GENETIC DIVERSITY AND SPATIAL SUBDIVISION OF POPULUS TREMULOIDES (SALICACEAE) IN A HETEROGENEOUS LANDSCAPE

Trembling aspen (Populus tremuloides Michx.), a clonal angiosperm, is the most geographically widespread tree in North America. It is generally thought that most extant populations in the western interior of Canada and the United States became established shortly after glacial retreat, but sexual recruitment then effictively ceased owing to inimical climatic conditions. Six populations of trembling aspen were studied in the prairie and montane environments of Waterton Lakes National Park, Alberta. Vegetative tissues were analyzed for electrophoretically detectable variation in 13 enzymes encoded by 14 polymorphic loci and three monomorphic loci. All populations maintained high levels of inter- and intrapopulation diversity (P = 0.891; H = 0.319; A = 2.4). The mean fixation index, F, was -0.102, indicating some deviation from Hardy-Weinberg expectations. Genetic differentiation (F_ST = 3.0) was apparent in this ecologically diverse, but geographically small-scale, spatial setting. It is suggested that the maintenance of diversity in the absence of frequent modern-day recruitment, and resistance to further geographic differentiation in this spatially heterogeneous environment reflect occasional seedling establishment through “windows of opportunity” and more importantly, the species' clonal morphology. The phalanx growth form and concomitant physiological integration between ramets combine to spread the risk of death and buffer the effects of selection over time and space.