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

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 >1min 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.