Abstract
AbstractQuaking aspen (Populus tremuloides) recruitment during the 1980s–90s was suppressed by Rocky Mountain elk (Cervus canadensis) herbivory on winter ranges in the Yellowstone region, and saplings (young aspen taller than 2 m) were rare. Following the 1995–96 reintroduction of gray wolves (Canis lupus), browsing decreased and sapling recruitment increased in Yellowstone National Park. We compared aspen data from inside the park to data collected in three winter ranges outside the park. For most areas, the percentage of young aspen browsed annually was 80–100% in 1997–98, decreasing to 30–60% in 2011–15. Sapling recruitment was inversely correlated with browsing intensity, and increased despite climate trends unfavorable for aspen. Browsing decreased with decreasing elk density, a relationship suggesting that densities greater than about 4 elk/km2 resulted in consistently strong browsing effects likely to suppress aspen recruitment. Changes in elk density and distribution were influenced by predators, as well as human hunters. Most evidence for trophic cascades involving large terrestrial mammals has been from protected areas within national parks. This study provides evidence of widespread changes in plant communities resulting from large carnivore restoration, extending outside a protected national park to areas with hunting, livestock grazing, and other human activities.
Highlights
Quaking aspen (Populus tremuloides) forms a small but important component of ecosystems in the northern Rocky Mountains, with a large effect on habitat diversity and biodiversity as one of the few deciduous tree species in the region
Most of the evidence for trophic cascades associated with wolves and elk has been from protected areas in national parks (Soule et al 2003, Hebblewhite et al 2005, Beschta and Ripple 2007, 2009, Kuijper et al 2013, Muhly et al 2013)
We examined evidence for trophic cascades involving aspen, elk, and wolves in the Yellowstone region, using surveys of aspen stands in elk winter ranges north, west, and east of the park, as well as inside the park boundary
Summary
Quaking aspen (Populus tremuloides) forms a small but important component of ecosystems in the northern Rocky Mountains, with a large effect on habitat diversity and biodiversity as one of the few deciduous tree species in the region. Rocky Mountain elk (wapiti, Cervus canadensis) forage on young aspen in fall and winter, and high levels of herbivory by elk can suppress heights of young aspen. This process over time can result in a loss of understory aspen and shrubs in aspen stands, and eventually loss of entire stands, as old trees die and are not replaced (Romme et al 1995, White et al 1998, Barmore 2003, Smith et al 2016). Human hunters may influence elk density and behavior (Proffitt et al 2013), possibly
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