The evolutionary diversification of columnar cacti in North America has resulted in a range of morphologies from large trees, shrubs, and epiphytes, to geophytes, and single-stemmed dwarf forms. This group of cacti include members of the tribes Pachycereeae, Leptocereeae, which are predominantly large to moderately-sized trees and shrubs, and the Hylocereeae which are primarily epiphytes. The South American genus Corryocactus (formerly placed in tribe Notocacteae) also has been allied to this clade. Comparative sequencing evidence from variation in plastid markers supports an origin for these North American cacti from within a sub-clade of the presently circumscribed tribe Browningieae of the central Andean region. The present study examined the primary patterns of phylogenetic divergence in this North American clade and reviews the systematic relationships between these tribes. Additionally, the phylogeny within each of the represented tribes was examined in the context of understanding the evolution of systematically useful morphological characters (vegetative and floral), and to evaluate present geographic distribution as related to phylogenetic divergence. Sequences of plastid non-coding regions of the psbA-trnH intergenic spacer and the rpl16 intron were used in concert with maximum parsimony techniques to develop a phylogenetic assessment of the various groups. It was discovered that the two subtribes of the Pachycereeae, Pachycereinae and Stenocereinae, were strongly supported, the latter containing the genus Echinocereus. The relationship between the tribes Leptocereeae and Hylocereeae was found to be closer than previously hypothesized. Bat pollination is prevalent throughout these clades, however multiple shifts to entomophily were also suggested.

Key words: Cactaceae, evolution, Hylocereeae, Leptocereeae, Pachycereeae, phylogeny