15560 (PTBG); Nu‘ololo, 1027 m elev., Jul 2013, K. R. Wood, M. Kirkpatrick & S. Perlman 15562 (PTBG); Nu‘ololo, 1052 m elev., 12 Sep 2013, K. R. Wood & W. Kishida 15667 (PTBG); Nu‘ololo, 1036 m elev., 12 Sep 2013, K. R. Wood & W. Kishida 15668 (PTBG); Nu‘ololo, 1061 m elev., 28 Sep 2013, K. R. Wood & M. Query 15671 (PTBG); Nu‘ololo, 1073 m elev., 22 Jul 2014, K. R. Wood, M. Kirkpatrick, S. Perlman & R. Aguraiuja 16001 (PTBG); loc. cit., 1073 m elev., 22 Jul 2014, K. R. Wood, M. Kirkpatrick, S. Perlman & R. Aguraiuja 16002 (PTBG).

Distribution, ecology, and threats. Melicope stonei is endemic to Kaua‘i, oldest of the high Hawaiian Islands at 5.1 million years, with an area of ca. 1430 km² (Sakai et al. 2002), and maximum elevation of 1598 m at the summit of Kawaikini. Floristically, Kaua‘i has the phenomenal distinction of having the highest level of plant diversity of all the Hawaiian Islands, which includes 249 single island endemic (SIE) taxa, 232 of which are flowering plants, and the remaining 17 being unique fern taxa (Palmer 2003, Wagner et al. 2012, Vernon and Ranker 2013, Ranker 2016, Wood et al. 2016b).

Earliest known collections of Melicope stonei were made by David Lorence and Timothy Flynn (National Tropical Botanical Garden) as far back as February of 1988 within the forests of Mākaha Valley, Kaua‘i. Over the past 29 years, 94 trees have been mapped by local botanists, with colonies extending into several valleys to the north of Mākaha, namely Ku‘ia, Miloli‘i, Mahanaloa, Nu‘ololo, and Pa‘aiki (Figure 3). Melicope stonei has a very narrow elevational range of 988 to 1097 m and a precariously small extent of occurrence of 1.5 km². Rich forest habitats still flourish in adjacent lower and higher elevational regions, yet it is evident that M. stonei prefers very tall (15–20 m) old growth Metrosideros polymorpha Gaudich. (Myrtaceae)- Acacia koa A. Gray (Fabaceae) mixed mesic forest with the occasional co-dominant Alphitonia ponderosa Hillebr. (Rhamnaceae). This plant community type only occurs on Kaua‘i and is exceedingly limited with some of the finest examples occurring around the forested flats of Kōke‘e State Park above Pa‘aiki, Mahanaloa, and Nu‘ololo. In these forests, M. stonei reaches heights of 10–12 m and is associated with a wide diversity of other tall understory trees such as Bobea brevipes A. Gray (Rubiaceae), Cryptocarya mannii Hillebr. (Lauraceae), Dodonaea viscosa Jacq. (Sapindaceae), Kadua affinis DC. (Rubiaceae), Melicope barbigera A. Gray, Myrsine lanaiensis Hillebr. (Primulaceae), Nestegis sandwicensis (A. Gray) O. Deg., I. Deg. & L.A.S. Johnson (Oleaceae), Planchonella sandwicensis (A. Gray) Pierre (Sapotaceae), Polyscias kavaiensis (H. Mann) Lowry & G.M. Plunkett and P. waimeae (Wawra) Lowry & G.M. Plunkett (Araliaceae), Psychotria mariniana (Cham. & Schltdl.) Fosberg (Rubiaceae), Santalum pyrularium Hook. & Arn. (Santalaceae), Syzygium sandwicensis (A. Gray) Nied. (Myrtaceae), Xylosma hawaiiense Seem. (Salicaceae), and Zanthoxylum dipetalum H. Mann (Rutaceae).

