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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) |
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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) |
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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. |
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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). |
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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) |
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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 |
