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16.16 Taxon Summary: arbuscula

2 Photographer: J. Obata 4 Scientific name: Hillebr. 6 Hawaiian name: None known Family: (Sunflower family) 8 Federal status: Listed endangered

10 Description and biology: Hesperomannia arbuscula is a shrub or small tree 2-3.3 m (6.6-10.8 ft) tall, and reportedly reaching up to 7.6 m (25 ft) tall (Degener 1946). The leaves measure 10- 12 18 cm (3.9-7.0 in) long, 5.5-11.5 cm (2.1-4.5 in) wide, and are covered with minute hairs. The flower heads, which resemble those of thistles, are borne at the stem tips, usually in clusters of 4- 14 5. The florets are yellow in color, and are perfect (possessing both male and female reproductive parts) and project beyond the bracts of the flower head. The 's achenes (a type of dry, seed- 16 like fruit) are 0.8-1 cm (0.3-0.4 in) long and are tipped by hair-like bristles about twice as long as the achene. 18 The flowers are visited by birds, and are presumably pollinated by them (Carlquist 1974). 20 Bristle-bearing achenes are characteristic of the wind-dispersed members of the sunflower family. However, the achenes of H. arbuscula are very large and heavy in comparison to 22 continental wind-dispersed members of the family, and seemingly would not be capable of being carried on the wind over long distances. Furthermore, this species usually grows in tight 24 colonies (Lau pers. comm. 2000), supporting the supposition that the seeds are not widely dispersed. Judging from observed growth rates and the size of the largest , the plants may 26 live 10 to 20 years, or more (Lau pers. comm. 2000).

28 Known distribution: Hesperomannia arbuscula is endemic to the Waianae Mountains and West Maui. The species is found throughout the Waianae Range, both on the windward and 30 leeward sides, at elevations of 597-914 m (1,960-3,000 ft). The currently known plants of H.

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arbuscula on West Maui, whose identity is in question, range from 488-762 m (1,250-2,500 ft) 32 in elevation.

34 Population trends: All of the population units that have been observed for a number of years have declined in numbers. Thirteen plants were counted in the Kapuna Gulch colony in 1991, 36 shortly after the colony was discovered, whereas only seven remained in 1998. The Waianae Kai population unit was reported to contain seven mature plants, eight saplings, and 12 seedlings 38 in 1978, but in 1999, only nine mature plants and one immature plant were left. In 1977 the Makaha population unit reportedly contained 12 mature plants, 25 saplings, and 25 seedlings, 40 while in 1999, only 13 mature plants and a single immature plant were counted. Finally, in 1977, the Kaluaa Gulch colony was reported to contain six mature plants and a single sapling, 42 but by 1985, the colony had completely disappeared.

44 Current status: A total of 39 individuals of H. arbuscula are known to remain in the Waianae Mountains. The seven individuals in Kapuna Gulch are within the Makua action area. The 46 questionable H. arbuscula on West Maui totals about 63 known individuals. The species' current population units are listed in Table 16.46 and their sites are plotted on Maps 16.24 and 16.25. 48 The population units proposed for management for stability, which include all of the current populations in the Waianae Mountains, are characterized in Table 16.47 and threats to the plants 50 at these sites are identified in Table 16.48. Since the identity of the West Maui plants is unclear, none of their population units are proposed for management for stability at this time. 52 Habitat: Hesperomannia arbuscula in the Waianae Mountains typically grows in mesic forests 54 on upper gulch slopes, or on ridge tops. The dominant trees at these sites are usually ohia (Metrosideros polymorpha), lama (Diospyros sandwicensis), and/or koa (Acacia koa). The 56 questionable H. arbuscula on West Maui occurs in wetter mesic forests to very wet rainforests, which are often dominated by ohia. 58 Taxonomic background: Hesperomannia is an endemic Hawaiian genus with two species 60 besides H. arbuscula: H. lydgatei, which is endemic to Kauai, and H. arborescens, which has been recorded on Oahu, Molokai, Lanai, and West Maui. 62 The type specimen of H. arbuscula, which was collected inland of Lahaina in the 1800's, is the 64 only firm basis for the inclusion of West Maui in the historic range of the species, as plants found since the collection of the type specimen are of dubious identity. Some taxonomists have 66 identified the plants as H. arbuscula, while others think that they actually represent H. arborescens instead of H. arbuscula, perhaps with the exception of the plants in Iao Valley. The 68 of Hesperomannia on West Maui is in need of further study.

