Increased willow heights along northern Yellowstone's Blacktail Deer Creek following wolf reintroduction

Gray wolves (Canis lupus) were reintroduced into Yellowstone National Park in 1995-1996. In August 2004 we measured plant architecture of Geyer willow (Salix geyeriana) stems along three 100-m reaches of Blacktail Deer Creek in Yellowstone's northern elk (Cervus elaphus) winter range to evaluate changes in patterns of browsing and height growth following wolf reintroduction. Average browsing intensities (n = 3 stream reaches) of 100% in 1997 decreased to 0%-55% by 2003, whereas average stem heights of 25-74 cm in 1997 increased to 149-268 cm by 2003, indicating that willow height growth was inversely related to browsing intensity In addition, average willow canopy cover over the streams increased from <5% in 1997 to 14%-73% in 2004. These findings were consistent with a hypothesis that increased willow heights following the 1995-1996 wolf reintroduction represent a trophic cascade involving wolves, elk, and deciduous woody vegetation.