Climatic Drivers of Mast Fruiting in European Beech and Resulting C and N Allocation Shifts

European beech shows mast fruiting at intervals of 2–20 years with a recent increase in frequency. It is not precisely known which climatic or endogenous factors are the proximate causes of masting. We recorded fruit mass production in 11 beech stands across a climate gradient over 4 years, analyzed the influence of climatic, edaphic, and stand structural parameters on fructification, and quantified carbon (C) and nitrogen (N) allocation to leaf and fruit mass production. The solar radiation total in June and July of the year preceding a mast year (JJ) was the parameter most closely related to fruit mass production, whereas no influence was found for drought. Radiation induced flowering and subsequent fruit production in beech apparently through a threshold response when the long-term mean of June-July radiation was exceeded by more than 5%. Full masting was associated with a significantly smaller leaf size and stand leaf area in the mast year and it significantly lowered foliar N content in the mast and post-mast year. We conclude that radiation totals and the N status of the foliage jointly govern the temporal pattern of masting in beech, presumably by controlling the photosynthetic activity in early summer. Anthropogenic increases in N deposition and atmospheric [CO] thus have the potential to increase masting frequency which can substantially alter forest productivity and forest biogeochemical cycles.