Niche diversification of Sphagnum relative to environmental factors in northern Minnesota peatlands

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Abstract
The peatlands sampled in this study are located in northern Minnesota and include 32 stands positioned in 7 sites in the Red Lake peatland and Lake Itasca areas. Surface water chemistry ranges from pH values of 4.6 to 7.4, with corrected conductivity of 53 to 476 .mu.ohms cm-1 and Ca of 6.9 to 44.5 ppm. Six broad physiognomic landscape units are delimited based on quantitative analysis of the vegetation; these range from ombrotrophic ovoid islands to strongly minerotrophic forested fens. These landscape units can best be further divided or themselves characterized by the dominant Sphagnum spp. Calculations of niche breadth across 8 microhabitat axes and niche overlap for 4 microhabitat axes (pH, corrected conductivity, height above H2O level and shade) for 13 spp. of Shagnum [S. angustifolium, S. centrale. S. contortum, S. fallax, S. fuscum, S. magellanicum, S. nemoreum, S. papillosum, S. rubellum, S. teres, S. subsecundum, S. warnstortii, S. wulfianum] suggest independent species utilization of the gradients. Among the mire-expanse species, niche breadths become narrower from hollow to hummock along Ca and pH gradients, whereas broadest niche breadths are present for midhummock species along the height gradient. For the taxonomically close species, S. fallax and S. angustifolium, niche overlap is greatest for conductivity and pH, whereas overlap is least along the height gradient. Quantification of niche breadth and niche overlap indicates considerable niche diversification along these microhabitat gradients, with individual species interacting independently to different gradients. These differing niche breadths along several different gradients suggest that these species of Sphagnum are largely equilibrium species, not opportunistic ones. In general, niche overlap is smaller in mire habitats where an abundance of bryophytes coexist with Sphagnum and highest in mire-expanse situations where Sphagnum is dominant.

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