Interspersed below this community’s canopy are rich assemblages of medium statured trees that include Antidesma platyphylla H. Mann var. hillebrandii Pax & Hoffm. (Phyllanthaceae), Chrysodracon aurea (H. Mann) P.-L. Lu & Morden (Asparagaceae), Claoxylon sandwicense Müll. Arg. (Euphorbiaceae), Coprosma foliosa A. Gray and C. waimeae Wawra (Rubiaceae), Cyanea leptostegia A. Gray (Campanulaceae), Elaeocarpus bifidus Hook. & Arn. (Elaeocarpaceae), Labordia helleri Sherff (Loganiaceae), Myrsine alyxifolia Hosaka, Pittosporum kauaiense Hillebr. (Pittosporaceae), Platydesma rostrata Hillebr. and P. spathulata (Rutaceae), Psychotria greenwelliae Fosberg, Psydrax odorata (G. Forst.) A.C. Sm. & S.P. Darwin (both Rubiaceae), Sophora chrysophylla (Salisb.) Seem. (Fabaceae), Streblus pendulinus (Endl.) F. Muell. (Moraceae), and Wikstroemia furcata (Hillebr.) Rock (Thymelaeaceae).

Common shrubs and smaller tree components are Leptecophylla tameiameiae (Cham. & Schlecht.) C.M. Weiller (Epacridaceae), Lysimachia kalalauensis Skottsb. (Primulaceae), Melicope anisata (H. Mann) T.G. Hartley & B.C. Stone, M. feddei (H. Lév.) T.G. Hartley & B.C. Stone, M. ovata (H. St. John & E.P. Hume) T.G. Hartley & B.C. Stone, and M. peduncularis (H. Lév.) T.G. Hartley & B.C. Stone.

Associated sedges (Cyperaceae) include Carex meyenii Nees, C. wahuensis C.A. Mey., and Gahnia beecheyi H. Mann, grasses (Poaceae) are Eragrostis grandis Hillebr., E. variabilis (Gaudich.) Steud., and Panicum nephelophilum Gaudich., and additional associated vegetation includes the herbaceous Dianella sandwicensis Hook. & Arn. (Xanthorrhoeaceae), and vines of Alyxia stellata (J.R. & G. Forst.) Roem. & Schult. (Apocynaceae), and Smilax melastomifolia Sm. (Smilacaceae).

Common to occasional pteridophytes of this habitat are Asplenium normale D. Don (Aspleniaceae) Adenophorus tamariscinus (Kaulf.) Hook. & Grev. and A. tripinnatifidus Gaudich. (Polypodiaceae), Cibotium nealiae O. Deg. (Cibotiaceae), Coniogramme pilosa (Brack.) Hieron. (Pteridaceae), Diplazium sandwicensis (A. DC.) Fosberg (Athyriaceae), Doodia kunthiana Gaudich. (Blechnaceae), Dryopteris glabra (Brack.) Kuntze, and D. unidentata (Hook. & Arn.) C. Chr. (Dryopteridaceae), Elaphoglossum paleaceum (Hook. & Grev.) Sledge (Dryopteridaceae), Microlepia strigosa (Thunb.) C. Presl (Dennstaedtiaceae), and Odontosoria chinensis (L.) J. Sm. (Lindsaeaceae).

Threats to Melicope stonei include habitat degradation by introduced pigs (Sus scrofa L.) and mule deer (Odocoileus hemionus Rafinesque), predation of seeds by rats (Rattus rattus L. and R. exulans Peale), environmental events such as hurricanes, fire (mostly by humans), and competition with invasive non-native plant species, including Adiantum hispidulum Sw. (Pteridaceae), Blechnum appendiculatum Willd. (Blechnaceae), Corynocarpus laevigatus J.R. Forst. & G. Forst. (Corynocarpaceae), Erigeron karvinskianus DC. (Asteraceae), Grevillea robusta A. Cunn. ex R. Br. (Proteaceae), Hedychium gardnerianum Ker Gawl. (Zingiberaceae), Kalanchoe pinnata (Lam.) Pres. (Crassulaceae), Lantana camara L. (Verbenaceae), Lophospermum confertus (R. Br.) P.G. Wilson & J.T. Waterhouse (Myrtaceae), Morella faya (Ait.) Wilbur (Myricaceae), Psidium cattleianum Sabine (Myrtaceae), Rubus argutus Link and R. rosifolius Sm. (Rosaceae), Setaria parviflora (Poir.) Kerguélen (Poaceae), and Sphaeropteris cooperi (Hook. ex F. Muell.) R.M. Tryon (Cyatheaceae) all of which possess the ability to spread rapidly and cover effectively large areas (Smith 1985).

