A RARE EPISODE OF SEXUAL REPRODUCTION IN ASPEN (POPULUS-TREMULOIDES MICHX) FOLLOWING THE 1988 YELLOWSTONE FIRES

A widely held tenet of Rocky Mountain ecology is that trembling aspen (Populus tremuloides Michx.) reproduces almost exclusively by vegetati ve root sprouting. However, a rare episode of sexual reproduction in a spen occurred following the extensive Yellowstone fires in Wyoming (US A) in 1988. Great numbers of aspen seedlings became established in bur ned areas in 1989, apparently because of the unusual combination of hi gh seed production, favorable weather conditions, and suitable substra te conditions that existed that year. The new aspen genets are elongat ing and are developing lateral root systems with numerous sprouts. Gen etic diversity of seedling populations on the Yellowstone Plateau is g reater than the diversity of mature clones that were sampled in northe rn Yellowstone National Park. Aspen has become established in many por tions of the park where it was absent before the 1988 fires. Thus, we appear to be witnessing an unusual ecological event in which a widespr ead, ecologically important species has very rapidly expanded its geog raphic range and genetic variability in response to a large-scale natu ral disturbance.