Methods: Eight crustose lichen species were surveyed on 165 Quercus robur trees, 17-478 years old, at three study sites. The occurrence patterns of these species were examined at two spatial scales: among trees and within trees. Occurrence patterns within trees were examined in 10 cm × 10 cm plots at all four cardinal aspects at different heights from 0.5 to 4.5 m above the ground.Results: At the tree level, age-related factors were the most important predictors of species occurrence. All species were more frequent on trees > 100 years than on younger trees. At the plot level, the frequency of occurrence increased with increasing bark crevice depth. The frequencies of all study species decreased with increasing cover of bryophytes.Bryophytes were in turn more frequent in plots that were exposed to rainwater and showed a low evaporation rate. Patches most exposed to rainwater were directed upwards, and the lowest evaporation rates occurred on the northern side of the trunks.Conclusions: For many crustose lichens the association with old oak trees seems at least partly to depend on their preference for the deep bark crevices that only occur on old trees. Trees represent epiphyte habitat patches that differ in size due to within-tree variability in habitat quality, such as bark crevice depth and microclimate. This study shows that variability at a finer scale, within habitat patches, contribute to explain species occurrence patterns at habitat patch level.

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This study provides a unique large dataset of total epiphytic lichen diversity (fruticose, foliose and crustose species) and composition on 1,294 trees of 17 tree species in wooded meadows in Sweden and Estonia, the Baltic region. The inventory (25,380 observations and 246 lichen taxa) clearly illustrated that Ulmus minor, Quercus robur and Fraxinus excelsior contributed most significantly to epiphytic lichen richness and number of red-listed species. In Sweden, average single tree a richness was 22.2 on Ulmus (only in Sweden), 21.6 on Quercus (25.0 in Estonia) and 19.8 on Fraxinus (16.7 in Estonia), respectively. Ulmus hosted on average one red-listed species per tree, compared with 0.7 on Fraxinus (0.6 in Estonia), 0.4 on Quercus (0.7 in Estonia) and only 0.05 on Betula (same in Estonia). Lichen species composition and the average number of red-listed lichens were influenced by tree diameter on Fraxinus and Quercus, whilst no such pattern was evident on Ulmus. Randomized species accumulation curves of the dominating tree species illustrated that Fraxinus, Quercus and Ulmus supported a dominated lichen communities where individual trees hosted a substantial part of the total richness. Betula, on the other hand, supported b dominated communities where individual trees tended to be dissimilar and, therefore, more of the total richness existed as species turnover among host trees. Lichen species composition was influenced by tree species, and most notably, lichen species on Ulmus had a strong consistent clumping in ordination graphs, with many rare and red-listed lichens. The broadleaved deciduous trees within the wooded meadows clearly contribute greatly to the biodiversity of the Baltic region.

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Summary We assessed the relative importance of habitat quantity, quality and isolation for the distribution and local abundance of two epiphytic lichens, Parmelina tiliacea, which is red‐listed in Sweden, and the common Pleurosticta acetabulum. We predicted that habitat isolation should constrain the distribution of the mainly vegetatively dispersed P. tiliacea more than the sexually dispersed P. acetabulum. All patches of habitat containing suitable host trees for the study species were mapped from aerial, infra‐red photographs. Presence and abundance of the lichens were recorded for a total of 3237 trees in 94 patches. Patch identity had strong influence on presence of both species on individual trees. Within patches, presence was positively correlated with tree size and was also influenced by tree species. At patch‐level, species presence was positively correlated with tree number and negatively correlated with the proportion of the boundary that bordered coniferous forest and with isolation. Abundance was mostly correlated with tree size or with the number of large trees at tree‐ and patch‐levels, respectively. For P. acetabulum, within‐patch abundance was also influenced by isolation from other patches. Presence at patch‐level was affected by isolation for both species, but this effect differed between the three isolation measures used. There was no clear evidence that the distribution of P. tiliacea was more constrained by isolation than that of P. acetabulum. Whilst this study emphasizes the importance of habitat quantity for species occupancy and local abundance, it shows that explanations of species occurrence in fragmented landscapes must also involve variables describing habitat quality and spatial configuration. Moreover it illustrates the difficulties in predicting species’ occupancy from general knowledge about their dispersal traits.

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Summary 1We assessed the relative importance of habitat quantity, quality and isolation for the distribution and local abundance of two epiphytic lichens, Parmelina tiliacea , which is red-listed in Sweden, and the common Pleurosticta acetabulum . We predicted that habitat isolation should constrain the distribution of the mainly vegetatively dispersed P. tiliacea more than the sexually dispersed P. acetabulum . 2 All patches of habitat containing suitable host trees for the study species were mapped from aerial, infra-red photographs. Presence and abundance of the lichens were recorded for a total of 3237 trees in 94 patches. 3 Patch identity had strong influence on presence of both species on individual trees. Within patches, presence was positively correlated with tree size and was also influenced by tree species. 4 At patch-level, species presence was positively correlated with tree number and negatively correlated with the proportion of the boundary that bordered coniferous forest and with isolation. Abundance was mostly correlated with tree size or with the number of large trees at tree-and patch-levels, respectively. For P. acetabulum , within-patch abundance was also influenced by isolation from other patches. 5 Presence at patch-level was affected by isolation for both species, but this effect differed between the three isolation measures used. There was no clear evidence that the distribution of P. tiliacea was more constrained by isolation than that of P. acetabulum . 6 Whilst this study emphasizes the importance of habitat quantity for species occupancy and local abundance, it shows that explanations of species occurrence in fragmented landscapes must also involve variables describing habitat quality and spatial configuration. Moreover it illustrates the difficulties in predicting species' occupancy from general knowledge about their dispersal traits.

Question: Which environmental and historical variables affect epiphytic lichen diversity in managed and unmanaged wooded meadows? Location: The island of Gotland located in the Baltic Sea east of the Swedish mainland. Methods: We examined total epiphytic lichen diversity (crustose, foliose and fruticose species) on 1148 trees in eight grazed, eight traditionally managed (mowing, hay gathering and pollarding) and seven unmanaged wooded meadows. In addition to management, data on site location, habitat structure, history and adjacent habitat were analysed. Results: Lichen species richness increased with wooded meadow area and was greater within managed sites than in unmanaged meadows. Historic crown cover (ca. 1930) also influenced present‐day lichen richness. Geographic location, distance to sea, wooded meadow area and average tree circumference were important determinants of lichen species composition. Tree circumference was the strongest overall predictor of the number of species on individual trees. However, tree circumference interacted with management regime, whereby cessation of management appeared to reduce species richness most on large trees. Traditionally managed sites, on average, supported the greatest richness of red‐listed species. Conclusions: Management regime, wooded meadow area and canopy cover were the main drivers of lichen species richness, while geographic location, wooded meadow area, distance to sea and average tree circumference were important determinants of species composition.