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 of naturally regenerated beech seedlings has been reported. This may be due to an increased masting. In this investigation in southern Sweden, mast year frequency, mast crop size and the relationships between mast year and climatic variables were studied. Our analysis shows that while the average mast year interval was 4–6 years from the end of the seventeenth century up to the 1960s, the mean interval has decreased to 2.5 years during the most recent 30 years, and there have been two consecutive mast years on two occasions during this later period. Mast years have often followed years in which the temperature in July and September was higher than the 30-year mean. However, there were significant variations in the amounts of beechnuts produced between different mast years, and beechnut production increased with increasing site index. Climatic changes, especially increases in temperature, may have been responsible for the higher frequency of mast years, but increased atmospheric nitrogen deposition may also have been a contributory factor.