CLEAVITT, NATALIE L. Department of Biological Sciences, University of Alberta, Edmonton, AB T6G 2E9. - Disentangling moss species limitations: the role of substrate specificity.
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
Key words: mosses, rarity, substrate specificity