| Abstract |
- To investigate the extent and causes of recent quaking aspen (Populus
tremuloides) recruitment in northern Yellowstone National Park, we measured browsing
intensity and height of young aspen in 87 randomly selected aspen stands in 2012, and
compared our results to similar data collected in 1997–1998. We also examined the
relationship between aspen recovery and the distribution of Rocky Mountain elk (Cervus
elaphus) and bison (Bison bison) on the Yellowstone northern ungulate winter range, using
ungulate fecal pile densities and annual elk count data. In 1998, 90% of young aspen were
browsed and none were taller than 200 cm, the height at which aspen begin to escape from elk
browsing. In 2012, only 37% in the east and 63% in the west portions of the winter range were
browsed, and 65% of stands in the east had young aspen taller than 200 cm. Heights of young
aspen were inversely related to browsing intensity, with the least browsing and greatest heights
in the eastern portion of the range, corresponding with recent changes in elk density and
distribution. In contrast with historical elk distribution (1930s–1990s), the greatest densities of
elk recently (2006–2012) have been north of the park boundary (~5 elk/km²), and in the
western part of the range (2–4 elk/km²), with relatively few elk in the eastern portion of the
range (<2 elk/km²), even in mild winters. This redistribution of elk and decrease in density
inside the park, and overall reduction in elk numbers, explain why many aspen stands have
begun to recover. Increased predation pressure following the reintroduction of gray wolves
(Canis lupus) in 1995–1996 played a role in these changing elk population dynamics,
interacting with other influences including increased predation by bears (Ursus spp.),
competition with an expanding bison population, and shifting patterns of human land use and
hunting outside the park. The resulting new aspen recruitment is evidence of a landscape-scale
trophic cascade in which a resurgent large carnivore community, combined with other
ecological changes, has benefited aspen through effects on ungulate prey.
- Keywords: Rocky Mountain elk, Trophic cascade, Predation risk, Gray wolves, Cervus elaphus, Large carnivore restoration, Canis lupus, Yellowstone National Park, Browsing effects, Populus tremuloides, Bison, Quaking aspen
- Keywords: Rocky Mountain elk, Trophic cascade, Predation risk, Gray wolves, Cervus elaphus, Large carnivore restoration, Canis lupus, Yellowstone National Park, Browsing effects, Populus tremuloides, Bison, Quaking aspen
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