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Prolonged periods of air pollution depleted soil Ca and increased N supply in forests of Eastern North America and Central Europe. We compared a cool rural to a warm urban forest site to explore the geographic range of foliar N, Ca, and N/Ca on naturally regenerated saplings of American beech and European beech in Nova Scotia (Canada) and Saarland (Germany), respectively. Foliar N/Ca of beech saplings remained variable due to geoclimatic gradients and bio-climatic site factors, tight internal cycling of Ca, and varying pollution history. Our data suggest that deep and extreme soil Ca depletion renders climatic factors the drivers of foliar N/Ca in beech forests. Global warming will therefore amplify the potential risks of changing nutrient supply ratios in regions with a prolonged history of severe acid rain. Existing forest site classifications usually capture regional differentiation of climatic factors but often fail to assess impacts of acid rain history on foliar Ca that will be required for regional differentiation of foliar nutrient ratios on acid soils. However, homogenization of foliar Ca can be expected in regions with a history of severe acid rain.