'Ohai

Sesbania tomentosa

Description

Sesbania tomentosa (commonly known as 'ohai; a member of the pea family) is typically a sprawling shrub with branches up to 45 ft (13.7 m) long, although it may also be a small tree up to 20 ft (6.1 m) in height. Every compound leaf, usually sparsely to densely covered with silky hairs, is comprised of 18-38 oblong to elliptic leaflets, each 0.6-1.5 in (1.5-3.8 cm) long and 0.2-0.7 in (5.1-17.8 mm) wide. The flowers, in clusters of two to nine, are salmon colored with yellow, orange-red, scarlet, or (rarely) pure yellow tinges. The petals are 0.9-1.8 in (2.3-4.6 cm) long, the upper pair sometimes of a lighter color than the other petals. The calyx is about 0.3-0.5 in (7.6-12.7 mm) long. Fruits are slightly flattened pods 2.8-9 in (7.1-22.9 cm) long and about 0.2 in (5.1 mm) wide; the fruits contain about 6-27 olive to pale or dark brown oblong seeds. S. tomentosa is the only endemic Hawaiian species in the genus, differing from the naturalized S. sesban by the color of the flowers, the longer petals and calyx, and the number of seeds per pod.

S. tomentosa, named for its silvery hairs, was first described by W.J. Hooker and G.A.W. Arnott in 1836 from collections made on Oahu. In 1920 Joseph F. Rock described S. tomentosa var. arborea, an arborescent form of the species, from a Molokai specimen. Variety molokaiensis was published in 1949 based on plants from West Molokai. Otto and Isa Degener elevated that variety to the specific level in 1978; at that time, the Degeners also described the new species S. hawaiiensis and S. hobdyi. In the currently accepted classification from 1990, S. arborea, S. hawaiiensis, S. hobdyi, and S. molokaiensis are synonymized with S. tomentosa. They note, however, that the arborescent form of the species found on the island of Molokai probably merits formal taxonomic recognition.

The preliminary findings of the pollination biology of S. tomentosa by a researcher at the University of Hawaii suggest that 1) although many insects visit Sesbania flowers, the majority of successful pollination is accomplished by native bees of the genus Hylaeus and 2) populations at Kaena Point are probably pollinator limited. Flowering at Kaena Point is highest during the winter to spring rains and then gradually declines throughout the rest of the year.

Habitat

S. tomentosa is found on sandy beaches, dunes, soil pockets on lava, and along pond margins. It commonly occurs in coastal dry shrublands and grasslands, but it is also known from open 'ohi'a forests and mixed coastal dry cliffs. Associated plant species include 'ilima, naupaka kahakai, Heteropogon contortus (pili), Myoporum sandwicense (naio), and Sporobolus virginicus ('aki'aki).

Distribution

On the privately owned island of Niihau, S. tomentosa was known from the south tip of the island at the headland west of Kaumuhonu Bay; at least one collection was made in 1947 at an elevation of 160 ft (48.8 m). S. tomentosa was found on Kauai between the town of Mana and Mana Point as recently as the late 1980s, but this population is no longer extant. This species was known historically on Oahu from the eastern portion of the island at Ulupau Crater, as well as on the islets of Kaohikaipu and Mokulua. The species also occurred on western Oahu at an unspecified location along the Waianae coast. On Molokai, S. tomentosa was known historically from Mahana on Mauna Loa, in the vicinity of the coast near Waiahewahewa Gulch, and on the island's western coast at Laau and Ilio points. The species also occurred at various locations on Lanai and at an unspecified location on Kahoolawe.

S. tomentosa currently occurs on at least six of the eight main Hawaiian Islands—Kauai, Oahu, Molokai, Maui, Hawaii, and Kahoolawe—and on the Northwestern Hawaiian Islands of Nihoa and Necker in the Hawaiian Islands National Wildlife Refuge. The extant S. tomentosa populations on the main Hawaiian Islands contained an estimated 2,000-3,000 individuals in 1997.

The largest population in the Northwestern Hawaiian Islands (described as relatively common in some areas) occurs on Nihoa, an island 0.3 sq mi (0.8 sq km) in size, and consists of several thousand individuals. Another population occurs on Necker Island, which is only 0.1 sq mi (0.3 sq km) in area; there, S. tomentosa grows on the tops of all hills on the main island at elevations of 150-276 ft (45.7-84.1 m). A few plants are also found on the Northwest Cape, giving an islandwide total of several hundred individuals.

