Interspecific Hybridization in Populus L. and Its Implications for the Ecology and Management of Riparian Ecosystems in the Southwestern USA

Interspecific hybridization in forest trees is common and can have important implications for ecology, evolution, and the conservation of forest habitats. Hybridization often results in greater genetic diversity and opportunities for backcrossing with one or both parents, which may introduce novel genotypes that influence biodiversity and ecosystem processes. However, the extent of hybridization, direction of backcrossing, and overall survival and performance of hybrids is often poorly understood, leading to inaccurate assessments of the role hybrids may play in forest ecology and conservation. Here, we investigate interspecific hybridization and the extent and direction of backcrossing between two species, Populus fremontii (S. Watson) and P. angustifolia (E. James ex Torr.), which are broadly distributed along riparian corridors in the riparian ecosystems of the southwestern United States. Using molecular assays of six putative hybrid zones and a common garden trial we test the following: (1) whether putative hybrids show evidence of genetic intermediacy relative to the parent species; (2) if confirmed hybrids exhibit higher genetic diversity than either parent species; (3) the extent and direction of backcrossing (uni- or bi-directional) within each site; and (4) whether hybrid derivatives show evidence of higher survival and performance in an experimental common garden consisting of both parents and hybrids that were propagated from the six sites. Our results confirm genetic intermediacy in all six sites, but with varying degrees of backcrossing, genetic diversity, and structure. All six locations reveal extensive bidirectional backcrossing to both parent species, a result that contrasts with previous findings, which suggest that backcrossing is predominantly unidirectional between the two species. Results from our common garden trial indicate that hybrids do not have higher survival or out-perform the parent species, suggesting that heterosis may be limited in this system, or that long-term assessments beyond the duration of our field experiment may be required. Results from this study improve our understanding of the frequency of hybridization, and the associated backcrossing in this system, and provide land managers with information on how hybrids may be employed for the long-term preservation of riparian habitats undergoing rapid environmental change.