Extent and patterns of hybridization among the three species ofPopulusthat constitute the riparian forest of southern Alberta, Canada

Abstract

Three species of Populus (Salicaceae) overlap and hybridize in southern Alberta, Canada. Variation in leaf morphology throughout the region identifies five genetically distinct zones. The drainage of the Oldman River contains a pure zone of balsam poplar (Populus balsamifera L.), a pure zone of plains cottonwood (Populus deltoides Marsh.), plus overlap and hybrid zones of balsam poplar – narrowleaf cottonwood (Populus angustifolia James) and balsam poplar – narrowleaf cottonwood – plains cottonwood. The drainage of the Red Deer River contains a pure zone of balsam poplar, a pure zone of plains cottonwood, plus an overlap and hybrid zone of balsam poplar – plains cottonwood. Zones on both drainages coincide with an elevational gradient. Overlap and hybrid zones extend at least 700 river km (the length obtained by measuring distances between sites of hybridization on a river and its tributaries) combined across the two drainages. Principal components analyses on leaf morphology identify a pattern of bidirectional intro gression between sect. Tacamahaca (balsam poplar, narrowleaf cottonwood) species. These analyses also identify a pattern of unidirectional introgression between Tacamahaca and sect. Aigeiros (plains cottonwood) species, with F1hybrids backcrossing only with the Tacamahaca parent. This pattern of unidirectional introgression subsequently was assessed and supported with data from the drainages of Ashley Creek and the Ogden River, Utah, which contain hybrid and overlap zones between narrowleaf cottonwood (Tacamahaca) and Fremont cottonwood, Populus fremontii S. Watson (Aigeiros). Given its unique nature in terms of extent, health, and composition, and given the effect of plant hybrid zones on the ecology and evolution of their associated organisms, it is hoped that the current study will facilitate future research targeting conservation of this riparian forest in its full genetic complexity.Key words: Populus, hybridization, riparian, cottonwood, introgression.