Dubautia - A Study in Diversity
In earlier articles in this series I talked about two members of the silversword alliance, Argyroxiphium and Wilkesia. The third, and largest, member of the alliance is Dubautia which we meet now. Dubautia represents one of the most interesting examples of diversity in the Hawaiian flora. At least 23 species are known with growth forms that include low shrubs, exemplified by D. scabra subsp. scabra (Photo 1); a variety of intermediate sized shrubs, such as D. linearis subsp. hillebrandii (Photo 2) and D. ciliolatasubsp. ciliolata (Photo 3); a small tree, D. knudsenii (Photos 4 and 5); and a liana, D. latifolia (Photo 6).
The considerable variation evident in five of the species is further accommodated by recognizing 14 subspecies within that set. Eleven of the 23 species of Dubautia occur only on Kaua‘i; several of these are very rare.
Two Hawaiian words most frequently associated with Dubautia are küpaoa, which means a strong and penetrating aroma (characteristic of the flowers of the genus); and na‘ena‘e, which also refers to a pleasant aroma, but is also used to refer to the genus as a whole. Two species are named based on their flower color: D. laxa is known in Hawaiian as na‘ena‘e pua melemele, literally, Dubautia with yellow (melemele) flower (pua); while D. paleata is called na‘ena‘e pua kea, referring to its white (kea) flowers. Dubautia raillardioides is known as na‘ena‘e ‘ula likely in reference to the reddish leaf bases as seen in Photo (7). ‘Ula is the Hawaiian word for red (among other things, including, not surprisingly, blood).
In addition to variation in habit (growth forms), species of Dubautia also exhibit a range of variation in leaf structure. Variants include plants with short and clustered leaves, as in D. scabra (Photo 1); plants with leaves in two ranks held sharply at right angles to the stem, as seen in D. arborea (Photo 8); and plants with long, grass-like leaves as in the case of D. raillardioides (Photo 7). It might be mentioned in passing that the liana habit (D. latifolia) is one of the rarest growth forms in the family (Asteraceae) and, thus, represents one of the more unexpected phenomena seen in Hawaiian botany.
Habitat specialization is another characteristic feature of Dubautia with some species being among the earliest colonists on new lava and cinder fields (D. scabra and D. ciliolata). Others can be found growing in grasslands that experience moderate rainfall (D. linearis subsp. hillebrandii); while still others occur in areas of high rainfall such as D. raillardioides and D. waialealae (not shown). The latter species occurs on Mt. Waialeale (Kaua‘i) famed for receiving the highest amount of rainfall on Earth. Dubautia raillardioides can be found growing along the trail leading to the Alakai Swamp on Kaua‘i, although the hike may be muddy and rainy.
The easiest way to observe a habitat specialist in this genus is to visit the cinder fields in the Kilauea Iki area of Volcano National Park on the Big Island. One of the attractions of the park is The Devastation Trail which leads from the parking lot to a viewing platform on the crater's rim and passes through a cinder field created in the eruption of Kilauea Iki in 1959. One of the pioneering species common in this area is D. scabra subsp. scabra whose flower heads we saw above in Photo (1). Photo (9) shows a single plant growing in cinders.
A broader view of the same area showing dead ‘öhi‘a trees (Metrosideros polymorpha) in the foreground and a D. scabra subsp. scabra plant in the center of the frame is shown in Photo (10). Photograph (10) was taken about 30 years ago when very few plants-of any species-were growing on the cinder field.
Visitors today would see scattered plants-mostly D. scabra and a Vaccinium (blueberry) species - reaching nearly to the top of the distant cinder pile. The tenacious nature of this species can be seen in Photo (11) where a tiny depression in a lava boulder is home to a new seedling (note 25 cent coin for scale).
It is generally felt that natural selection favored features that provided plants with the capacity to take advantage of the varied habitats available on new volcanic islands, with much less emphasis on the development of barriers to gene flow between species. The result over the relatively short lifetime of the silversword alliance is that, while morphology and ecology have diverged, selection for breeding barriers has occurred to a much lesser extent. In fact, crossing experiments performed under controlled greenhouse conditions have demonstrated that all possible combinations of crosses among the three genera yielded viable offspring.
Photo (12) shows the offspring of an artificial cross between a species of Argyroxiphium and a Dubautia. Notice that the tall growth form resembles the former and the leaf morphology the latter. While there are comparatively few opportunities for crossing in Nature, such crosses do occur where different species (of any of the three genera) occur in the same area. For example, hybrid individuals involving crosses between Argyroxiphium sandwicense subsp. macrocephalum and Dubautia menziesii are commonly encountered in Haleakala crater as seen in Photo (13).
It was thought for a time that D. menziesii occurred on both Mauna Kea (Big Island) and on Haleakala (East Maui). A reexamination of specimens from Mauna Kea, however, revealed that the plants were first generation hybrids (F1 hybrids) between D. arborea and D. ciliolata subsp. glutinosa. Photo (14) shows one of the hybrid individuals growing in the silversword exclosure on the slopes of Mauna Kea. Dubautia arborea is known only from Mauna Kea where it was once much more common, feral animals having destroyed much of its natural habitat. The other putative parent, D. ciliolata subsp. glutinosa, is endemic to Mauna Kea as well.
Copyright 2007 Bruce A. Bohm - used by permission