A LACK OF HETEROSIS IN NATURAL POPLAR HYBRIDS FROM SOUTHERN ALBERTA

Shoot cuttings were collected from 56 native poplars along 11 rivers a nd creeks in southwestern Alberta, a region where Populus angustifolia , Populus balsamifera (sspp. balsamifera and trichocarpa), and Populus deltoides form a disectional, trispecific hybrid swarm. Cuttings were rooted and established in an irrigated nursery plot. After 3 years, s tem diameters and heights were measured from 381 trees and whole shoot s were harvested from 74 trees to establish a regression equation rela ting basal diameter and shoot wet weight. Shoot wet weight varied by m ore than an order of magnitude across the clones, and P. angustifolia clones tended to grow more slowly than clones of the other two species . There was no significant correlation between shoot growth rate and t he degree of hybridization, as determined by a prior multivariate anal ysis of leaf form that has been substantiated by chemotaxonomic data. Thus, the interspecific hybrids were not generally more vigorous than clones with leaf forms typical of the parental species P. deltoides or P. balsamifera. Conversely, in the same environment, artificial hybri ds of P. deltoides from Illinois and P. balsamifera (ssp. trichocarpa) from Washington expressed heterosis. It is theorized that introgressi ve genetic exchange accompanying the natural interbreeding of these th ree poplar species in southern Alberta minimizes their genetic distanc e, preventing heterosis.