|Listed||October 10, 1996|
|Description||Gnarled shrub; bears one to three erect flowers.|
|Habitat||Exposed ridge tops and cliff faces in mesic vegetation.|
|Threats||Habitat degradation and/or destruction by feral goats; potential impacts from military activities; competition with alien plants; fire.|
Lepidium arbuscula is a gnarled shrub of the mustard family (Brassicaceae or Cruciferae) that grows to a height of 2-3.9 ft (0.6-1.2 m). The leathery and hairless leaves, 1-2.4 in (2.5-6 cm) long and 0.3-0.7 in (7.5-18 mm) wide, have toothed margins and are spatula-shaped to oblong-elliptic or elliptic. The un-branched flowering stalk contains one to three erect flowers. The white, pale yellow, or greenish petals are 0.08-0.1 in (2-2.5 mm) long. The fruit is a capsule that is broadly egg-shaped to almost circular. This species has been observed in flower in February. This species is distinguished from others in the genus by its height.
More than 100 years ago, a plant was collected in the Waianae Mountains that was named L. arbuscula for its treelike habit. This species has been maintained in the most recent treatment of Hawaiian members of the genus.
L. arbuscula generally grows on steep slopes, exposed ridge tops, and cliff faces in mesic shrub-lands composed of a variety of native herbs and grasses, at elevations of 755-3,000 ft (230-910 m). This species is typically associated with native and non-native plants such as 'a'ali', Christmasberry, kawelu, pamakani, ko'oko'olau, Carex meyenii, and molasses grass.
L. arbuscula had historical occurrences at 11 populations in the Waianae Mountains. Ten populations now remain on state, City and County of Honolulu, and federal lands, the last of which occur at Lualualei Naval Reservation, Makua Military Reservation, and Schofield Barracks Military Reservation. These populations range from Kuaokala in the northern Waianae Mountains to Lualualei-Nanakuli Ridge in the southern Waianae Mountains. Less than 900 individuals of this species remained in 1997. Three populations numbered 100-600 individuals; three other populations in Manini Gulch and Mohiakea Gulch, however, numbered less than 10 individuals.
The primary threats to L. arbuscula are habitat degradation and destruction by feral goats; potential impacts from military activities; competition with the alien plants Christmas berry, lantana, Maui pamakani, molasses grass, silk oak, strawberry guava, Hamakua pamakani, and firetree; and potential fire. The population at the head of Kapuhi Gulch also is threatened by its proximity to a road.
One of the largest populations of L. arbuscula is directly threatened by feral goat trampling of plants and seedlings, as well as by goat-induced substrate erosion. L. arbuscula is not known to be unpalatable to goats and grows in areas where they have been reported; direct predation is therefore a possible threat.
Dense stands of strawberry guava threaten L. arbuscula, as do the dense thickets formed by Christmas berry. The mat-forming weed Hamakua pamakani is also a threat, as are lantana, a thicket-forming shrub, and silk oak. Molasses grass threatens L. arbuscula through the smothering mats it forms and the intense fires it can fuel, while fire-tree threatens one of the largest populations of this endangered species.
Populations of L. arbuscula that occur on land leased and owned by the U. S. Army face the threat of being damaged through military activity, either by troops in training maneuvers or by the construction, maintenance, and utilization of helicopter landing and drop-off sites.
Accidentally or maliciously set fires in areas of habitation near the Lualualei Naval Reservation and the Makua Military Reservation could easily spread and pose a possible threat to more than half of the individuals of L. arbuscula that occur on both reservations.
L. arbuscula has populations in recreational areas, near trails, and close to roads, making it very vulnerable to human disturbance.
Conservation and Recovery
The U. S. Army has adopted a fire management plan that includes realigning targets and establishing firebreaks. Implementation of the plan may aid in protecting this species from fire. Completion of a boundary fence on the south and southeast perimeter of the Makua Valley and continued goat control efforts, though limited, should help to protect the Makua-Keaau ridge plant from further goat damage.
This species is being propagated at the National Tropical Botanical Garden.
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
U. S. Fish and Wildlife Service. 1998. "Recovery Plan for Oahu Plants." U. S. Fish and Wildlife Service, Portland, Oregon. 207 pp.