Edge effects on liverworts and lichens in forest patches in a mosaic of boreal forest and wetland

In studies of edge effects, it is difficult to separate edge from size effects because size and edge are strongly correlated in most cases. In our study we separated these two effects by examining small forest patches of two different size classes (<1 ha and 4-6 ha ), with patches chosen so that area and perimeter were not correlated within each class. We conducted our study in a mosaic of old-growth Picea abies forest and wetland, consisting of forested moraine hills (islands) in a Sphagnum-bog matrix. On each island, we demarcated a 0.1-ha sample plot and measured site and forest characteristics. In each plot, we examined all fallen logs for the occurrence of epixylic hepatics (liverworts), and all standing trees for the occurrence of calicioid lichens. We examined correlations between species and area, shape, and distance to nearest edge. The effect of shape on forest-interior conditions was analyzed with a core-area model. Site and forest characteristics were similar between the island size groups. The majority of hepatics and lichens occurred more frequently on larger islands, and the cover of hepatics tended to be higher on larger islands. A rough estimation of depth-of-edge influence was made at 50 m, but the two species groups did not respond similarly to edge effects. The lichens showed several important correlations with island shape on small islands, suggesting that circular small islands may have a forest structure approaching interior conditions. Hepatics tended to respond to small islands as edge environments. We conclude that the response of species to edges is strongly species-specific and context-dependent. This points to the limitations of extrapolating results from studies on depth-of-edge influence to forest-management situations.