Natural intersectional hybridization between North American species of Populus (Salicaceae) in sections Aigeiros and Tacamahaca. I. Population studies of P. × parryi

Abstract

Populus fremontii and P. trichocarpa are western trees of sections Aigeiros Duby (cottonwoods) and Tacamahaca Spach (balsam poplars), respectively. They have distinctive ranges and ecological tolerances but are broadly sympatric in California and Nevada. Here, the two species often display a narrow zone of overlap between otherwise separate elevational ranges. They may hybridize in such areas to produce the tree that Sargent first named P. × parryi. I have found hybrids in most investigated populations in which both parent species are present. Putative hybrids from four populations are morphologically intermediate between their parents and possess an additive chromatographic profile combining the flavones of cottonwoods with the flavonols of balsam poplars. A pictorialized scatter diagram demonstrates this and morphological and chemical hybrid indices quantify the relationship. Hybrids from two populations, each lacking one of the parent species, resemble hybrids from the other two populations, in which all three taxa are present. Morphological and chemical evidence for advanced-generation hybrids is equivocal. The putative hybrids are less fertile and are more susceptible to at least some pests than are the parent species. These factors may now limit hybridization to the first generation but do not preclude long-term intersectional introgression. Since balsam poplars and cottonwoods have been in contact and have hybridized in this region for at least 12 million years, hybridization may have had significant effects on either or both species as they evolved from their Tertiary ancestors.