Partitioning the role of climate, pollution and old-growth woodland in the composition and richness of lichen epiphytes in Scotland

This paper presents a study to partition the role of three regional-scale drivers woodland extent and continuity, pollution regime, and climatic setting - in explaining the composition and richness of lichen epiphytes in Scotland. To do this we used partial canonical correspondence analysis and multiple least squares regression, to examine lichen communities across 170 study sites. First, our results demonstrate the importance of climate in explaining species composition. This highlights the relatively clean-air environment of Scotland within a European setting, and emphasizes the important consideration of regional context in the development of bioclimatic species-response models. This result contrasts with a previous similar study which collapsed complex environmental data into summary gradients, and which therefore discounted climate as a key factor. Second, we show a functional decoupling between composition and species richness, which was optimally explained by old-growth woodland extent and pollution, and only weakly explained by climate. The difference in explanatory variables between composition and richness is a focal issue in determining the processes by which species compositional change, driven by rapid and deep climate change, may indirectly impact species richness. For example, this impact may occur through an imbalance in rates of species extinction (for sensitive range-edge species) and establishment in a fragmented landscape (for dispersal-limited colonists), though operating against the 'stabilizing effect' of microclimatic setting.