Argyroxiphium grayanum
Encyclopedia
Argyroxiphium grayanum, commonly known as the greensword, is a species of flowering plant
in the sunflower
family, Asteraceae
, and a member of the silversword alliance
, a group of over 50 species which are diverse in morphology and habitat but are genetically closely related.
The silversword alliance provides a convincing natural case study in evolution by adaptive radiation
, with the greensword representing one extreme of the genus' plasticity. Some Argyroxiphium, including the well-known Haleakala
and Mauna Kea silversword
s, live in harsh alpine desert
-like conditions of heat, sun, wind, and aridity, and are drought-adapted plants capable of storing water as a gel in leaf structures which are normally air pockets in other plants. However, A. grayanum is a bog plant adapted to very different conditions — excessive moisture, lack of regular sunlight, and cool temperatures, and its leaves are non-succulent like those of the related genus Dubautia
.
plant endemic to the island of Maui
in Hawaii
. Its growth form is typically a low shrub up to 2 m high, erect, with an erect single-stemmed monocarpic rosette shape, though in the interior of bogs it typically grows as a dwarf shrub under 30 cm high. It has green, 5-11-nerved, narrowly elliptic-ligulate leaves which are broadest above the middle. It occurs only in and around montane cloud forest bogs at elevations ranging from about 1,200 to 2,050 m. The sites receive from about 300 to over 1,000 cm precipitation per year. It is most abundant along the upper rim of Kīpahulu Valley on East Maui and near the summit of Puu Kukui
on West Maui. The latter region is also home to a related species, the Eke silversword (A. caliginis
). Despite their close relationship and shared habitat, the two species differ in several ways beyond the coloring of their lance-shaped leaves, with silversword possessing a distinctive sheen.
Most Argyroxiphium species generally produce one inflorescence, after which the plant dies. Neither A. grayanum nor the sympatric
Eke silversword (A. caliginis) follow this pattern in a strict sense. Both species flower infrequently compared to the mass flowerings of the Haleakala silversword
, and produce multiple branches such that only some rosettes of a given plant die back in any given year. A. caliginis additionally reproduces by way of runners or prostrate stems which root and spread.
, was formerly found on East Maui only, but is now apparently extinct.
) found in western North America. That several spontaneous hybrids have been observed supports this hypothesis. If true, the adaptive radiation of the tarweed/silversword/greensword into extremely diverse morphologies and habitats is an extraordinary case history of evolution.
Flowering plant
The flowering plants , also known as Angiospermae or Magnoliophyta, are the most diverse group of land plants. Angiosperms are seed-producing plants like the gymnosperms and can be distinguished from the gymnosperms by a series of synapomorphies...
in the sunflower
Sunflower
Sunflower is an annual plant native to the Americas. It possesses a large inflorescence . The sunflower got its name from its huge, fiery blooms, whose shape and image is often used to depict the sun. The sunflower has a rough, hairy stem, broad, coarsely toothed, rough leaves and circular heads...
family, Asteraceae
Asteraceae
The Asteraceae or Compositae , is an exceedingly large and widespread family of vascular plants. The group has more than 22,750 currently accepted species, spread across 1620 genera and 12 subfamilies...
, and a member of the silversword alliance
Silversword alliance
Silversword alliance refers to an adaptive radiation of over 50 species in the composite or sunflower family, Asteraceae. The group is endemic to Hawaii, and is derived from a single immigrant to the islands...
, a group of over 50 species which are diverse in morphology and habitat but are genetically closely related.
The silversword alliance provides a convincing natural case study in evolution by adaptive radiation
Adaptive radiation
In evolutionary biology, adaptive radiation is the evolution of ecological and phenotypic diversity within a rapidly multiplying lineage. Starting with a recent single ancestor, this process results in the speciation and phenotypic adaptation of an array of species exhibiting different...
, with the greensword representing one extreme of the genus' plasticity. Some Argyroxiphium, including the well-known Haleakala
Haleakala Silversword
The Haleakalā silversword is a rare plant, part of the family Asteraceae. The silversword in general is referred to as āhinahina in Hawaiian...
and Mauna Kea silversword
Mauna Kea silversword
The Mauna Kea silversword is a highly endangered flowering plant endemic to the Big Island of Hawaii. It is the "crown jewel" of the volcanic mountain, Mauna Kea, from which it derives its common name, and where it was once common. Extraordinary plant conservation efforts are being made to...
s, live in harsh alpine desert
Hawaiian tropical high shrublands
The Hawaiian tropical high shrublands are a tropical savanna ecoregion in the Hawaiian Islands. They cover an area of on the upper slopes of the volcanoes Mauna Kea, Mauna Loa, Hualālai, and Haleakalā. It includes open shrublands, grasslands, and deserts...
