Background and aims – Large variation in annual berry production occurs in Vaccinium myrtillus L., and the variation has been associated with population cycling of many herbivores. Because of its ecological significance, variation in berry production has frequently been related to climatic variables. Methods – In an attempt to elucidate the possible causality of such relations, we have examined the seasonal and climatic control of growth rhythm and flowering performance of V. myrtillus in the field and in a controlled environment. We then reanalysed two long-running time series of berry production, with selected climatic factors as explanatory variables. Key results – Variables retained in the regression models were maximum temperature in June and mean temperature in August–September the year before fruiting, and maximum snow depth in April, minimum temperature in May, and hydrothermal ratio in June–July the current year. These factors could all be directly linked to critical plant processes and events such as timing of floral initiation, winter hardening, avoidance of frost injury during bloom, and drought avoidance in summer. Demonstration of superficial winter dormancy explained the vulnerability of V. myrtillus to winter frost injury and the need for stable snow cover. A highly significant lunisolar index corresponds to oscillations in ionizing cosmic radiation that by some unknown mechanism(s) may affect plant growth and development. Conclusions – We conclude that the explanatory variables obtained in the regression models for annual berry production are causally related to specific physiological mechanisms controlling crucial events in the annual life cycle of V. myrtillus .

BERRY PRODUCTION; BILBERRY; CLIMATE; FLOWER FORMATION; GROWTH RHYTHM; TIME SERIES; VACCINIUM MYRTILLUS