Linkages between wolf presence and aspen recruitment in the Gallatin elk winter range of southwestern Montana, USA

Abstract

The decline of aspen (Populus tremuloides) in Yellowstone National Park (YNP) has been attributed to conifer encroachment, climatic influences, ungulate herbivory and more recently, a lack of grey wolves (Canis lupus) via trophic cascades. The current study explores evidence of a trophic cascade on aspen decline. We conducted the study in the Gallatin elk winter range of YNP, an area where aspen have not previously been studied. We approximated aspen origination dates from an age–diameter relationship to examine potential correlations between wolf presence and absence, elk (Cervus elaphus) herbivory and aspen recruitment. A comparative analysis was also conducted between the aspen data collected in the winter range and aspen data collected within two elk exclosures. Within the elk exclosures, aspen successfully recruited since fence construction in the 1940s. Outside the exclosures, aspen recruitment into mature stems began to decline in the 1920s (during wolf extirpation), completely ceased after the 1950s and has only been observed since the 1990s (post-wolf reintroduction). While a host of interacting biophysical factors may influence aspen recruitment and growth, the correlative results between aspen recruitment and historical elk browsing activities, coincident with the presence and absence of wolves, are consistent with a top–down trophic cascade.