Masting behaviour in beech: linking reproduction and climatic variation

The question of what triggers masting in beech (Fagus) has been a source of uncertainty and curiosity. Analysing seed production series from Europe (Fagus sylvatica L.), eastern North America (Fagus grandifolia Ehrh.), and Japan (Fagus crenata Blume), for various periods (lasting between 6 and 34 years) over the last 150 years, we find a close relationship between masting (mast year) and preceding growing season climate events (mast year(-1) and mast year(-2)) in eastern North America and Europe, with tentative indications of this pattern in Japan. A drought in the early summer preceding masting (mast year(-1)) is a very strong predictor in Europe and eastern North America, but drought events were not found for the Japan series. The predictive power is increased in all three regions if there has been an unusually moist, cool summer the year before the drought (mast year(-2)). We suggest that, in this initial moist summer (mast year(-2)), carbohydrate buildup within the trees primes them for floral induction the following year (year(-1)). In the European and eastern North American series, a drought event in the early part of the following summer (mast year(-1)) acts as a proximal trigger for the release of those reserves into flower initiation and then seed production.