This species, known as Haleakalā silversword, is endemic to the crater and upper slopes of Haleakalā volcano, where this photo was taken.
The genera Argyroxiphium, Dubautia, and Wilkesia are known as the silversword alliance, and all 30 or so species are endemic to the islands of Hawai’i and Maui. They are believed to have evolved from a Californian tarweed that managed to reach Hawai’i millions of years ago (perhaps carried by a bird or borne by the wind). Together, the silverswords provide an excellent example of the principle of adaptive radiation, in which individuals that originate from a common ancestor evolve into different forms due to different selective pressures (e.g. Darwin’s finches). Argyroxiphium sandwicense subsp. macrocephalum grows in barren volcanic landscapes at high elevations, and so has evolved characteristics that help it survive large temperature fluctuations and drought. For example, its thick leaves store water, while its parabolic shape reflects sunlight inwards, warming the growing point at its centre. This form is remarkably different from that of Dubautia waialealae, which grows as a cushion-like, rounded clump that is very well-suited to the extremely wet, bog environment in which it grows. For more on the adaptive radiation of the silversword alliance, see this previous BPotD post about Argyroxiphium sandwicense subsp. sandwicense.
The silvery rosettes formed by Haleakalā silversword’s leaves are beautiful, but this species becomes truly impressive when it flowers. Each individual plant flowers only once in its lifetime, at the age of 10-50 years, and dies shortly after (this type of life cycle is termed monocarpic). During this stage, each rosette sends up a monstrous raceme, reaching a height of up to 3 meters and containing up to 600 maroon, nodding flower heads. I imagine it is an otherworldly experience to arrive face to face with one of these towering inflorescences after spending much of the day hiking up a volcano; this photo of a hiker and a Haleakalā silversword really shows the size and stark habitat of this incredible species.