Effects of microhabitat and growth form on bryophyte mortality associated with leaf litter burial in a boreal spruce forest

Questions Do mortality rates of boreal bryophytes associated with leaf litter burial vary with degree of shelter on forest floor microsites in spruce-dominated forests? Do erect and prostrate species respond similarly? What is the relative importance of deciduous leaves (from aspen and birch) vs conifer needles as bryophyte mortality agents? Location Boreal spruce forests of central Sweden. Methods A transplant study examining mortality was set up with four different bryophyte species: two creeping, prostrate liverworts (Calypogeia integristipula and Lepidozia reptans) and two more erect mosses (Rhytidiadelphus triquetrus and Hylocomiastrum umbratum). We tested for differences in mortality rates associated with leaf litter burial in three microhabitats differing in degree of shelter on the forest floor, i.e. sheltered (large boulders, tree bases of spruce) and unsheltered (open forest floor). Results After two growing seasons, 12.7% of the bryophyte transplants (40 out of 316, of which 37 transplants were prostrate liverworts) were dead, presumably due to litter burial. These two prostrate liverworts displayed significantly higher mortality rates close to boulders and at tree bases compared to unsheltered forest floors. Furthermore, although only comprising a small portion of the canopy (<10%), deciduous litter accounted for 53% of all transplants that were buried by litter, compared to 47% for conifer needle litter. Conclusion We find fine-scale spatial patchiness in the responses of bryophytes to litter fall. Mortality of prostrate liverworts associated with litter burial varies among microhabitats on the forest floor, with significantly higher mortality at sheltered microsites (here exemplified by boulders and tree bases) compared to erect moss species. Bryophytes close to microtopographic structures (e.g. boulders and trees) are more strongly exposed to litter burial, but such structures can also function as refuges under disturbance events such as clear-cuts and windthrows. In addition, they may contain convex surfaces that accumulate less litter than flat or convex forest floor surfaces. The observed large effect of litter burial associated with bryophyte mortality, and the variation among microhabitats and species growth forms, suggest that incorporation of litter fall is of vital importance for our understanding of the dynamics of forest bryophyte communities.