Threats

The primary threats to S. tomentosa are 1) habitat degradation caused by axis deer and cattle, 2) competition with various alien plant species, 3) fire, and 4) destruction by off-road vehicles.

Two populations of S. tomentosa on Lanai were last seen in the mid-1950s. These areas have long supported axis deer.

One population of S. tomentosa located east of Moomomi Preserve on Molokai is potentially threatened because it grows in a cattle grazing area. In the Kamaoa-Puueo and South Point regions of the Big Island, cattle continue to graze in habitat currently occupied by most of the populations of S. tomentosa.

Predation by cattle, deer, and goats is a probable threat to this plant at sites where these animals have been reported.

The growth of koa haole, kiawe, buffelgrass, molasses grass, and Bermuda grass pose serious threats to S. tomentosa.

All-terrain vehicles have driven over S. tomentosa plants growing west of the Nakalele Point light-house on West Maui. Continued off-road activity threatens to destroy a significant portion of that population. On the Big Island, a dirt road runs through a population of S. tomentosa located in the Kamaoa-Puueo region. Off-road activity could damage a significant portion of that population as well.

S. tomentosa is an exceptionally attractive species that grows well in low elevation urban areas. Over-collection for scientific or horticultural purposes could become a potential threat to this species.

Fire is also a threat to two populations of S. tomentosa on Molokai.

Conservation and Recovery

S. tomentosa has benefited from extensive conservation actions, many of them conducted by the State of Hawaii. Individuals on Oahu have been protected from off-road vehicles within the Kaena Point Natural Area Reserve through the construction of rock barriers; an extensive propagation and outplanting effort is ongoing. Several hundred seedlings were propagated at the Division of Forestry and Wildlife's Makiki Rare Plant Facility and outplanted at Kaena Point from 1992-1994. Approximately 250 individuals survived and became established, but very little natural reproduction was observed. Rat and mice controls were initiated in 1994 and 1995, respectively, and resulted in dramatic increases in seedling germination and survival. S. tomentosa has also been propagated in the Division of Forestry and Wildlife's nursery on Maui at Kahului, and approximately 30 seedlings were planted within a protective enclosure at Mokolea Point. One seedling survived as of June 1995. Propagation and outplanting were also conducted in Hawaii Volcanoes National Park in the 1970s. The status of these plants is unknown because the out-planted individuals have not been visited since they were planted. Further outplanting and surveying is planned for Kilauea Point National Wildlife Refuge on Kauai using seedlings propagated by the Kauai Division of Forestry and Wildlife.

S. tomentosa 's pollination biology and the effects of population isolation and fragmentation are being studied by the Department of Zoology at the University of Hawaii at Manoa.

More than 8,000 seeds were in storage at the National Tropical Botanical Garden in 1997. Eighty-eight individuals are in cultivation at Lyon Arboretum, the National Tropical Botanical Garden, and Waimea Arboretum. Seed germination tests indicate 65% germination of fresh seeds and less than 20% germination after a minimum of 45 days in storage.

A research project on cultivation of S. tomentosa was conducted at Hawaii Community College on the island of Hawaii. The effects of different treatments of scarification and heat on seed dormancy was examined, as was the effect of various fertilizer regimes on seedling growth rate, vigor, and flowering. Results of the project should help determine optimal conditions for seed germination and seedling growth of S. tomentosa.

Although S. tomentosa occurs in more than 27 populations and up to 7 of the populations have more than 300 individuals, reproduction sufficient to sustain current numbers is occurring only in the Nihoa and Necker populations. The most important recovery actions for S. tomentosa include 1) research into the reasons for the poor reproductive success of the species, 2) implementation of defensive measures to combat rats, mice, and alien insects, and 3) the development of techniques to conserve native pollinators.

Contacts

U. S. Fish and Wildlife Service
Regional Office, Division of Endangered Species
Eastside Federal Complex
911 N.E. 11th Ave.
Portland, Oregon 97232-4181
Telephone: (503) 231-6121
http://pacific.fws.gov/

Pacific Remote Islands Ecological Services Field Office
300 Ala Moana Blvd., Room 3-122
P.O. Box 50088
Honolulu, Hawaii 96850
Telephone: (808) 541-1201
Fax: (808) 541-1216

Reference

U. S. Fish and Wildlife Service. 10 November 1994. "Endangered Status for 12 Plants from the Hawaiian Islands." Federal Register 59 (217): 56333-56351.