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

We conducted an analysis of aspen (Populus tremuloides) overstory recruitme nt on the northern range of Yellowstone National Park (YNP) using informati on provided in a monograph published by Warren (Warren, E.R., 1926. A study of beaver in the Yaney region of Yellowstone National Park, Roosevelt-Wild l. Ann. 1, 1-191), increment cores collected from riparian aspen stands in 1998, and an extensive random sample of aspen increment cores collected ove r YNP's entire northern range in 1997 and 1998. We summarized aspen size cl asses reported by Warren and estimated overstory origination dates of the s tands he described using a linear regression based on our riparian aspen di ameter/age relationship. Applying our regression results to Warren's diamet er measurements, we predicted that the stands measured by Warren contained aspen that originated between approximately 1751 and 1920. The random set o f aspen increment cores were used to analyze the age distribution of the cu rrent aspen overstory on YNP's northern range. These increment core data sh owed that approximately 10% of the current overstory aspen originated befor e 1871, 85% between 1871 and 1920, and 5% after 1921. Based upon our analys is of the Warren data and our aspen increment cores, we conclude that succe ssful aspen overstory recruitment occurred on the northern range of YNP fro m the middle to late 1700's until the 1920's, after which it essentially ce ased. Rocky Mountain Elk (Cervus elaphus) browsing has been identified as s ignificantly impacting aspen overstory recruitment on YNP's northern range. We hypothesized why elk browsing has a different influence on aspen now th an it did historically. We discussed several potential social and ecologica l factors and hypothesize that a main factor is YNP's loss of significant p redator/prey relationships in the early 1900's, especially the influence of gray wolves (Canis lupus). We found that aspen overstory recruitment cease d during the same years that wolves, a significant source of elk predation, were removed from YNP. Wolves may positively influence aspen overstory rec ruitment through a trophic cascades effect by reducing elk populations, mod ifying elk movement, and changing elk browsing patterns on aspen. (C) 2000 Elsevier Science Ltd. All rights reserved.