Usually the life cycle of most plants and crops last for 6 months and they flower within weeks. But, there is a plant genus of plant in the sunflower family Asteraceae known as Argyroxiphium. Its members are known by the common names of silversword or greensword due to their long, narrow leaves and the silvery hairs on some species. It belongs to a larger radiation of over 50 species, including the physically different genera Dubautia and Wilkesia. This grouping is often referred to as the silversword alliance. Botanist P. H. Raven referred to this radiation as “the best example of adaptive radiation in plants”.
DESCRIPTION OF THE PLANT
These perennials are endemic to Hawaii, occurring only on the islands of Maui and Hawaii in an extremely localized distribution. They are primarily found above 1,500 m (4,900 ft) in elevation in alpine deserts or bogs, indicating an adaptation to low-nutrient soils. The Kaʻū or Mauna Loa silversword (A. kauense) is the most adaptable: it can be found in rocky lava flows, bogs, and open forest. The evolutionary root of Argyroxiphium stems from Californian Tarweeds by way of DNA evidence which shows they share almost 80% of their DNA evidence. The Tarweed ancestor is thought to have traveled to Hawaii almost 10 million years ago, at which point the Hawaiian flora began diversifying at incredible speed.
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 intervals 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.
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 Haleakala silversword really shows the size and stark habitat of this incredible species.
THE POSSIBILITY OF EXTINCTION
Maui’s silverswords (Argyroxiphium sandwicense subsp. macrocephalum) had been declining for centuries, victims of feral goats and tourists eager to uproot living souvenirs. Even before the silversword was declared a federally threatened species in 1992, conservationists had fenced the barren slopes of their habitat, rid the area of goats, and planted silversword seeds. The efforts seemed to be working until the 1990s, after which the Maui species declined by 60%. Plants that sit farther down the volcano have suffered the most, even though they live in wetter conditions.
In 2016, Paul Krushelnycky, an ecologist at the University of Hawaii in Honolulu, noted that this recent drop coincided with more frequent changes in the trade winds, east-to-west winds that flow up the volcano. Increasingly, the trade winds’ cool, moist air has been trapped midslope by warmer air, creating hotter, drier conditions for the uphill plants. Such inversions have always been common, Krushelnycky notes, but now they are even more frequent, likely because of climate change.
To understand why silverswords at lower elevations are the most vulnerable, Krushelnycky and his colleagues grew seeds taken from plants living at low, medium, and high elevations in greenhouses and small outdoor plots. They watered some of them regularly and some of them irregularly, to compare growth under different conditions. They also measured how many plants survived. The researchers thought that plants from the highest elevations—where conditions were driest—would do better in the artificial “drought” scenarios, thanks to their presumed adaptations to dry living.
Seeds planted at lower elevations and subjected initially to moist conditions were the least likely to survive later droughts, regardless of which plants the seeds came from, the team reports in Ecological Monographs. “This suggests that traits acquired in response to the wetter growing conditions, not genetic differences, made them less drought resistant,” Krushelnycky says. Many researchers worry that increasing fluctuations in climate conditions can have a detrimental effect on ecosystems, says Bruce Baldwin, a botanist at the University of California, Berkeley, who was not involved with the work. But that “can be difficult to demonstrate convincingly,” he says. The new findings, he adds, show that silverswords’ early adaptations to their environments “can later work against them.”
Rather than transplanting silverswords from high elevations—where conditions are harsher—to lower ones, Krushelnycky says it may be more productive to transplant them to places with more consistent moisture, like lower grade slopes. Toward that end, he and his colleagues are planting silverswords in a wide variety of habitats to see which are best.