Effects of Beaver, Castor canadensis, herbivory on streamside vegetation in a northern Ontario watershed

Based on the life-form and their utility to Beavers (Castor canadensis), we classified the riparian plants around Beaver impoundments into five catego ries: Alder (Alnus spp.) - dam construction; Trembling Aspen (Populus tremu loides) primary food; White Birch (Betula papyrifera) and Willows (Salix sp p.) - secondary foods; shrubs - occasionally used for food and dam construc tion; and conifers - occasionally used for dam construction. To study the i nfluence of Beaver herbivory on shoreline woody plants, we randomly chose e ight recently active (< 5 years since establishment), seven recently abando ned (< 12 years since abandonment), and eight old abandoned (> 12 years sin ce abandonment) dam sites. We found that Beavers concentrated the majority of their herbivory to within 20 m of the impoundment edge. Three explanatio ns are proposed: (1) thermoregulatory restrictions; (2) directionally leani ng trees within riparian habitats; and (3) reduction of predation risk. Aft er 12+ years of abandonment, we found the Beaver's preferred food tree, the Trembling Aspen, had not re-established along streams adjacent to abandone d dam sites. In contrast, less desirable food trees and conifers showed inc reased recovery. If this selective herbivory continues unchecked, the ripar ian habitat along the Swanson River will become dominated by conifers and t hus unsuitable for sustaining Beavers. We believe that fire is needed to re juvenate the failing Trembling Aspen stocks.