We studied young riparian cottonwoods (Populus spp.) and associated woody plants along Soda Butte Creek and the Lamar River in northeastern Yellowstone National Park (YNP) to examine the potential influence of wolf/elk interactions upon plant growth. After a period of approximately 70 years without wolves in YNP, they were reintroduced in the winter of 1995-1996. When we compared woody plant heights shown in photographs taken prior to 1998 with those shown in 2001-2002 photos, we found an increase in the height of riparian woody plants for six of the eight sites within the study area. Plants were tallest (1-4 m) at point bar, stream confluence, and island sites, while heights remained relatively low (<1 m) along straight river reaches in a wide valley setting. We measured differences in both browsing intensity and cottonwood height for sites with relatively high predation risk (low visibility and/or the presence of escape barriers) and compared them to nearby sites with relatively low predation risk (open areas). In general, the high-risk sites had lower browsing intensities (percent of stems browsed) and taller plants than low-risk sites. Although the young cottonwoods on high-risk sites were growing taller each year over the last 4 years, there was little change in the plant heights for low-risk sites. For a stand of young cottonwoods growing adjacent to a gully (potential escape barrier), we found a linear correlation (r(2) = 0.76) between cottonwood height and adjacent gully depth; as gully depth increased, the percent of stems browsed decreased and cottonwood height increased. While the release of cottonwood and willows within the study area is in a very early stage, results provide rare empirical evidence illustrating the indirect effects of a top carnivore in a terrestrial food chain and supports theories on: (1) predation risk effects and (2) trophic cascades (top-down control). Wolf reintroductions into the Yellowstone environment may thus represent a management action that was needed to help insure the restoration of riparian species and preservation of biodiversity. (C) 2003 Elsevier Science B.V. All rights reserved.
