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

摘要

Ungulate browsing and lack of overstory disturbance have historically prevented aspen regeneration on the Northern Yellowstone Winter Range (NYWR). Aspen clones regenerate if sprouts are produced that grow into recruitment stems (＞2 m tall) and replace the mature overstory. Beaver rein-troduced in 1991 to Eagle Creek on the NYWR facilitated aspen restoration by removing overstory trees and increasing sprouting. However, intense ungulate browsing, primarily from the Northern Yellowstone elk herd, was preventing aspen recruitment in Eagle Creek as of 2005. Since 2005, wolf predation has contributed to a 56% decrease in this elk herd. We investigated the effects of beaver reintroduction, ungulate herbivory, and predator-mediated declines in elk numbers on aspen regeneration in Eagle Creek from 1997 to 2012. Aerial photos of Eagle Creek in 2005 and 2011 showed that the aspen overstory has not been replaced 21 years after beaver reintroduction (p ＞ 0.05). Sprouting and recruitment were investigated using 4-m radius circular plots (n = 31) established throughout Eagle Creek in 1997 and monitored annually until 2012. Beaver activity stimulated sprouting in 71% of these plots. In 2012, 77% of the plots had ≥1 recruitment stem and 75% of the paired plots associated with exclosures (n = 16) had aspen stems with an average height ≥2 m. Recent increases in aspen recruitment in Eagle Creek indicate that aspen communities are regenerating. This has likely resulted from decreased ungulate browsing pressure on aspen saplings from 2005 to 2012. These findings are consistent with the predictions of a density-mediated trophic cascade following wolf reintroduction.