Genetic variation in postfire aspen seedlings in Yellowstone National Park

A rare episode of regeneration of aspen (Populus tremuloides Michx.) by see ds occurred in Yellowstone National Park (YNP), Wyoming, USA, following ext ensive fires that occurred in 1988. In 1997, we sampled 410 aspen seedlings from 23 local populations distributed widely across YNP to determine how g enetic diversity varies with elevation, substrate, plant competition, ungul ate browsing, and geographical location. We employed 132 randomly amplified polymorphic DNA (RAPD) markers based on six primers to show genetic relati onships within and among the postfire aspen seedling populations. Measures of genetic variation, including estimates of percentage polymorphic loci, e xpected heterozygosity, and Nei's Fs,, indicated that most of the variation occurred within rather than among local populations. There was no indicati on of geographical differentiation among sampled populations based on hiera rchal estimates of Nei's Fs, neighbour-joining, or correlations between gen etic distance and geographical distance. Even genetically distant populatio ns shared nearly 90% of the same markers. Within plots, the amount of genet ic variation decreased slightly in response to increased percentage vegetat ive cover, mean seedling basal diameter, and mean seedling height. Geologic al substrate, density of lodgepole pine (Pinus contorta var. latifolia Doug l.) seedlings, browsing intensity, and elevation were not significantly rel ated to levels of genetic variation within the seedling plots. These data s uggest mat genetic variation and geographical structure among seedling popu lations may occur over time as the transition from seedling-dominated stand s to clone-dominated stands occurs.