Natural intersectional hybridization between North American species of Populus (Salicaceae) in sections Aigeiros and Tacamahaca. II. Taxonomy

Abstract

Five of six possible hybrid combinations of the two native North American species of Populus sect. Aigeiros Duby with the three native species of sect. Tacamahaca Spach have been found in nature. Putative hybrids occur wherever species of the two sections are sympatric and sometimes even beyond the range of one or both parents. Parents of the sixth possible combination are not, however, sympatric and this explains the apparent absence of the corresponding hybrid. Although the five hybrids can easily be recognized as a group, each parentage yields hybrids of unique morphology, all of which have previously been provided with binomials. Variability within each hybrid reflects variation in its parents. These hybrids are morphologically intermediate between their putative parents and have an additive chromatographic profile combining leaf flavonoids of each pair. Trees of P. × generosa (P. deltoides × P. trichocarpa) and P. × jackii (P. deltoides × P. balsamifera) from natural populations closely resemble artificial hybrids between their presumed parents. Three natural hybrids, P. × acuminata (P. deltoides × P. anguslifolia), P. × jackii, and P. × parryi (P. fremontii × P. trichocarpa), are all cultivated as shade trees. Keys distinguish the five hybrids from each other and from their parents; each hybrid is described; its nomenclature is clarified; and its range is mapped in relation to those of its parents.