Intraspecific diversity in trembling aspen (Populus tremuloides, Salicaceae) in Waterton Lakes National Park, Alberta: a biogeographic perspective

Trembling aspen (Populus tremuloides Michx), a clonal angiosperm, is the most geographically widespread tree in North America. It is widely thought that most extant populations in the western interior of Canada and the United States became established shortly after glacial retreat, but sexual recruitment effectively ceased soon thereafter owing to inimical climatic conditions. Six populations of P. tremuloides 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 16 loci, 14 of which were polymorphic. Six populations maintained high levels of interand intra-population 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 (FsT = 3.0) was apparent in this ecologically diverse, but geographically small spatial setting. Some of this structure was attributed to the effects of selection. The pattern of allele frequencies was analyzed in relation to a surrogate measure of fitness, mean annual increment of ramets, which revealed the presence of a significant, albeit weak, positive correlation between growth and heterozygosity. This was interpreted as evidence of balancing selection acting to maintain diversity. A chemical analysis of dormant twigs for crude protein, macro- and micro-chemical elements, and fiber demonstrated that there were considerable differences between clones within populations and between sites. The relative intensity of browsing by mule deer (Odocoileus hemionus) and elk (Cervus elaphus) also varied substantially between clones and between sites. Correlation analysis of browsing intensity with chemical composition demonstrated that herbivory by ungulates was not linked to protein, elemental composition, or fiber. Moreover, it is unlikely that selective feeding is cost effective given the nature of chemical differences between clones. It is postulated that while herbivores may affect genetic diversity, the precise effects are random with respect to the assayed chemical components. In conclusion, it is suggested that the maintenance of diversity in the absence of modem-day recruitment, and resistance to geographic differentiation in a spatially heterogeneous environment is largely due to clonality, specifically, the species' phalanx growth habit and concomitant physiological integration between ramets, the combination of which spreads the risk of death and buffers the effects of selection over time and space.