Résumé
Large herbivores and deer in particular can have dramatic effects on ecosystem functions and act as keystone species in many forest systems. At high densities, deer are assumed to determine the structure and composition of forest understory. The introduction in the late 19th century of the Sitka black-tailed deer (Odocoileus hemionus sitkensis) on Haïda Gwaii archipelago (British Columbia, Canada) gave us the opportunity to illustrate the effects of predator-free deer populations on previously unbrowsed old-growth forest understory vegetation and the potential for recovery after a prolonged reduction of deer density. We installed and monitored a network of twenty 25m² enclosures from 1997 to 2005. We compared changes in species richness, apparition events, and vegetation cover by height strata in plots that were fenced or not. Protection from deer had no effect on total species richness but created a stable situation that favoured species appearance and establishment. Species tended to appear first inside the enclosures. The exclusion of deer had a positive effect on overall vegetation cover in height strata directly accessible to deer, but not above the browse line (>1.5m). Deer limited maximum height reached by the species outside the enclosures. Analysis of the vegetation cover by species groups (trees, shrubs, herbs) did not reveal any effect of the exclusion of deer. However, some species as Cornus canadensis, Menziesia ferruginea, Blechnum spicant, Lysichiton americanus exhibited a positive response to the protection from deer whereas more dominant species like Gaultheria shallon and Vaccinium sp. were not influenced. As expected, deer exclusion did result in some recovery of understory in Haïda Gwaii forest understory. Furthermore, fenced plots may be considered as sources for surrounding patches. However the slow vegetation growth recorded associated to potential interspecific competition with browse resistant species could prevent the understory recovery if a significant reduction of deer abundance is not maintained over a long period of time.