Argyroxiphium sandwicense subsp. sandwicense, the Mauna Kea silversword, is a highly endangered flowering plant endemic to the island of Hawaiʻi (Big Island) of Hawaii. It is the "crown jewel" of the volcanic mountain Mauna Kea, from which it derives its English name. The Hawaiian name is ʻahinahina; it applies to silverswords more broadly. The Mauna Kea silversword was once common on the volcano, and extraordinary conservation efforts are being made to preserve the species.
Silverswords – Argyroxiphium Closely related to the Haleakala silversword (Argyroxiphium sandwicense subsp. macrocephalum) and in the family Asteraceae, the Mauna Kea silversword is a member of the silversword alliance, a group of approximately 50 species in three genera, all endemic to the Hawaiian islands. Their diverse morphologies belie extremely close genetic kinship and suggest extremely rapid evolution from a single precursor species. The silversword alliance is considered the most dramatic example of adaptive radiation among plants in Hawaii, illustrating the role of isolation and distinctive ecological conditions in promoting evolution.
Description The Mauna Kea silversword is an erect, single-stemmed and monocarpic or rarely branched and polycarpic basally woody herb, producing a globe-shaped cluster of thick, spirally arranged, sword-shaped silvery-green floccose-sericeous, linear-ligulate to linear-lanceolate leaves growing in a rosette. The epigeal or nearly epigeal rosette may become 0.6 m (2 ft) or more in diameter with individual leaves up to 0.3 m (1 ft) long and is usually less than 1.3 cm (1⁄2 in) wide. The leaves are completely covered with a dense layer of long silvery hairs. The leaves of all silverswords have an unusual and important ability to store water as a gel in intracellular spaces where other plant leaves contain air. The flowering stalk, which appears a few weeks before flowering, is narrow, but may reach nearly 3 m (9.8 ft) in height. It is composed of numerous, very sticky stalklets bearing up to 600 flowering heads about 2.5 cm (31⁄32 in) in diameter, each containing 500 individual flowers. Each head has about a dozen pink to maroon petal-like ray flowers around its periphery. The fruits resemble those of a sunflower, but are long and slender and are usually crowned with about 6 unequal scales less than 0.3 cm (1⁄8 in) in length. The plant lives for many years until it flowers; estimates of its life span range from 5–15 years to 40 years. Flowering occurs mostly from mid-June to November. Atypical plants possess branches that flower and die independently from the main plant, so that these individuals die only after the last branch flowers.
Other subspecies The other subspecies, A. sandwicense subsp. macrocephalum, is found in and around Haleakalā crater. They differ primarily in the inflorescence shape—broader in the Haleakalā plants (less than 4 times as long as wide), and narrower on Mauna Kea (4.3–8.6 times as long as wide). The Haleakalā subspecies also generally has more ray florets, 11–42 versus 5–20 for Mauna Kea.
Adaptation Argyroxiphium are thought to have evolved from a now-extinct species from which the current California tarweed (genus Raillardiopsis) evolved, diverging from a common mainland ancestor about 5 million or 6 million years ago. Molecular DNA research strongly supports this hypothesis. It is assumed that the colonizing individual reached Hawaiʻi on a bird's feathers, forcing "the need to seriously consider extreme long-distance dispersal of intercontinental magnitude" as the species had to overcome a dispersal barrier of at least 3,900 km (2,400 mi) of open ocean, a distance 60% greater than that separating Africa and South America. The Mauna Kea silversword evolved to survive extraordinarily harsh subalpine conditions where virtually no other plants could grow. It possesses no protective mechanism against grazing mammals, however, which did not exist in Hawaiʻi during the millions of years of the plants' adaptation. Sherwin Carlquist, who first established the ancestral role of California tarweeds in the evolutionary history of Hawaiian silverswords, speculated that their mainland precursor species may have had a resin unappetizing to herbivores. This is a familiar story to Hawaiian botanists; two frequently mentioned examples of a similar loss of defensive characteristics are the endemic Hawaiian raspberry (Rubus hawaiensis), which evolved to have no thorns, and endemic Hawaiian nettles that lost their sting. Without selective pressure to maintain those mechanisms, these species failed to conserve whatever protective traits their ancestors might have possessed. Protective advantage, however, favored the tarweeds and silverswords in their unique ability to store water as intracellular gel, an adaptation that would have made it possible for the species to live in arid environments.
Original habitat Historical records suggest the Mauna Kea silversword was once dominant on Mauna Kea, adapting particularly well to wind-swept exposed alpine desert at altitudes of 2,600–3,800 m (8,500–12,500 ft). In this region, humidity is virtually zero and precipitation is generally in the form of winter snow. Mean annual rainfall is less than 51–102 cm (20–40 in) per year, chiefly during the winter, with extreme drought during the summer. There is no shade from the hot tropical sun nor protection from nighttime temperatures which fall below freezing any time of year. The soil is thin, rocky, volcanic cinder. At the upper ranges of this habitat, no other plants grow. In this seemingly inhospitable environment, Argyroxiphium thrived. Earliest accounts of the range of the Mauna Kea silversword do not suggest rarity; dead stalks were gathered for firewood by explorers in 1825. More than one anecdotal estimate made in the 1920s about the earlier population was that plants numbered in the thousands; one report was that the plants covered the slope so that the sun glared off them. The first documentation of its decline dates to 1892, when it was said to have been once abundant but is "now nearly extinct except in the most rugged and inaccessible places."
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