Boulders increase resistance to clear-cut logging but not subsequent recolonization rates of boreal bryophytes.

The extent to which a plant assemblage might recolonize a disturbed system is in general related to the availability of propagule sources and sites with appropriate conditions for establishment. Both these factors might be sensitive to aspects of spatial heterogeneity. Microtopographic variation may enhance initial resistance by reducing the impact of the disturbance and facilitating establishment of incoming propagules by providing shaded "safe-sites". This study explores the influence of microtopographic heterogeneity (caused by variation in surface boulder cover) on the recolonization of closed-canopy forest floor bryophytes using a chronosequence of 75 spruce-dominated forests in south-central Sweden (2-163 years after clear-cutting). We found that high boulder cover did increase survival and subsequent persistence in young forests at both investigated scales (i.e. 1,000 and 100 m(2)), although this pattern became less evident on the smaller spatial scale. Species accumulation in boulder-poor subplots was not different when surrounded by boulder-rich compared with boulder-poor subplots suggesting short-distance recolonization from boulder-created refugia to be of little importance during recolonization. To conclude, it seems that boulders increase initial resistance to clear-cutting for this bryophyte guild, but that the subsequent recolonization process is more likely to depend on external propagule sources and factors affecting establishment such as the microclimate in the developing stand.