Conservation status. IUCN Red List Category. When evaluated using the World Conservation Union (IUCN) criteria for endangerment (IUCN 2001), Melicope stonei 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 ranking system of criteria and subcriteria: CR B1ab(i,ii,iii,iv,v)+2ab(i,ii,iii,iv,v); C2a(i); which reflects a severely limited Extent of Occurrence (EOO) and Area of Occupancy (AOO) of less than 1.5 km² and a wild population of less than 100 individuals with all facing a continuing decline in their area of extent and quality of habitat (see Distribution, ecology, and threats). Seeds from several individuals of M. stonei have been collected and submitted to the NTBG Horticultural Department for cultivation.

Morphology, related taxa, and phylogenetic placement. Melicope stonei stands apart from all other described Hawaiian Melicope species by its combination of being large monoecious trees up to 12 m tall with distinct carpels and short-villous ramiflorous inflorescences. There are three other Hawaiian species that usually have ramiflorous cymes arising on stems below the leaves, namely M. clusiifolia (A. Gray) T.G. Hartley & B.C. Stone from Kaua‘i, O‘ahu, Moloka‘i, Lana‘i, Maui, and Hawai‘i, M. haleakalae (B.C. Stone) T.G. Hartley & B.C. Stone from Maui, and M. waialealae (Wawra) T.G. Hartley & B.C. Stone from Kaua‘i, yet they differ from M. stonei in having leaves in whorls of 4–8 with abaxial surface glabrous or with some hairs on midrib or sometimes loosely villous throughout the surface, and having carpels connate at their base or nearly throughout their length in fruit (as compared to M. stonei having leaves opposite with abaxial surface densely tomentose and with carpels distinct in fruit). Other Hawaiian species that may occasionally have the cymes arising below the leaves include M. ovata from Kaua‘i, M. pseudoanisata (Rock) T.G. Hartley & B.C. Stone, from Maui and Hawai‘i, and M. quadrangularis (H. St. John & E.P. Hume) T.G. Hartley & B.C. Stone, also from Kaua‘i. The latter two differ with carpels connate and leaves glabrous or with some hairs on midrib (Wagner et al. 1999), and the former, M. ovata, lacks the short-villous peduncles, pedicels, sepals, and petals found on M. stonei. Morphologically, these species have little else in common with M. stonei.

In habit Melicope stonei appears most similar to trees of M. barbigera from Kaua‘i and M. knudsenii (Hillebr.) T.G. Hartley & B.C. Stone from Kaua‘i and Maui, both of which can reach heights of 12 m. Melicope barbigera differs from M. stonei in having new growth grayish appressed puberulent; slightly folded leaves with waxy scurf and commonly having abaxial leaf surface densely long-villous along each side of midrib (Fig. 2G); cymes axillary, not ramiflorous; peduncles 20–25 mm long; nectary disk and ovary puberulent (as compared to M. stonei with new growth tomentose, yellow-tan; leaves neither folded nor having a waxy scurf, rarely with abaxial surface densely long-villous along each side of midrib (Fig. 2E–F); cymes axillary and ramiflorous; peduncles 1–5(–10) mm long; with nectary disk and ovary glabrous). Melicope knudsenii differs in having leaf bases weakly cordate; flowers perfect or unisexual, (3–)20–200; cymes axillary, not ramiflorous; and peduncles (10–)30–60 mm long; (as compared to M. stonei with leaf bases rounded; flowers male or female, plants monoecious, 3–5(–7); cymes axillary and ramiflorous; and peduncles 1–5(–10) mm long) (Wagner et al. 1999).

