A Multivariate Study of Morphological Characters for Echinocactus horizonthalonius and E. texensis (Cactaceae) and Description of a New Subspecies, E. horizonthalonius subsp. australis

Abstract

A study was conducted to ascertain the taxonomic validity of the endangered taxon Echinocactus horizonthalonius var. nicholii and whether there may be other groups of populations worthy of subspecies status within the range of the species. To test the hypothesis that individuals of E. horizonthalonius var. nicholii are morphologically distinct from those of the typical variety, a multivariate analysis was done to compare the degree of morphological variation or phenotypic plasticity of stem characters within populations to the variation among populations of E. horizonthalonius throughout its known range. Populations of E. texensis were sampled for outgroup comparison. Discriminant function analysis (DFA) assigning individuals by population showed loose groupings of geographically correlated populations. The DFA assigning individuals to regions or potential subspecific taxa indicated high percentages of correct classification for individuals within populations grouped into the Chihuahuan Desert, Sonoran Desert, and Central Mexican Plateau regions. Taxonomically, these groups correspond to E. horizonthalonius subsp. horizonthalonius (Chihuahuan Desert), E. horizonthalonius var. nicholii (Sonoran Desert), and unnamed taxon of E. horizonthalonius (Central Mexican Plateau). Because these morphological entities are correlated with regional distributions, they are placed here under subspecies, including a newly described taxon, E. horizonthalonius subsp. australis. Because no type for E. horizonthalonius could be located, a neotype is designated.