White spruce (Picea glauca (Moench) Voss.) is considered as the tree most characteristic of the North American boreal forest biome (Takhtajan 1986). Yet, in boreal eastern North America (Québec–Labrador Peninsula) it is rather a secondary, companion species (c. 10 % of the forest cover) of the balsam fir (Abies balsamea (L.) Mill.)–white birch (Betula paryrifera Marsh.) dominated forest assemblage (Jurdant 1959; Lafond 1964, 1974), typical of the closed-crown boreal forest south of 49°N (Fig. 13.1). Noticeable exceptions however exist in eastern North America where white spruce is locally the dominant species. On the maritime coast of Hudson Bay and Labrador, monospecific white spruce stands are initiating the colonization (primary succession) of barren surfaces (Payette and Filion 1985; Caccianiga and Payette 2006; Payette 2007; Laliberté and Payette 2008). Other maritime white spruce populations are forming a strip along the Acadian coast where it is a pioneer species of the secondary succession following forest openings by wind blowdowns (Davis 1966). White spruce also readily colonizes abandoned agricultural fields in maritime Canada and New England (Davis 1966; Nienstaedt and Zasada 1990). On Anticosti island in the Gulf of St. Lawrence, selective grazing of balsam fir by overpopulated white-tailed deer (Odocoileus virginianus Zimmerman) resulted in a shift from former balsam fir stands to white spruce dominated stands over the last century (Potvin et al. 2003).