Substrate specificity has been pinpointed as key both in explaining plant species' distributions and in differentiating types of plant rarity. The relative importance of substrate specificity to moss occurrence and rarity was evaluated for the rare moss species, Mielichhoferia macrocarpa, Mnium arizonicum and Didymodon johansenii and the taxonomically allied common species, Bryum pseudotriquetrum, Mnium spinulosum and Didymodon rigidulus var. icmadophilus, respectively. Substrate pH and percent organic matter were measured within five sites for each species. Sensitivity to these two substrate parameters was tested by a fragment regeneration experiment on native and non-native substrates. Evidence from field plot data and establishment experiments further resolved the role of substrate specificity in limiting M. macrocarpa and D. johansenii. There was no clear trend relating rarity to substrate specificity. The rare M. macrocarpa and common D. rigidulus var. icmadophilus were determined to be most specific for substrate chemistry. These two species were the only species to have a significant substrate effect in the regeneration experiment. Although M. macrocarpa is sensitive to substrate chemistry, quadrat data and field establishment experiments both suggest that competition may be limiting its occurrence more directly. In contrast, Didymodon johansenii is less sensitive to substrate chemistry than the common D. rigidulus var. icmadophilus, yet it is clearly habitat specific as it never occurs on logs without a silt layer and occurs significantly more often on larger logs of older decay stages. Significantly higher regeneration on plots with versus without silt confirm the importance of a silt layer to D. johansenii and further suggest that silt initially may be most important in terms of securing the fragments to log substrates. Overall, these studies highlight that 1) substrate specificity is only one factor explaining species' occurrences and 2) observed substrate limitation sometimes may result from physical rather than physiological constraints on establishment.

Key words: mosses, rarity, substrate specificity