Effect of gap size and regeneration niche on species coexistence in bryophyte communities

Gaps in the dominant bryophyte cover on forest floor logs are produced by decay and animal activity. Differential exploitation of disturbance gaps promotes the coexistence of two epixylic moss species, Tetraphis peliucida and Dicranum flagellare. Dicranum flagellare predominates in small, dry gaps located on log tops while T. peliucida achieves greatest dominance on larger, moist gaps on log sides. Reciprocal transplant experiments demonstrate that adult survivorship does not explain the species distribution. Regeneration patterns are found to be responsible for the observed niche partitioning. Patterns of gap utilization are correlated with contrasting reproductive modes. Dicranum flagellare relies primarily on asexual propagules and T. peliucida utilizes a mixed sexual and asexual reproductive strategy. Propagule dispersal is distinctly patterned with regard to microtopography. Tetraphis peliucida gemmae occur primarily in gaps on the sides of logs, while propagules of D. flagellare predominate on log tops. Colonization success by asexual propagules differs with substrate. Gemmae of T. pellucida germinate well only under moist conditions, while D. flagellare can tolerate drier substrates. Relative abundance of the two species varies with availability of the appropriate regeneration niche, which is a function of the local disturbance regime. Rodent activity may both produce the gaps favored by D. flagellare and facilitate its dispersal to them. Dominance of T. pellucida is associated with larger gaps produced by decay. The interaction of the regeneration niche and the disturbance regime are found to be primary determinants of community structure on this unstable substrate.