Life history, reproductive biology, and conservation of the Mauna Kea silversword, Argyroxiphium sandwicense DC (Asteraceae), an endangered plant of Hawaii

The Mauna Kea silversword is an endangered giant rosette plant endemic to the alpine cinder deserts of the volcano Mauna Kea, Hawaii Island, Hawaiian Islands. The species became extinct in most of its original range between 1825 and 1950 due to browsing by feral ungulates. In 1988, there were 39 plants in a single remnant naturally occurring population. Between 1973 and 1982, silverswords were greenhouse-grown and outplanted in three exclosures on Mauna Kea.;The life history and reproductive biology of the Mauna Kea silversword was studied from 1985 to 1988. Plants grow for 7 to over 50 years before flowering, fruiting, and dying. The three re-established and one naturally occurring populations have more adults than seedlings. Mortality was 15 percent for seedlings and almost zero for established juveniles and adults. About 74 percent of the naturally occurring plants were unbranched, monocarpic plants, and about 80 percent of re-established plants were branched, polycarpic plants. Branching is apparently a trait that has been enhanced in the re-established populations by inadvertent selection.;Flowering capitula were visited by native Nesoprosopis bees, non-native flies, and native noctuid moths. Achenes were parasitized by larvae of the native fly, Tephritis arborea. Achenes in bagged capitula set about 0.3 percent seeds. Because flowers typically self-pollinate, low seed set suggests that the silversword is self-incompatible. About 24 percent of the achenes in pollen-augmented capitula set seeds, and 3 percent of the achenes in naturally pollinated capitula set seeds. In 1988, distance from the nearest flowering plant and seed set were significantly negatively correlated. Seed set was apparently limited by the number or activity of pollinating insects.;Simulations using transition matrices indicated that adult survival contributed most to population growth when seedling production was low. The probability of two plants flowering simultaneously in a single year, which is needed for cross-fertilization, is related to population size. The two smallest re-established populations (11 and 17 plants in 1988) and the naturally occurring population are expected to decline. The largest re-established population (261 plants in 1988) is composed primarily of siblings and half siblings. Inbreeding depression or increased frequency of shared self-incompatibility alleles may hamper reproduction in this population in the future.