Rolling stones and mosses: effect of substrate stability on bryophyte communities in streams

Effects of 3 disturbance parameters (% of the streambed in motion at bankfu ll discharge [P-BF], catchment-specific discharge [SPECQ(BF)], and frequenc y of bankfull floods [N-BF]) on aquatic bryophyte richness and community co mposition were assessed. Relationships between taxonomic richness and P-BF and SPECQ(BF) were strongly curvilinear, such that richness was low in both stable and highly unstable streams, and in streams of both low and high SP ECQ(BF). Richness was highest in streams with moderate P-BF and SPECQ(BF). Interspecific competition may have reduced species richness at stable sites . In contrast, only a few bryophyte taxa were able to grow at unstable site s. Low richness at sites with a low SPECQ(BF) most likely reflects the into lerance of some species to desiccation. Low richness at sites with high SPE CQ(BF) may reflect differences in the resistance of different taxa to high discharges. Community composition (expressed as ordination scores), and pre sence/absence of individual bryophyte species and life forms also differed among streams of different P-BF and SPECQ(BF). Thalloid or weft liverworts were common in streams with low P-BF and high SPECQ(BF) whereas cushion-for ming mosses were common in streams of high P-BF and low SPECQ(BF). A concep tual habitat template model for bryophyte occurrences in streams is present ed, which shows how communities changed predictably along the axes of P-BF and SPECQ(BF).