Morphological Variation in Hawaiian Wikstroemia (Thymelaeaceae)

Abstract

An analysis was made of morphological variation in the taxonomically complex Ha- waiian genus Wikstroemia. Previous authors had recognized 12 or 26 species. Multivariate analyses indicate that variation is continuous and that natural groups are not distinguishable on the basis of macromorphology. Two micromorphological characters, pollen exine sculpturing and seed coat morphology, were examined for their potential taxonomic utility; however, neither of these char- acters contributed any useful taxonomic information at the species level. The polyforate, crotonoid pollen grains from populations on the six major islands of Hawaii and from five extra-Hawaiian species are morphologically uniform. In contrast, seed coats showed intra- and interpopulation variability, but the variation patterns are not consistent at the population level or above. The genus Wikstroemia Endl. consists of 55- 70 species of shrubs and small trees distributed throughout the Pacific Basin, southeast Asia, and Australia (Peterson 1990). The continuous na- ture of morphological variation in Hawaiian Wikstroemia has led to much taxonomic confu- sion in attempts to distinguish species using morphological criteria. Previous authors have recognized 12 (Peterson 1990) or 26 species (Skottsberg 1972), all endemic. However, nei- ther of these treatments utilized distinctive, non- overlapping suites of characters. The aim of this study was to test the morphological separate- ness of the species recognized by Peterson us- ing multivariate statistical techniques and phe- netic methods and, if those species lacked distinctness, to determine whether distinct morphological taxa could be recognized in Ha- waiian Wikstroemia. Two micromorphological characters, pollen exine sculpturing and seed coat morphology, were examined for their po- tential taxonomic usefulness. In Hawaii, Wikstroemia has a very broad eco- logical range, occurring on rocky seaside cliffs and in dry forest, rain forest, and alpine habitats (defined by Carlquist 1980, as above 2000 m). The plants range in habit from decumbent or erect shrubs to trees. Leaf shape varies from the