-like conditions of heat, sun, wind, and aridity, and are drought-adapted plants capable of storing water as a gel in leaf structures which are normally air pockets in other plants. However, A. grayanum is a bog plant adapted to very different conditions — excessive moisture, lack of regular sunlight, and cool temperatures, and its leaves are non-succulent like those of the related genus Dubautia
Dubautia
Dubautia is a genus of flowering plant in the sunflower family, Asteraceae, that is endemic to Hawaii. It contains more species than the other two genera in the silversword alliance, including cushion plants, shrubs, trees, and lianas.-Selected species:...
.
Description
A. grayanum is a perennialPerennial plant
A perennial plant or simply perennial is a plant that lives for more than two years. The term is often used to differentiate a plant from shorter lived annuals and biennials. The term is sometimes misused by commercial gardeners or horticulturalists to describe only herbaceous perennials...
plant endemic to the island of Maui
Maui
The island of Maui is the second-largest of the Hawaiian Islands at and is the 17th largest island in the United States. Maui is part of the state of Hawaii and is the largest of Maui County's four islands, bigger than Lānai, Kahoolawe, and Molokai. In 2010, Maui had a population of 144,444,...
in Hawaii
Hawaii
Hawaii is the newest of the 50 U.S. states , and is the only U.S. state made up entirely of islands. It is the northernmost island group in Polynesia, occupying most of an archipelago in the central Pacific Ocean, southwest of the continental United States, southeast of Japan, and northeast of...
. Its growth form is typically a low shrub up to 2 m high, erect, with an erect single-stemmed monocarpic rosette shape, though in the interior of bogs it typically grows as a dwarf shrub under 30 cm high. It has green, 5-11-nerved, narrowly elliptic-ligulate leaves which are broadest above the middle. It occurs only in and around montane cloud forest bogs at elevations ranging from about 1,200 to 2,050 m. The sites receive from about 300 to over 1,000 cm precipitation per year. It is most abundant along the upper rim of Kīpahulu Valley on East Maui and near the summit of Puu Kukui
Puu Kukui
Puu Kukui is a mountain peak in Hawaii. It is the highest peak of Mauna Kahalawai . The summit rises above the Puu Kukui Watershed Management Area, an private nature preserve maintained by the Maui Land & Pineapple Company...
on West Maui. The latter region is also home to a related species, the Eke silversword (A. caliginis
Argyroxiphium caliginis
The Eke Silversword is a species of flowering plant in the sunflower family, Asteraceae. It is found only in two bogs in West Maui, Hawaii. The species is threatened by damage to the bogs by rooting feral pigs, but the main population at Eke Crater is now protected by fencing....
). Despite their close relationship and shared habitat, the two species differ in several ways beyond the coloring of their lance-shaped leaves, with silversword possessing a distinctive sheen.
Most Argyroxiphium species generally produce one inflorescence, after which the plant dies. Neither A. grayanum nor the sympatric
Sympatry
In biology, two species or populations are considered sympatric when they exist in the same geographic area and thus regularly encounter one another. An initially-interbreeding population that splits into two or more distinct species sharing a common range exemplifies sympatric speciation...
Eke silversword (A. caliginis) follow this pattern in a strict sense. Both species flower infrequently compared to the mass flowerings of the Haleakala silversword
Haleakala Silversword
The Haleakalā silversword is a rare plant, part of the family Asteraceae. The silversword in general is referred to as āhinahina in Hawaiian...
, and produce multiple branches such that only some rosettes of a given plant die back in any given year. A. caliginis additionally reproduces by way of runners or prostrate stems which root and spread.
Other greensword species
Another greensword species A. virescensArgyroxiphium virescens
Argyroxiphium virescens was a species of flowering plant in the Asteraceae family.It was found only in the Hawaiian Islands.Its natural habitats were subtropical or tropical moist montanes and subtropical or tropical high-altitude shrubland....
, was formerly found on East Maui only, but is now apparently extinct.
Adaptive radiation and the silversword alliance
Based on biosystematics and molecular studies, all of the silversword alliance species are thought to have evolved from a single ancestor, related to the tarweed (Carlquistia muiriiCarlquistia
Carlquistia is a monotypic genus of flowering plants in the aster family containing the single species Carlquistia muirii. Formerly named Raillardiopsis muirii, the plant was reexamined in the 1990s and moved to a new genus of its own, separate from similar and closely related genera, such as Madia...
) found in western North America. That several spontaneous hybrids have been observed supports this hypothesis. If true, the adaptive radiation of the tarweed/silversword/greensword into extremely diverse morphologies and habitats is an extraordinary case history of evolution.