Insular patterns of calicioid lichens in a boreal old-growth forest-wetland mosaic

Fragmentation of the forested landscape poses a threat to many aspects of biodiversity associated with old-growth forests. Studies of the effects of forest fragmentation are often complicated by the variation in composition and age of patches and the matrix. This study used a system of isolated stands where patch age and composition were similar and the matrix variability negligible. The patches were composed of old-growth Picea abies stands of varying size and shape in a wetland matrix. The study organisms were epiphytic crustose calicioid lichens (also known as Caliciales), many of which are very substrate-specific and restricted to old-growth stands. The aim of the study was to measure the effect of patch size, patch isolation, habitat and substrate quality on the species richness and composition of epiphytic calicioids. Twenty-four patches ranging from 0.4 to 15.9 ha in size were studied. All species of calicioid lichens were registered in 0.1 ha plots in each patch. Isolation was measured as the percentage of available habitat within 400 m of a patch. Twenty-two species were found with an average of 9.48 +/- 0.26 (SE) species per patch and 2.92 +/- 0.18 (SE) species per tree. Species richness at patch level correlated with stand structure, primarily tree density, while number of species per tree (reflecting population size) was strongly correlated with island size and several stand variables. There was no effect of isolation on species richness. Species composition was influenced by both substrate variables and patch size. The species composition on the islands showed a significant nestedness, i.e. species composition on species-poor islands constituted a non-random subset of the species composition on species-rich islands. We propose that the explanation for the strong relationship between species richness at tree level and stand size is an edge effect which implies that unaffected interior areas only occur on large islands. The different microclimate of the patch edge enables only the hardiest species to establish large populations there whilst shade and moisture demanding species are restricted to the interiors of larger islands.