Population substructure in continuous and fragmented stands of Populus trichocarpa

Population substructure has important implications for both basic and applied genetic research. We used 10 microsatellite markers to characterize population substructure in two ecologically and demographically contrasting populations of the model tree Populus trichocarpa. The Marchel site was a continuous stand growing in a mesic habitat in western Oregon, whereas the Vinson site consisted of three disjunct and isolated stands in the high desert of eastern Oregon. A previous study revealed that pollen-mediated gene flow is extensive in both populations. Surprisingly, model-based clustering, principal components analysis and analyses of molecular variance provided overwhelming support for the existence of at least two intermingled sub-populations within the continuous Marchel population (F(ST)=0.026, P