Silversword, Ka'u

Argyroxiphium kauense

division: Magnoliophyta

class: Magnoliopsida

order: Asterales

family: Asteraceae

status: Endangered, ESA

range: USA (Hawaii)

Description and biology

Ka'u (pronounced KAH-oo) silversword (also called Mauna Loa silversword) is a monocarpic plant (one that bears fruit and flower only once and then dies) of the aster family. It grows to a height ranging from 1 inch to 2 feet (3 to 70 centimeters) and has long flowering stems that grow from 2 to 8 feet (0.7 to 2.5 meters) long. The Ka'u silversword is a giant rosette (clustered) shrub with leaves covered with gray or silver hairs. The leaves are long and narrow, measuring about 8 to 16 inches in length and only about .2 inch (0.5 centimeters) in width. When the Ku'a silversword grows, its rosette widens in diameter and its stem gets taller. This growth occurs for several years before the plant produces a flowering stalk, called an inflorescence. The inflorescence has many branches, and each branch has a flowering head made up of 3 to 11 ray flowers that are about one-half inch (1 centimeter) long. Each of the ray flowers contains 50 to 200 florets that are only about 0.2 inch (0.6 cm) long. The flowers, which

range in color from white or yellow to deep red, bloom in August and September. The fruits the plant produces are dry and black. Once the fruit sets, the plant dies. Scientists believe that flying insects, probably native bees, pollinate the species.

Habitat and current distribution

The Ka'u silversword grows in moist open forest areas or bogs, generally at altitudes between 3,330 and 9,100 feet (1,625 and 2,774 meters). It also occurs in some dry areas in which smooth lava is covered by a shallow layer of soil. There are only three known populations of this species and they are on Mauna Loa, a large volcanic mountain on the island of Hawaii. They are located on private land on Kahuku Ranch and on state-owned land in the Upper Waiakea and Kapapala Forest Reserves. The population of the species in the wild has been estimated to be under 500 individual plants. There are an additional 1,000 individual plants that were planted at Kapapala and another 1,000 planted in Hawaii Volcanoes National Park.

History and conservation measures

At one time the Ka'u silversword probably existed across the island of Hawaii, in a band stretching from the southwest of the island to the northeast slope of Mauna Lao. It also probably existed on Hualalai. It is interesting to note that the plant is endemic (native to and occurring only in) to a volcanic mountain and has survived its lava flows with a healthy population for many years.

Today, because of its low population, the Ka'u silversword is extremely vulnerable to extinction. Browsing (feeding) animals are a particular threat. Cows and feral animals (once-tame animals that have escaped into the wild) such as pigs, goats, and mouflon sheep have greatly reduced its numbers and prevent it from recovery. The animals tend to eat older, more mature plants. Since the plant flowers only one time in its life, their browsing significantly reduces reproduction. With so few remaining plants, the gene pool (the number of units that pass on inherited traits) is quite low, which may weaken the species. Since there are only three surviving populations—and those three are located on a volcano—there is strong risk that lava flow or fire could entirely wipe out the species. Pests, drought, disease, and the introduction of alien species also threaten the Ka'u silversword.

Protective fencing has been built around existing populations of the species since the 1970s to keep feral animals from browsing and trampling the plants. Controlling the ungulates (hooved animals) around the Ka'u silversword habitat is crucial to its survival. Research into the effects of nonnative plants and habitat requirements is ongoing.