Interactive effects of human activities, herbivory and fire on quaking aspen (Populus tremuloides) age structures in western Wyoming

Aim Fire suppression, elk (Cervus elaphus Erxleben) browsing and drought have been suggested as possible explanations for low aspen (Populus tremuloides Michx.) ramet regeneration in elk winter range of the Greater Yellowstone Area (GYA) during the twentieth century. This study analyses the relationship between the rates of aspen regeneration, biophysical factors and human land use since 1830. This approach reveals the importance of indirect human impacts, especially through fire and elk management strategies, on forest structure. Location This study was conducted in and around the winter range of the Jackson Hole elk herd in western Wyoming, USA. Aspen stands in this region represent the ecotone between coniferous forest and sagebrush-grassland vegetation. Methods Age structures of aspen stands were reconstructed from tree-rings in order to determine how variation in drought, elk populations and fire occurrence may have affected aspen ramet regeneration since 1830. The effects of recent prescribed fires and elk browse on aspen regeneration were also evaluated by measuring stem height, browse intensity and age of ramets <2 m tall in prescribed burns and in unburned areas. Results Aspen ramet regeneration has occurred consistently but at low frequencies since 1830, with three peaks of regeneration: 1860-85, 1915-40 and 1955-90. Periods of frequent ramet regeneration coincided with low to moderate elk populations and aspen regenerated only sporadically when elk populations were high. Based on a comparison of the age structures with a tree-ring reconstruction of Palmer drought severity index (PDSI), observed PDSI and annual precipitation, drought variability appears unrelated to episodes of aspen regeneration. Recent regeneration patterns suggest that fire does not enhance the recruitment of tree-sized aspen stems in the presence of elk browse, although sample sizes were small. Since 1900, elk hunting and spatially clustered elk feeding in the study area has facilitated low but consistent regeneration of aspen ramets in the twentieth century. In contrast, extremely low ramet regeneration has been observed in national parks of the Rocky Mountains, where elk have been managed according to the 'natural regulation' policy (no hunting or feeding) since 1969. Main conclusions Over time, different management strategies have altered the interactions between fire, herbivory and aspen regeneration suggesting that management history and the causes of change in management, should be explicitly included in ecological studies and future management strategies. These results also point to the value of using the spatial and temporal variation in human interactions with ecological systems as a method for understanding ecological relationships.