Reduced Cottonwood Recruitment following Extirpation of Wolves in Yellowstone’s Northern Range

Cottonwoods (Populus spp.) represent an important tree component of riparian plant communities within the winter range of the northern Yellowstone elk (Cervus elaphus) herd, yet young cottonwoods are highly palatable to ungulates. To assess potential herbivory impacts associated with wild ungulates following the removal of gray wolves (Canis lupus), long-term cottonwood recruitment and stand dynamics were studied. While results indicated that the establishment of young cottonwoods is an ongoing process in Yellowstone’s northern range, their relatively small stature and hedged appearance were indicative of high browsing pressure by wild ungulates, principally elk. At five study sites, the diameter at breast height (dbh) of all cottonwood trees $5 cm was determined; increment cores were also obtained for developing tree age vs. dbh relationships. At the La Duke Spring and Devils Slide Sites, which represent refugia sites since elk have limited access (due to local terrain conditions and cultural developments), cottonwood recruitment has been occurring during the last half century, indicating that climatic factors have not prevented cottonwood recruitment during that time. However, over this same period cottonwood recruitment no longer occurred at sites within the Soda Butte and Lamar Valleys. The overall decline in cottonwood recruitment at these locations occurred following the extirpation of wolves from Yellowstone National Park in the 1920s. At the Buffalo Ranch Site, where ranching of buffalo [bison] (Bison bison) and activities associated with culling of elk were centered, cottonwood recruitment continued after the loss of wolves but stopped following the curtailment of ungulate management activities at these facilities in the late 1960s. Taken together, data from the five sites reported herein along with the results of other studies provide compelling evidence that the absence of wolves in the northern Yellowstone has had a major impact on the recruitment of cottonwood and other woody browse species.