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

Abstract

Based on the life-form and their utility to Beavers (Castor canadensis), we classified the riparian plants around Beaver impoundments into five categories: Alder (Alnus spp.) - dam construction; Trembling Aspen (Populus tremuloides) - primary food; White Birch (Betula papyrifera) and Willows (Salix spp.) - secondary foods; shrubs - occasionally used for food and dam construction; and conifers - occasionally used for dam construction. To study the influence of Beaver herbivory on shoreline woody plants, we randomly chose eight recently active (< 5 years since establishment), seven recently abandoned (< 12 years since abandonment), and eight old abandoned (> 12 years since abandonment) dam sites. We found that Beavers concentrated the majority of their herbivory to within 20 m of the impoundment edge. Three explanations are proposed: (1) thermoregulatory restrictions; (2) directionally leaning trees within riparian habitats; and (3) reduction of predation risk. After 12+ years of abandonment, we found the Beaver's preferred food tree, the Trembling Aspen, had not re-established along streams adjacent to abandoned dam sites. In contrast, less desirable food trees and conifers showed increased recovery. If this selective herbivory continues unchecked, the riparian habitat along the Swanson River will become dominated by conifers and thus unsuitable for sustaining Beavers. We believe that fire is needed to rejuvenate the failing Trembling Aspen stocks.