Rolling Stones and Mosses: Effect of Substrate Stability on Bryophyte Communities in Streams

Effects of 3 disturbance parameters (% of the streambed in motion at bankfull discharge [PBF], catchment-specific discharge [SPECQBF], and frequency of bankfull floods [NBF] on aquatic bryophyte richness and community composition were assessed. Relationships between taxonomic richness and PBF and SPECQBF were strongly curvilinear, such that richness was low in both stable and highly unstable streams, and in streams of both low and high SPECQBF. Richness was highest in streams with moderate PBF and SPECQBF. Interspecific competition may have reduced species richness at stable sites. In contrast, only a few bryophyte taxa were able to grow at unstable sites. Low richness at sites with a low SPECQBF most likely reflects the intolerance of some species to desiccation. Low richness at sites with high SPECQBF may reflect differences in the resistance of different taxa to high discharges. Community composition (expressed as ordination scores), and presence/absence of individual bryophyte species and life forms also differed among streams of different PBF and SPECQBF. Thalloid or weft liverworts were common in streams with low PBF and high SPECQBF, whereas cushion-forming mosses were common in streams of high PBF and low SPECQBF. A conceptual habitat template model for bryophyte occurrences in streams is presented, which shows how communities changed predictably along the axes of PBF and SPECQBF.