Four specimens of Melicope stonei (Wagner & Wood 6891; Wood 7696; Wood & Aguraiuja 7697; Wood, Query & Kirkpatrick 15101) have been included in phylogenetic analyses of Hawaiian Melicope (Harbaugh et al. 2009; Appelhans et al. 2014b,c) and they all represent paratypes of this new species. The specimens were labeled as M. knudsenii in these studies according to their original conferred determination. Phylogenetic analyses showed that the former section Apocarpa is paraphyletic with respect to the former section Pelea (unpublished results) and that M. stonei forms a clade with the “Apocarpa” species M. adscendens and M. ovata (Fig. 4). The monophyly of this clade is supported by posterior probability (1.00pp) and bootstrap (98bs) values and the three species, of which three to four specimens have been sampled, were resolved as monophyletic entities (Appelhans et al. 2014c; Fig. 4).

Melicope adscendens (H. St. John & E.P. Hume) T.G. Hartley & B.C. Stone, from Maui, and the previously mentioned M. ovata from Kaua‘i, which are the closest relatives of M. stonei, (Appelhans et al. 2014c; Fig. 4) do not have clear morphological similarities with the new species. While M. stonei is a tall tree, M. adscendens is a sprawling shrub and M. ovata is a shrub or small tree with sprawling branches. Melicope adscendens has considerably smaller leaves 1.5–6.5 × 1–4 cm and does not have a densely tomentose abaxial leaf surface, being minutely puberulent, becoming glabrate (as compared to M. stonei with leaves (5–)8–30 × (4–)6–11 cm with densely tomentose abaxial leaf surface). Melicope ovata has sepals glabrous or sparsely minutely ciliate externally, and petals glabrous externally (Wagner et al. 1999), (as compared to M. stonei with sepals and petals densely short-villous externally).

A striking character of Melicope stonei is the ramiflorous inflorescences, which it most notably shares with M. clusiifolia, M. haleakalae, M. ovata, M. pseudoanisata, M. quadrangularis, and M. waialealae, and can occur on rare occasions with other Hawaiian Melicope. This character is homoplasious and has evolved probably three or four times in Hawaiian Melicope: once in M. stonei and M. ovata of the Apocarpa group, once in the ancestor of M. clusiifolia, M. haleakalae and M. waialealae, who form the monophyletic Pelea group (Appelhans et al. 2014c; Fig. 4), and once or twice in the ancestor(s) of M. pseudoanisata and M. quadrangularis, which are part of an unresolved group of species belonging to the intermixed former sections Cubicarpa and Megacarpa (unpublished results; Fig. 4).

Morphologically, Melicope stonei is most similar to M. barbigera and the Kaua‘i population of M. knudsenii (see above). Melicope knudsenii has been shown to be polyphyletic and the Maui form does not belong to this group (Appelhans et al. 2014c; Fig. 4). As a result, a future publication is now in preparation which will resurrect the Maui form (i.e., M. multiflora Rock) as a species distinct from the Kaua`i M. knudsenii. While M. barbigera and the Kaua‘i form of M. knudsenii are resolved as immediate sisters in the phylogenetic analyses, M. stonei is not closely related to them and belongs to another clade of the paraphyletic Apocarpa group (Appelhans et al. 2014c; Fig. 4). It should be noted that the variable abaxial midrib pubescence of M. stonei ranging from villous on the sides to uniform pubescence (Fig. 2E–F) could represent natural variability or may suggest possible undetected hybridization between M. stonei and M. barbigera. Other putative Apocarpa hybrids collected and observed in the Kōke‘e forests of Kaua‘i include: M. barbigera × M. haupuensis (H. St. John) T.G. Harley & B.C. Stone (i.e., Wood & Query 14696 [PTBG]) observed around Awa‘awapuhi and Honopū (825–1050 m); M. haupuensis × M. pallida (Hillebr.) T.G. Hartley & B.C. Stone (i.e., Wood et al. 7725 [PTBG]) around Awa‘awapuhi (1050 m); and M. ovata × M. stonei (i.e., Wood 17237 [BISH, K, MO, NY, PTBG, UC, US], Wood 17245 [BISH, PTBG, UC, US], Wood 17246 [BISH, PTBG, US]), in the valleys of Mākaha to Nu‘ololo (988–1097 m).