Long-term aspen dynamics, trophic cascades, and climate in northern Yellowstone National Park

Abstract

We report long-term patterns of quaking aspen (Populus tremuloides Michx.) recruitment for five ungulate exclosures in the northern ungulate winter range of Yellowstone National Park. Aspen recruitment was low (<3 aspen·ha−1·year−1) in the mid-1900s prior to exclosure construction due to herbivory by Rocky Mountain elk (Cervus elaphus Linnaeus, 1758) but increased more than 60-fold within 25 years after exclosure construction despite a drying climatic trend since 1940. Results support the hypothesis that long-term aspen decline in Yellowstone’s northern range during the latter half of the 20th century was caused by high levels of ungulate herbivory and not a drying climate. Gray wolves (Canis lupus Linnaeus, 1758) were reintroduced during 1995–1996. For the period 1995–2012, we summarized annual predator–prey ratios, ungulate biomass, and drought severity. The average density of young aspen increased from 4350 aspen·ha−1 in 1997–1998 to 8960 aspen·ha−1 in 2012; during the same time period, those >1 m in height increased over 30-fold (from 105 to 3194 aspen·ha−1). Increased heights of young aspen occurred primarily from 2007 to 2012, a period with relatively high predator–prey ratios, declining elk numbers, and decreasing browsing rates. Consistent with a re-established trophic cascade, aspen stands in Yellowstone’s northern range have increasingly begun to recover.