Plant diversity, composition, and rarity in the southern boreal peatlands of Manitoba, Canada

Plant diversity and rarity have been relatively well studied for bryophytes in Canadian western boreal peatlands, but little information exists for vascular plants. Diversity, community composition, and rarity of bryophytes and vascular plants were determined and relationships examined among these and environmental variables in five peatland types at Duck Mountain, Manitoba: wooded bogs, black spruce swamps, wooded moderate-rich fens, open moderate-rich fens, and open extreme-rich fens. Total diversity was 298 species comprising 86 bryophytes and 212 vascular plants. Mean diversity followed a unimodal distribution over a bog - rich fen gradient. Wooded moderate-rich fens (59.0) and black spruce swamps (53.4) had the highest mean diversity, whereas wooded bogs (32.3) and open extreme-rich fens (34.7) had the lowest mean diversity. Occurrences of locally rare species followed the same general pattern, and provincially rare vascular plants were found primarily in wooded moderate-rich fens and black spruce swamps and were mostly orchids. Reasons for these patterns are complex, but high diversity appears to be related to high habitat heterogeneity and moderate environmental variables, e.g., pH and alkalinity, and low diversity appears to be related to environmental extremes, e.g., pH and alkalinity. Boreal wooded moderate-rich fens and black spruce swamps have comparatively high plant diversity and rarity and require consideration if the focus is biodiversity conservation. This will become increasingly important in landscapes where development pressures are high.