The effect of introduced Sitka black-tailed deer, Odocoileus hemionus sitkensis Merriam, on the forest understorey plant communities of Haida Gwaii, British Columbia: Pattern, process, and recovery

Abstract

The introduction of Sitka black-tailed deer, Odocoileus hemionus sitkensis Merriam, to Haida Gwaii (Queen Charlotte Islands, B.C., Canada) in the late 19th century provides a valuable opportunity to understand the long-term effects of deer populations on the vegetation of the North American temperate rain forest. We conducted two island-based experiments to investigate the effect of Sitka black-tailed deer on the forest understorey vegetation of this archipelago. In the first experiment we used a set of seven small islands (<15 ha) with different browsing histories (more than 50 years of deer presence, less than 20 years of deer presence, and no evidence of any deer presence) to test the effects of deer on plant cover, species richness and community composition. Browsing history was inversely proportional to both vegetation cover and plant species richness. Modification of the forest understorey plant communities followed a series of steps towards a greatly simplified community of plants possessing mechanisms to keep developing plant tissue inaccessible to deer. In the second experiment we utilized the cull of Sitka black-tailed deer from two large islands (295 ha and 170 ha) to investigate the release of forest plant communities from deer browsing. Using a paired-island approach, deer were culled on two experimental islands but remained on three adjacent control islands. Clear increases in species richness and cover as well as changes in the community composition of the forest understorey of experimental islands in the five years following the initiation of culls suggested a quick return to the forest understorey communities thought to exist before deer modification. However, failure of key shrub species to establish, coupled with the development of closed canopy stands of Sitka spruce, Picea sitchensis, suggests possible alternate stable-states for some communities.