A morphometric analysis and taxonomic appraisal of the Hawaiian Silversword Argyroxiphium sandwicense Dc. (Asteraceae)

Abstract

A difference of opinions presumably related to a paucity of critical field observations has led to a persistent uncertainty regarding the taxonomic status of the Haleakala and Mauna Kea silverswords, Argyroxiphium sandwicense s.l. In order to assess the differentiation of these silverswords, morphometric techniques were used to examine the pattern of variation of 45 characters between the two populations. Also, qualitative features were evaluated. A framework for a priori comparisons between the two populations of A. sandwicense was provided by including two additional species in the study, i.e. A. kauense and A. virescens var. paludosa. The F tests of one way analysis of variance indicate that the means of each of 18 characters differ significantly (P -< 0.05) between the two populations of A. sandwicense. Based on the presence of quantitative differentiation and geographical isolation and the near absence of qualitative differentiation between the two populations, it is proposed to recognize them as two different subspecies: A. sandwicense ssp. macrocephalum and A. sandwicense ssp. sandwicense. Argyroxiphium virescens var. paludosa and A. kauense are as distinct from each other as each is from A. sandwicense in many quantitative and qualitative characters. The correlation analyses of different pairs of characters show that the more useful characters are: inflorescence proportions, leaf proportions, number of ray florets per capitulum, and capitulum diameter. Individuals of either subspecies can be recognized by the proportion of the inflorescence. However, to separate all four taxa, based on quantitative characters, at least three of the foregoing features are needed. An affinity index based on the number of times a given combination of populations exhibited adjacent mean values for the vegetative and inflorescence characters analyzed suggests a linear pattern of relationships as follows: A. virescens var. paludosa - A. sandwicense ssp. macrocephalum - A. sandwicense ssp. sandwicense - A. kauense. However, the capitulum characters do not support this pattern of relationships. They indicate a closer affinity between A. virescens var. paludosa and A. kauense. This situation suggests the possibility of convergent evolution of reproductive features adapted to similar reproductive modes in similar extreme environments. Furthermore, the intermediate morphological and ecological position of A. kauense raises the possibility that hybridization between A. sandwicense ssp. sandwicense and a greensword was involved in the origin of A. kauense.