No Common Name
|Listed||June 27, 1994|
|Description||Earth-growing fern with black, shiny frond stalks.|
|Habitat||Deep shade or open understory in dry-land forest.|
|Threats||Competition from alien plants; habitat destruction by people; fire; limited numbers.|
Diellia unisora, a member of the spleenwort family (Aspleniaceae), grows from a slender erect rhizome or underground stem 0.2-1.2 in (0.5-3 cm) tall that is covered with the bases of the leaf stalks and a few small black scales. Stalks of the fronds are black and shiny, and are 0.8-2 in (2-5 cm) long. The fronds are linear, 3-12 in (3-30 cm) tall by 0.2-1.2 in (0.5-3 cm) wide, with 20-35 pinnae (leaflets) per side, and gradually narrowing towards the apex. The pinnae are usually strongly asymmetrical, unequally triangular, with mostly entire (smooth) margins. There usually is a single marginal sorus (the spore-producing body) running along the upper margin of the underside of the pinna. This species is distinguished from others in the genus by a rhizome completely covered by the persisting bases of the leaf stalks and a few very small scales, by sori mostly confined to the upper pinna margins, and by delicate fonds gradually and symmetrically narrowing toward the apex.
Diellia unisora is a terrestrial fern that typically grows in deep shade or open understory in dryland forest at an elevation of 1,750-2,500 ft (533-762 m). Associated plant species include strawberry guava, Christmas berry, 'ohi'a, and a mixture of alien and native grasses, forbs, and shrubs.
Diellia unisora was known historically on Oahu from steep, grassy, and rocky slopes on the western side of the Waianae Mountains. Four populations were extant in 1997; scattered over a distance of 2 mi (3.2 km) on state and private land in the southern Waianae Mountains, they contained approximately 700 individuals. The South Ekahanui Gulch occurrence had six plants, Palawai Gulch had 90, Palikea had four, and the Pualii-Napepeiauolelo Ridge had 600.
The major threats to Diellia unisora are competition from the alien plants Christmas berry, molasses grass, huehue haole, strawberry guava, and Blechnum occidentale ; habitat degradation by feral pigs; potential plant damage from the two-spotted leafhopper, and a risk of extinction from naturally occurring events or reduced reproductive vigor due to the small number of remaining individuals.
Conservation and Recovery
The only three known populations of the Diellia unisora are on Lualualei Naval Reservation (Department of Defense) and on privately owned land. Both of these habitats must be protected. The privately owned habitat should be acquired and designated as an ecological reserve, or conservation easements negotiated. In addition, the critical habitat of the rare fern should be fenced to exclude mammalian herbivores, particularly goats. The abundance of threatening non-native plants, especially Christmas berry, huehue haole, and strawberry guava, should be aggressively reduced in the habitat of the endangered fern. Its populations should be monitored, and research undertaken into its biology and habitat needs. Work should be undertaken on the propagation of Diella unisora in captivity, with the long-term goal of out-planting to supplement the perilously small wild population.
U.S. Fish and Wildlife Service
Regional Office 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. 27 June 1994. "Endangered Status for Three Plants from the Waianae Mountains, Island of Oahu, HI." Federal Register 59 : 32933-32939.