70 There are marked morphological differences between some of the populations of H. arbuscula in the Waianae Mountains, with the differences in their leaf characteristics most readily apparent 72 (Lau pers. comm. 2000).

74 Outplanting considerations: Until recently, it was thought that on Oahu, H. arborescens was restricted to the wet forests of the Koolau Mountains, but in 2000, five plants were discovered 76 growing in mesic forest at Palikea Gulch within Mt. Kaala NAR. These few Waianae Range

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individuals of H. arborescens are morphologically very different from plants of all other 78 currently known populations in the Koolau Mountains, Molokai, and West Maui, and it is therefore very important that they be conserved. It is not known whether the ranges of the two 80 Hesperomannia species in the Waianae Mountains originally overlapped, whether the two occurred in different habitats, and whether any hybridization was taking place between the two. 82 These uncertainties, coupled with the importance of the distinctive H. arborescens of the Waianaes with respect to the conservation of the genus as a whole, necessitate a cautious 84 approach in the establishment of H. arbuscula reintroduction sites in the northern Waianae Mountains. An outplanting line has been drawn well to the west of Palikea Gulch, limiting 86 potential reintroduction sites of H. arbuscula to areas west of the line.

88 The distinctive morphological differences between the various Waianae Range populations should be maintained as much as possible by preserving the various stocks separately. However, 90 if future research clearly shows that the species' populations are suffering from inbreeding depression, controlled experiments on the consequences of mixing the morphologically different 92 stocks should be initiated.

94 Threats: The major threats to H. arbuscula in the Waianae Mountains include feral pigs and goats, which degrade the species’ habitat, and harm the plants by feeding on them, trampling 96 them, or uprooting them while rooting for food. Invasive alien plants threaten H. arbuscula by altering the species’ habitat and competing with it for sunlight, moisture, nutrients, and growing 98 space. Also, the spread of highly flammable alien grasses increases the incidence and destructiveness of wildfires. The Waianae Kai plants, which constitute the second largest 100 population unit, are vulnerable to human disturbance. A major hiking and hunting trail runs right through the population unit, and right alongside two of the plants. Some of the questionable H. 102 arbuscula populations of West Maui may also be threatened by axis deer, whose numbers on Maui have been increasing over the last decade. 104

106 Table 16.46 Current Population Units of Hesperomannia arbuscula. The numbers of individuals include mature and immature plants, and do not include seedlings. 108 Population units proposed for management are shaded. Island Population Unit Name Total No Management Number of Management Proposed Individuals Proposed Oahu: Kaaikukai 1 0 1 Kapuna 7 0 7 Makaha 14 0 14 North Palawai 7 0 7 Waianae Kai 10 0 10 Maui: Honokohau 25 25 0 Iao 3 3 0 Kapilau 2 2 0 Waihee 33 33 0

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110 Table 16.47 Site Characteristics for Population Units of Hesperomannia arbuscula Proposed for Management for Stability. 112 Population Unit: Site Characteristics: Habitat Quality Terrain Accessibility Existing Fence

Kaaikukai Low Steep High None

Kapuna High – Medium Moderate High None

Makaha High – Medium Moderate High None

North Palawai Medium - Low Moderate to Steep Medium None

Waianae Kai High – Medium Moderate to Steep High None

114 Table 16.48 Threats to Population Units of Hesperomannia arbuscula Proposed for Management for Stability.

Population Threats: Unit: Pigs Goats Weeds Rats Black Slugs Other Fire Fire Erosion Human Twig and Arthro- Ignition Fuels Distur- Borer Snails pods bance Kaaikukai Medium N/A High Unknown Unknown Unknown Unknown High Medium Low Medium A A A A Kapuna High Low Medium Unknown Unknown Unknown Unknown Very high Medium Low Medium A A A A Makaha High Medium High Unknown Unknown Unknown Unknown Very high Medium Medium Medium A A A A North Palawai Medium N/A High Unknown Unknown Unknown Unknown High Medium Medium Medium A A A A Waianae Kai High Medium High Unknown Unknown Unknown Unknown Very high Medium Medium High A A A A 116

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