Effects of beaver reintroduction and ungulate browsing on aspen recovery in the Eagle Creek drainage of the northern Yellowstone winter range

Abstract

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 were reintroduced to the Eagle Creek drainage on the NYWR in 1991 in an attempt to facilitate recovery of riparian aspen communities by removing aspen overstory and increasing sprouting. However, intense ungulate browsing, primarily from the Northern Yellowstone elk herd, was preventing aspen recruitment in Eagle Creek in 2005. Wolf predation has contributed to a 56% decrease in this elk herd from 2005 to 2012. I investigated the effects of beaver reintroduction and ungulate herbivory on aspen recovery in the Eagle Creek drainage in 2012. Aerial photos taken of Eagle Creek in 1990, 2005, and 2011 showed that although beaver activity stimulated aspen sprouting, the mature overstory of many aspen stands has not been replaced 21 years after beaver reintroduction (p>0.05). Sprouting and recruitment were investigated using 4-m radius circular vegetation plots (n=31) established in aspen stands throughout Eagle Creek in 1997 and monitored annually until 2012. Beaver activity stimulated increased sprouting in 71% of these plots, and 77% of the plots had > or = 1 recruitment stem in 2012. Prolonged flooding and high browsing levels contributed to lack of recruitment in 23% of the plots (p<0.05). In 2012, 75% of the paired plots associated with aspen exclosures had unfenced aspen stems with an average stem height > or = 2 m. Recent increases in aspen recruitment in Eagle Creek indicate that aspen communities are regenerating. This is likely the result of decreased 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.