Population Biology of the Clonal Moss Hylocomium Splendens in Norwegian Boreal Spruce Forests. II. Effects of Density

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1 Hylocomium splendens is a perennial clonal moss in which new modules may emerge annually by ramification of the current mature segment or by regeneration, the time-delayed development from dormant buds. Demographic information on 14 776 growing points was obtained by following 119 Norwegian boreal spruce forest populations for 5 years. The dry weight of each mature segment was estimated from morphological field measurements. 2 The mean size of mature segments was positively correlated with the density (cover) of the bryophyte layer; a density increase from 0 to 100% was associated with a doubling in mean size. This relationship is attributed to the improved moisture conditions, and hence to prolonged periods with positive net photosynthesis, in dense stands. Mean size was not significantly related to the density of Hylocomium splendens growing points, suggesting that interactions in bryophyte stands are mainly with neighbours regardless of species. 3 The frequency of regeneration declined with increasing density, notably when the effects of segment size were corrected for. Negative density-dependence of regeneration is considered to be due to reduction of light-induced bud initiation in dense stands. 4 The size-corrected mortality (termination and loss) rate was not related to density. Danger of burial in the moss carpet, causing about one tenth of all terminations and losses, increased strongly with density and affected c. 5% of all growing points at high densities. 5 At high densities, Hylocomium splendens populations are not regulated by a negative relationship between size and density, but by a combination of reduced branching (ramification and regeneration) and increased danger of burial in the bryophyte carpet. However, such high densities are infrequent in the forest floor (affecting less than 10% of the area investigated). It is suggested that differences in history due to finescale disturbance and microenvironmental variation are reflected in size differences of individual Hylocomium splendens segments, and that this is the major factor that keeps boreal forest bryophyte populations at low densities.