Relationships between productivity, number of shoots and number of species in bryophytes and vascular plants

1 We measured species density, biomass and shoot density for both bryophytes and vascular plants in 90 small plots in 18 calcareous fens. In addition, we recorded leaf area index and litter mass of vascular plants. Our goals were: (a) to compare the relationship between biomass and species density for the two taxonomic groups, (b) to test whether biomass and species density of bryophytes and vascular plants are related to their shoot density, and (c) to assess the degree to which biomass, shoot and species density of bryophytes are correlated with characteristics of the vascular plant layer. 2 For bryophytes there was a positive linear relationship between biomass and species density. Vascular plant species density was not related to biomass. Furthermore, bryophyte biomass and species density were linearly and positively related to bryophyte shoot density. For vascular plants, only biomass but not species density was related to shoot density. 3 We concluded that a bryophyte favourability gradient existed along which biomass and shoot and species density increased. This gradient was attributed to positive interactions within dense bryophyte stands, high clonal fragmentation, absence of competitive hierarchies and to the limited ability of larger bryophyte species to replace small species along this favourability gradient. 4 Since species density for vascular plants varied independently from biomass and shoot density, there was no such favourability gradient as for bryophytes. Large size variation, predominantly negative interactions between species, and clonal integration of species (e.g. tussock-forming grasses and sedges) may be responsible for the different behaviour of the two taxonomic groups. 5 Bryophyte favourability decreased with increasing vascular plant biomass. Concerning tight availability, we found highest bryophyte favourability at intermediate levels where the combination of radiation and moisture seems to be optimal for bryophytes. No relationship was found between bryophyte favourability and vascular plant shoot density and litter mass. 6 The negative relationship between bryophyte favourability and vascular plant biomass is important for bryophyte conservation. Stands of low vascular plant production are those with the potential for highest species richness, and should therefore receive conservation priority.