Fairy slipper (Calypso bulbosa) on Anticosti Island: the occurrence of a rare plant in an environment strongly modified by white-tailed deer

Abstract

The introduction of white-tailed deer ( Odocoileus virginianus Zimm.) at the end of the 19th century, and their subsequent proliferation on Anticosti Island (Quebec) has had a strong impact on forest vegetation. Only 50% of the original balsam fir ( Abies balsamea (L.) Mill.) cover remains on the island, and fir has been progressively replaced by white spruce ( Picea glauca (Moench) Voss) because of browsing. This phenomenon may have an impact on rare species, such as the fairy slipper orchid ( Calypso bulbosa (L.) Oakes; Orchidaceae), associated with old fir stands. In this study, we mapped fairy slipper populations on Anticosti Island and collected data on its habitat. We developed a statistical model to identify key environmental characteristics predicting the presence or the absence of the species. We hypothesized that on Anticosti Island, the fairy slipper is restricted to old-growth fir stands located on calcareous soils; consequently, this orchid is endangered because its habitat is gradually disappearing. A total of 69 sampling stations were surveyed along three transects crossing the island. The fairy slipper was present in 19 of the 69 stations. A fir stand, with a large quantity of dead logs and low levels of tree and moss cover, is more likely to harbour a fairy slipper population than any other forest stand. It is possible that on Anticosti Island, the presence of the fairy slipper is associated with forest age and structure (the latter being strongly influenced by deer) rather than with fir per se. Moreover, deer, by browsing shrubs and fir seedlings and saplings, have apparently had a positive impact on the fairy slipper population by eliminating competition between this orchid and other understory plants. On the other hand, deer have hampered the regeneration of fir forests; old fir stands that were eliminated by blowdowns, insect outbreaks, fires or logging have been replaced by spruce stands. This may have an impact on the long term survival of the fairy slipper, unless spruce stands soon provide suitable habitats for the species as they mature.