Factors associated with the presence of flowering individuals of Arethusa bulbosa (Orchidaceae) in peatlands of southern Quebec1

摘要

Arethusa bulbosa is a northeastern North American orchid species rare in most of its range. This study sought to determine the factors associated with the presence of Arethusa within pcatlands and its absence from a priori suitable sites. Twenty-fourplots with Arethusa were compared with paired plots from which the species was absent and with 22 plots in pcatland where the species has never been present. Vegetation and abiotic variables were recorded within the plots, and anthropogenic variables were identified using aerial photographs. The IndVal method was used to determine whether some species were characteristic of Arethusa habitat, and discriminant function analyses were performed to determine which factors were related to occupied and unoccupied sites. No characteristic species were identified when comparing paired plots. Occupied sites were mainly characterized by lower light availability at ground level (46%; mean), higher Floristic Quality Assessment Index (35), and smaller shrubs (55 cm)than unoccupied paired plots (61%, 32, and 61 cm respectively) from the same pcatland. Water pH was the main factor discriminating occupied sites (5.5) from the unoccupied sites in pcatlands where the species has never been present (3.9), pH being usually higher in plots with Arethusa. Ten species were characteristic of plots with Arethusa (including Larix laricina, Sphagnum palustre, and Aulacomnium warnstorjii) when compared to a priori suitable sites. Overall, our results suggest that Arethusa settles in poor to moderate Sphagnum-dominatcd fen habitats rather than typical open bog habitats, as usually assumed.