Effects of weather conditions on mast year frequency in beech (Fagus sylvatica L.) in Sweden

During the last two decades, an increase of the frequency ofnaturally regenerated beech seedlings has been reported. Thismay be due to an increased masting. In this investigation insouthern Sweden, mast year frequency, mast crop size and therelationships between mast year and climatic variables werestudied. Our analysis shows that while the average mast yearinterval was 4–6 years from the end of the seventeenthcentury up to the 1960s, the mean interval has decreased to2.5 years during the most recent 30 years, and there have beentwo consecutive mast years on two occasions during this laterperiod. Mast years have often followed years in which the temperaturein July and September was higher than the 30-year mean. However,there were significant variations in the amounts of beechnutsproduced between different mast years, and beechnut productionincreased with increasing site index. Climatic changes, especiallyincreases in temperature, may have been responsible for thehigher frequency of mast years, but increased atmospheric nitrogendeposition may also have been a contributory factor.