Impact of introduced white-tailed deer and native insect defoliators on the density and growth of conifer saplings on Anticosti Island, Québec1

Abstract

:Invaders can have negative impacts on native flora, fauna, and ecosystems, especially on remote islands where they compete with indigenous plants and animals. Since their introduction on Anticosti Island (Québec) in the late 1890s, white-tailed deer (Odocoileus virginianus) have had detrimental effects on preferred forage species. To determine the impact of deer browsing on tree growth and forest structure, we compared stem density, age, and size of conifer species across a range of open, semi-forested, and forest stands, where browsing severity on balsam fir (Abies balsamea), a preferred species, was variable. Most balsam fir saplings showed signs of browsing, with maximum impact at low sapling density. At most stands, balsam fir saplings were smaller than white spruce (Picea glauca) and black spruce (Picea mariana) saplings and developed bonsai-like growth forms. Browsing was light on white spruce; no browsing was recorded on black spruce. Tree-ring analysis was used to differentiate the influence of deer browsing from the possible effects of past insect activity on conifer species. Periods of radial growth reduction coincided with documented spruce budworm (Choristoneura fumiferana) and hemlock looper (Lambdina fiscellaria) infestations during the 20th century. The combined influence of insect defoliation and deer browsing on fir was evidenced by contrasted patterns in stem growth above and below browsing height. Balsam fir sapling mortality in the 1980s and early 1990s was likely due to severe deer browsing rather than insect defoliation. Reduced sapling growth and recruitment of balsam fir to the canopy will likely modify the forest composition from balsam fir- to white sprucedominated stands.