Habitats of circumboreal–subarctic sphagna: I. A quantitative analysis and review of species in the Caribou Mountains, northern Alberta

A quantitative analysis of the habitats of 14 species of Sphagnum found in the Caribou Mountains of northern Alberta and a literature review are the basis for a discussion of the habitats of these species as they occur in circumboreal–subarctic regions. Despite the implication inherent in the name, the Caribou Mountains are not mountains; the formation is an extensive, elevated plateau, which is underlain by more-or-less continuous permafrost. Three habitat types predominated in our study area: an upland "treed-tundra" (which is dominant over much of the plateau) with scattered Picea mariana and a hummocky bottom layer of sphagna; rounded "thaw-pocket" depressions dominated by carpets of Sphagnum and some carices; and streams defined by a dense zone of shrubs with the narrow water channel bordered by more-or-less firm lawns of Sphagnum species. Three gradients, which are considered to have a predominant influence on the occurrence of Sphagnum species, are wet to dry, ombrotrophic to minerotrophic, and shaded to exposed. The relative importance of each of these factors varies with each species of Sphagnum. Sphagnum jensenii, S. majus, S. riparium, and S. lindbergii occur exclusively in weakly minerotrophic to ombrotrophic, aquatic habitats where populations form loose, floating carpets. Such habitats are generally not much shaded. Such species as S. angustifolium, S. teres, and S. russowii are characteristic of somewhat less moist conditions, but all have a broad amplitude along the ombrotrophic to minerotrophic gradient. However, S. angustifolium is most abundant under poorly minerotrophic conditions where there is less shade, while S. teres predominates under highly minerotrophic conditions and is more-or-less equally tolerant of shade and exposure. Sphagnum warnstorfii apparently is restricted to habitats which are highly minerotrophic, but is tolerant of both shade and exposure. The sphagna most characteristic of densely shaded, woodland habitats where there is little peat development are S. squarrosum, S. wulfianum, and S. girgensohnii. All three species generally form low mounds. Sphagnum fuscum and S. nemoreum form well-defined hummocks and are most prevalent under ombrotrophic conditions.