Aspen and Willow Restoration Using Beaver on the Northern Yellowstone Winter Range

Abstract

Aspen (Populus tremuloides) on the northern Yellowstone winter range has declined over the last half-century. Beaver (Castor canadensis) were reintroduced in Eagle Creek in 1991 in an attempt to reverse this trend. In 2005, we assessed the efficacy of this project by quantifying the long-term effects of beaver on aspen stands and the riparian area in this drainage. Between 1990 and 2005, the canopy cover of mature aspen decreased more than 62%, whereas immature aspen cover more than tripled, resulting in a total aspen canopy cover decrease (p < 0.05) from 43 to 25% (a loss of 7.25 ha). Willow canopy cover increased from 10 to 14% during the same period. The impacts of beaver on aspen stands were estimated by comparing vegetative changes among control sites with less than 10% beaver use (n = 5), active beaver sites (n = 6), sites inactive for 1-3 years (n = 7), sites inactive for 4-6 years (n = 4), and sites inactive for 7-11 years (n = 5). Aspen sprout and sapling densities were greater (p = 0.01) on sites which were active and inactive for 1-3 years compared to the other sites. Aspen ramets were not able to grow taller than 2 m on sites without beaver activity for 4-11 years due to ungulate herbivory. Although beaver stimulated the growth of aspen sprouts and saplings, ungulate herbivory prevented successful aspen recovery in the Eagle Creek drainage of the northern Yellowstone winter range 14 years after beaver reintroduction.

Aspen Restoration Using Beaver on the Northern Yellowstone Winter Range under Reduced Ungulate Herbivory

“…These photos were obtained from the U.S. Department of Agriculture and had a 1‐m resolution and 5‐m accuracy. The 29‐ha 2 digital polygon used in McColley et al's () study was used to delineate the riparian corridor of Eagle Creek in the photos. Three hundred random points per photo were generated within the polygon and visually classified into a cover type category according to the dominant cover type at each point.…”

“…McColley et al () found that beaver activity in Eagle Creek decreased the mature aspen canopy cover by 62% from 1990 to 2005, while immature aspen cover more than tripled during this same time period. Willow ( Salix spp.)…”

“…Aspen stands regenerate if they produce suckers that grow into recruitment stems (>2 m tall) and replace senescent mature overstory stems (Kay ). Studies in 1991 (St. John ), 2005 (McColley et al ), 2006 (Kimble et al ), and 2004 (Kauffman et al ) have repeatedly shown that ungulate browsing, primarily from the Northern Yellowstone elk herd, is the most important factor preventing aspen recruitment on the NYWR, despite a 50% reduction in the Northern Yellowstone elk herd population size from 1995 to 2005 (NYCWWG ). These studies suggest that elk population declines could enhance the recruitment of aspen suckers into the overstory, facilitating NYWR aspen restoration.…”

“…Beavers were historically common in NYWR riparian aspen communities (Warren ), but were likely trapped out of Eagle Creek before the 1970s (McColley et al ). Owing to beaver reintroduction, Eagle Creek is now the only NYWR drainage containing aspen, willow, beaver, and elk, and provides the only opportunity to evaluate the effects of beaver reintroduction on aspen recovery in an area that traditionally had high ungulate densities.…”

“…Aspen sucker density increased in Eagle Creek due to beaver activity, but ungulate browsing prevented aspen suckers from growing to recruitment height, preventing aspen regeneration. McColley et al () determined that during the winter of 2005–2006, the elk density in Eagle Creek was 8.3 elk/km 2 , indicating that aspen in Eagle Creek were still exposed to heavy elk browsing 10 years after initial wolf reintroduction. Northern range elk population declines since 2005 indicate that aspen in Eagle Creek may now be exposed to lower browsing pressures.…”