Impact of Moose Browsing on Boreal-type Forests of Isle Royale National Park

Abstract

Measurable long-range effects of moose browsing were found in a boreal-type forest at Isle Royale National Park. Overall forest density decreased while average basal area per tree increased. Total basal area and reproduction density were unaffected. Balsam fir densities were drastically reduced, especially in younger age classes. For the most heavily used site, the oldest firs affected are linked to the time of moose arrival on Isle Royale. Mountain ash densities were also drastically reduced. White spruce, unpalatable to moose, increased in density, possibly due to reduced competition from fir. Birch and aspen densities were relatively unaffected. Their stems were mostly out of reach of moose when these animals first arrived. Reduction in fir and mountain ash densities accounts for the reduction in total tree density. Total cover in the shrub layer was unaffected by browsing, but averace stem height decreased. Taxus canadensis has almost disappeared on all browsed sites. Cover of low vascular plants and the ratio of windfalls to standing trees increased. INTRODUCTION The forests of Isle Royale National Park show several striking effects of moose (Alces alces andersoni) browsing. Many shrubs and tree saplings are truncated 1-1.5 m above the ,ground. Balsam fir (Abies balsamea) saplings appear scraggly and misshapen. Older firs exhibit a distinct browse line '2.5-3.5 m above the ground. Moose appear to be a maior ecological factor influencinz the successional development of the forest as well as the characteristics of the climax. The purpose of this study was to determine the extent of this influence. It is one phase of a comDrehensive research proiect aimed at investigating all maior aspects of the ecology of the upland boreal forest type in the park. It has been supnorted by the United States National Park Service and the Michigan Technological University. The effects of moose on a forest were studied by Berzerud and Manuel (1968) in the Noel Paul Drainage of Newfoundland where the forest type closely resembles that of Isle Royale. The emphasis in their study was on evaluating the effects on reproduction of tree sgecies and on determining the moose-carrying capacity of the area. Pimlott (1965) studied the effect of both moose and deer browsing upon major tree species in both Newfoundland and Anticosti Island. STUDY AREA Isle Royale National Park is an archipelago consisting of a large island about 72 km long and 14 km wide at its widest point and surrounded bv approximately 200 small islands. It lies in the western portion of Lake Superior 24 km SE of the Canadian shore and 72 km NW of Michigan's Keweenaw Peninsula. Uptilted layers of basaltic