The Silver Sword Alliance A Group Of Hawaiian Plants That Evolved From An Ancestral California Tarweed

One of the most fascinating members of the sunflower family (Asteraceae) is the infamous silver sword (Argyroxiphium sandwicense ssp. macrocephalum) that grows in the cinders of Haleakala Crater on the island of Maui. The rosettes of sword-shaped leaves are covered with silvery hairs that reflect light and heat and provide insulation against the intense solar radiation and extreme aridity of this 10,000 foot (3,000 m) volcanic mountain. In addition, the leaves contain air spaces filled with a gelatinous substance that absorbs and stores large quantities of water during the intervels between rains. This stored water is especially important when the plant blooms, because the fast-growing flower stalk requires a lot of moisture as it develops into a massive inflorescence. Another subspecies of silver sword (A. sandwicense ssp. sandwicense) grows on the upper slopes of Mauna Kea on the island of Hawaii.

In general growth form the silver sword superficially resembles the chaparral yucca (Yucca whipplei) of southern California, although they belong to entirely different plant families (the Asteraceae and Agavaceae). Both species are monocarpic and flower after about six to twelve years. Like the chaparral yucca, a single flower stalk grows out of the rosette of basal leaves, and then the plant dies. Only in the silver sword, the individual flowers resemble purplish daisies, unlike the lily-like flowers of chaparral yucca. Also unlike the chaparral yucca, the silver sword does not require a symbiotic moth for pollination. A silver sword in full bloom is truly one of the botanical wonders of the world.

Several additional species that are closely-related to the silver sword also grow on Haleakala, including the green sword (Argyroxiphium virescens) of fog-shrouded, boggy meadows and a low-growing, shrubby species (Dubautia menziesii) of windswept, alpine slopes.

On the rim of scenic Waimea Canyon on the island of Kauai is Wilkesia gymnoxiphium, another striking silver sword relative with a tuft of grasslike leaves at the top of a slender, hollow stem. In full bloom this plant produces a tall flower stalk with hundreds of sticky, daisylike flowers.

All these Hawaiian species, plus about two dozen additional ones, form the "Silver Sword Alliance," a truly remarkable example of adaptive radiation. According to S. Carlquist, B.G. Baldwin and G.D. Carr (2003), three California perennial tarweeds of the Madiinae are closely related to the Silver Sword Alliance, including Carlquistia muirii (Raillardella muirii), Anisocarpus scabridus (Raillardella scabrida) and Kyhosia bolanderi (Anisocarpus bolanderi). These taxonomic affinities are based on chromosome comparisons, hybridization studies and comparative chloroplast DNA. The Silver Sword Alliance apparently evolved from an ancestral, self-pollinating, California tarweed that colonized these islands millions of years ago. Presumably the ancestral tarweed seeds reached the Hawaiian islands by drifting or rafting in the ocean currents. Extensive research on the silver sword and its relatives by Dr. Gerald Carr at the University of Hawaii (and others) indicates a close genetic and biochemical affinity with the tarweed subtribe (Madiinae) of the sunflower tribe (Heliantheae) of the sunflower family (Asteraceae). It is quite remarkable how an ancestral tarweed got to the Hawaiian islands and gave rise to such a striking group of plants known as the Silver Sword Alliance.

There are several indigenous genera of tarweeds which are able to compete with aggressive naturalized weeds in disturbed areas of California. For example, the genus Hemizonia is widespread in coastal foothills and valleys of California, including many annuals and a few perennial species. These California tarweeds are generally small plants with sticky, aromatic foliage. They are adapted to a Mediterranean climate with dry, hot summers. One common annual species (H. fasciculata) is fairly common in San Diego County in heavy, clay soils. It blooms during the summer months when the ground is so hard and dry that it is difficult to penetrate with a shovel. In fact, it actually grows like an aggressive weed in disturbed areas near Palomar College, and even colonizes disturbed ground which has been bulldozed.

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