ABSTRACT
The genus Populus consists of about 29 species organized into six major sections that occur primarily in the Northern Hemisphere (Eckenwalder 1996). Species from the different sections of the genus have diverse ecological characteristics. Two of the most economically important sections (Aigeiros and Tacamahaca) contain species collectively known as cottonwoods. These occur mostly in riparian zones and are characterized by primarily ruderal life history, dominating early successional stages and thriving on flood-mediated disturbance (Braatne et al. 1996; Karrenberg et al. 2002). The other major section of the genus (section Populus, also known as Leuce), contains species commonly known as aspens, which are characterized by extensive clonal growth, and which can occur in very diverse sites, from mixed upland forests to boreal regions (Barnes 1969; Peterson and Peterson 1992; Romme et al. 2001). The cottonwood sections of the genus are highly interfertile and readily hybridize, and the aspen species are also highly interfertile, but the cottonwoods are reproductively isolated from the aspens. This isolation is reflected by the strong ecological, morphological, and genetic distinctions between these groups of species (Stettler et al. 1980). All Populus
species have 38 chromosomes in their diploid genomes (Blackburn and Heslop Harrison 1924), with exceptions apparently occurring regularly within species due to meiotic nondisjunction (Einspahr et al. 1963; Bradshaw and Stettler 1993).
