Ecological - Second Order
Fire & Wildlife
Most stands of trembling aspen (Populus tremuloides) in northern Yellowstone National Park appear to have become established between 1870 and 1890, with little regeneration since 1900. There has been controversy throughout this century regarding the relative roles of browsing by elk (Cervus elaphus) and fire suppression in preventing aspen regeneration. Fires in 1988 burned 22% of the northern ungulate winter range in the park, and created an unusual opportunity to investigate interactions between fire, ungulate browsing, and aspen regeneration. We tested two hypotheses. (1) The fires would stimulate such prolific sprouting of new aspen stems in burned stands that many stems would escape ungulate browsing and regenerate a canopy of large aspen stems. (2) Browsing pressure would be so intense that it would inhibit aspen canopy regeneration in the burned stands, despite prolific sprouting, but increased forage production in the burned areas would attract elk so that they would not seek out remote aspen stands, and hence, aspen regeneration would occur in unburned aspen stands remote from the burned areas. We sampled aspen sprout density, height, growth form, and browsing intensity in six burned aspen stands six unburned stands close (< 1km) to the burned area, and six unburned stands remote (> 4 km) from the burned area. Density of sprouts was generally greater in the burned stands than in the unburned stands in spring 1990 (2 yr after the fires), but was approaching the density of unburned stands by fall 1991. There were no significant differences in browsing intensity (percent of aspen sprouts browsed by ungulates) in 1990 or 1991 among burned, unburned close, or unburned remote stands, nor were there differences in relation to growth form (juvenile vs. adult sprouts). Unbrowsed sprouts generally were lower than the depth of the snowpack, suggesting that elk browsed nearly all sprouts that were accessible. The age distribution of 15 aspen stands across the northern winter range indicated that regeneration of large canopy stems had been episodic even prior to the establishment of the park in 1872. The period 1870-1890, when the present-day aspen stands were generated, was historically unique: numbers of elk and other browsers were low, climate was relatively wet, extensive fires had recently occurred, and large mammalian predators of elk (e.g., wolf, Canis lupus) were present. This combination of events has not recurred since 1900. The recent paucity of aspen regeneration in northern Yellowstone National Park cannot be explained by any single factor (e.g., excessive elk numbers or fire suppression) but involves a complex interaction among factors.