Historic aspen recruitment, elk, and wolves in northern Yellowstone National Park, USA

We conducted an analysis of aspen (Populus tremuloides) overstory recruitment on the northern range of Yellowstone National Park (YNP) using information provided in a monograph published by Warren (Warren, E.R., 1926. A study of beaver in the Yaney region of Yellowstone National Park, Roosevelt-Wildl. Ann. 1, 1±191), increment cores collected from riparian aspen stands in 1998, and an extensive random sample of aspen increment cores collected over YNP's entire northern range in 1997 and 1998. We summarized aspen size classes reported by Warren and estimated overstory origination dates of the stands he described using a linear regression based on our riparian aspen diameter/age relationship. Applying our regression results to Warren's diameter measurements, we predicted that the stands measured by Warren contained aspen that originated between approximately 1751 and 1920. The random set of aspen increment cores were used to analyze the age distribution of the current aspen overstory on YNP's northern range. These increment core data showed that approximately 10% of the current overstory aspen originated before 1871, 85% between 1871 and 1920, and 5% after 1921. Based upon our analysis of the Warren data and our aspen increment cores, we conclude that successful aspen overstory recruitment occurred on the northern range of YNP from the middle to late 1700's until the 1920's, after which it essentially ceased. Rocky Mountain Elk (Cervus elaphus) browsing has been identi®ed as signi®cantly impacting aspen overstory recruitment on YNP's northern range. We hypothesized why elk browsing has a di€erent in ̄uence on aspen now than it did historically. We discussed several potential social and ecological factors and hypothesize that a main factor is YNP's loss of signi®cant predator/prey relationships in the early 1900's, especially the in ̄uence of gray wolves (Canis lupus). We found that aspen overstory recruitment ceased during the same years that wolves, a signi®cant source of elk predation, were removed from YNP. Wolves may positively in ̄uence aspen overstory recruitment through a trophic cascades e€ect by reducing elk populations, modifying elk movement, and changing elk browsing patterns on aspen. # 2000 Elsevier Science Ltd. All rights reserved.