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The ecological, morphological, and physiological diversity of species in the Hawaiian silversword alliance is exceptional. The 28 species, which belong to the endemic genera Argyroxiphium, Dubautia, and Wilkesia, have a wide variety of geographical distributions and elevational ranges within the archipelago. They grow in habitats as varied as dry scrub and woodland, wet scrub and forest, cinder and lava, and bog. Ecological diversity is also evident among sympatric species. At a site of sympatry on the island of Hawaii, for example, D. ciliolata and D. scabra are restricted to different lava flows, even though individuals of the two species may grow within a meter of one another. The 28 species have growth forms as varied as rosette shrubs, shrubs, trees, and lianas. They have a wide range of leaf sizes and shapes, with the Dubautia species exhibiting significant variation in leaf turgor maintenance capacities. Morphological and physiological diversity is also evident among sympatric species. At a site of sympatry on the island of Maui, for example, A. sandwicense and D. menziesii exhibit different suites of morphological and physiological traits enabling them to cope with the severe environmental conditions. The patterns of diversity and the genomic relationships among the 28 species suggest that a variety of factors may have played important roles in their adaptive radiation. The Hawaiian silversword alliance is a premier example of adaptive radiation in plants (Carlquist, 1980; Carr et al., 1989). The alliance includes 28 species in three endemic genera: Argyroxiphium, Dubautia, and Wlilkesia (Carr, 1985). The species grow in a wide range of habitats and have a wide variety of growth forms. They are also closely related, as evidenced by the high frequency of spontaneous interspecific and intergeneric hybrids in nature, coupled with the ease of production of artificial hybrids in the laboratory (Carr & Kyhos, 1981, 1986). The detailed analysis of the hybrids and parental taxa using cytogenetic, electrophoretic, and molecular approaches has provided compelling evidence that the silversword alliance is a genetically cohesive group whose origin and diversification probably trace to a single colonizing ancestor (Baldwin et al., 1988; Carr et al., 1989). Our objective in this review is to provide insight into the ecological, morphological, and physiological diversity of species in the silversword alliance. With respect to ecological diversity, we compare the geographical distributions, habitats, and elevational ranges of the 28 species, then analyze the local distributions of several sympatric Dubautia species. With respect to morphological and physiological diversity, we compare the growth forms, leaf sizes, and leaf shapes of the 28 species, then examine the turgor maintenance capacities of the Dubautia species and the water and temperature balances of two sympatric Argyroxiphium and Dubautia species. Our primary theme is that the ' This paper is dedicated to Dr. Sherwin Carlquist in honor of his pioneering research on the Hawaiian silversword alliance. The research was supported by NSF Grant DEB-82064 11 and a gift from the Atlantic Richfield Foundation to the senior author. We thank Lani Stemmermann for invaluable companionship, botanical insight, and technical assistance in the field. We also thank the staff of Haleakala National Park, particularly R. Nagata and L. Loope, for critical logistical support, and J. Canfield and N. Friedman for generous photographic assistance. 2 Department of Ecology & Evolutionary Biology, University of Arizona, Tucson, Arizona 85721, U.S.A. < Department of Botany, University of Hawaii, Honolulu, Hawaii 96822, U.S.A. 4 Department of Plant & Soil Sciences, University of Maine, Orono, Maine 04469, U.S.A. Department of Botany, University of California, Davis, California 95616, U.S.A. ANN. MISSOURI BOT. GARD. 77: 64-72. 1990. This content downloaded from 152.1.161.146 on Thu, 27 Mar 2014 19:39:59 PM All use subject to JSTOR Terms and Conditions Volume 77, Number 1 Robichaux et al. 65 1990 Adaptive Radiation of Hawaiian Silversword Alliance

Adaptive Radiation of the Hawaiian Silversword Alliance (Compositae- Madiinae): Ecological, Morphological, and Physiological Diversity