Effects of herbivores on ground flora and nutrient dynamics along Lake Superior's North Shore Highlands

Abstract

Impacts due to ungulate herbivores such as moose and white-tailed deer have been of interest to researchers and resource managers for years. Recent studies have begun to suggest the presence of alternative stable states in these systems where the existence of hysteresis could result in significant changes to the understory community that would be costly and time consuming to undo. However, before such assertions can be made, the presence of different states needs to be established. Our study investigated changes to the understory community and nutrient dynamics of twelve paired moose and white-tailed deer exclosure and control plots located along a 95-mile section of the Lake Superior North Shore Highlands. Exclosure ages ranged from 11 to 24 years old. During the summer of 2010, we measured understory and shrub species percent cover, sapling densities, litter depth, and bulk soil composition inside and outside of exclosures at each site. Nitrogen availability was also measured at three sites using resin bags. Browsing significantly affected understory vegetation at all sites except one, as well as ammonium (NH4) availability and percent of nitrogen and carbon in the mineral soil. The directions of these effects varied by site and seemed to be related to differences in canopy composition, age of exclosure, and relative ungulate population size. Future studies should monitor long-term changes through time, as some trends may be important early on in forest recovery while others do not appear until years later. Although our study design was not able to detect evidence of hysteresis or affirm the presence of alternative stable states, we believe it does suggest evidence of instability and non-linear ecosystem impacts of ungulate herbivory.