Adaptive Radiation of the Hawaiian Silversword Alliance

Ecological and Physiological Adaptations

Last Modification: March 22, 1997

Very dry site of the facultatively drought-deciduous Dubautia linearis ssp. hillebrandii. This and adjacent sites occupied by this species receive an average of as little as 25 cm of precipitation per year. (photo 1980, Nahona o Hae, Hawai'i)
In contrast with the preceding species, Dubautia waialealae occurs in a site that is sometimes regarded as the wettest spot on earth, receiving average annual precipitation well exceeding 1000 cm. (photo 1983, Wai'ale'ale summit, Kaua'i)
Collectively, the species of the silversword alliance cover essentially the complete range of dryest to wettest terrestrial habitats in Hawaii. Even some individual species have adapted to a very broad range of moisture conditions.
Members of the silversword alliance also occupy habitats along a continuous gradient in elevation from near sea level (75 m) to the upper reaches of vegetation (3750 m).
Occurrence at extremely high elevations means species not only have to endure drought conditions but also freezing temperatures. Even in Hawaii, the plants occurring at the upper extremes in elevation must be able to withstand freezing temperatures that could come at any night of the year. In this case, near the summit of Haleakala (c. 3,000 m), Dubautia menziesii is blanketed by snow which could persist for several days. (photo 1968, Haleakala, E. Maui)
This curve shows the relationship between tissue turgor pressure and tissue relative water content for Dubautia menziesii (n=13) and D. knudsenii (n=14). In general, species of Dubautia with 13 pairs of chromosomes are able to survive in dryer sites compared to those occupied by species with 14 pairs of chromosomes, apparently, in part, due to differences in tissue elastic properties illustrated here. Additional Reading

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