Study done with Parmelia sulcata, P. caperata and Xanthoria parietina attached to roofing slates. These results suggest that elevated nutrient levels can affect lichen growth rates and competition between thalli, leading to alterations in lichen community structure., Nutrient Enrichment, Competition, Parmelia, Xanthoria, Flavoparmelia, Saxicolous, British Isles, Fertilizer