PATTERNS OF BRYOPHYTE ASSOCIATIONS AT DIFFERENT SCALES IN A NORWEGIAN BOREAL SPRUCE FOREST

Patterns of associations between 36 bryophytes and their relationships with trends in alpha- and beta-diversity were studied at five spatial scales (from 1 m2 to 1/256 m2) in a Norwegian boreal spruce forest. The range and dispersion of alpha-diversity values in the data were significantly higher than in comparable model data sets, indicating varation from favourable to unfavourable conditions for bryophytes at all scales, particularly with increasing cover of the upper layers. The number of positive associations was significantly higher than predicted from a random distribution, for all sample plot sizes except the smallest. For the most frequent species, this number decreased with decreasing sample plot size. The excess of positive associations was due to the presence of (1) a-diversity trends, as demonstrated by a stochastic simulation, and (2) beta-diversity (variation along environmental gradients). A microscale coenocline from dominance of large forest floor mosses to small liverworts, is added to two broad-scale coenoclines demonstrated earlier. Facilitation - positive density-dependence caused by more favourable moisture conditions within dense stands - is discussed as a possible third cause of excess positive associations. The number of significant negative associations was generally low, and deviated neither from theoretical values assuming random distribution of species, nor from predicted values accounting for alpha-diversity trends. The low proportion of negative associations, even in the presence of beta-diversity trends, suggests that interspecific competition is not important in this vegetation. Several alternatives to competition are discussed; (1) static mechanisms for avoidance of competition, (2) mobility, and (3) the importance of density-independent mortality, in particular due to fine-scale disturbance. Simulation studies for assessing the effects of a-diversity trends on species associations are emphasized.