Summer solstice orchestrates the subcontinental-scale synchrony of European beech (Fagus sylvatica) mast seeding

Tree mast seeding i.e., synchronous, highly variable seed production among years, has wide consequences for ecosystem functioning. In a large-seeding year, a pulse of resources is made available to wildlife over the large spatial scale by virtue of the synchronous reproduction by millions of trees. But how trees synchronize masting over the vast geographical areas? A major mechanism governing the annual allocation of resources to seed production is weather variation. If individuals respond to the same “weather cue” across extensive regions, masting synchrony can emerge. This requires, however, the timing of the cue window being well conserved across species range, but mechanisms facilitating such stability remain unknown. Here, we investigated factors driving masting synchrony in European beech, which extends to the thousands of kilometres throughout its geographic range. We used a moving window analysis to determine how correlations between annual seed production and mean temperatures in 61 populations of European beech sampled across the species' range fluctuate at a fine temporal scale. We found that correlation coefficient values between seed production and temperature rapidly increased after the summer solstice, compared to before it. Moreover, using temporally-restricted permutation tests we showed that this abrupt increase in masting-weather correlations at the solstice is not driven purely by chance. Beech achieves high spatial synchrony in seed production by anchoring the weather cue window to the summer solstice - the longest day of the year that occurs simultaneously across the whole Northern Hemisphere. Beech abruptly opens its temperature-sensing window on the solstice, hence widely separated populations all start responding to weather signals in the same week. This enables cohesive timekeeping across distant populations inhabiting diverse climatic regions and creates a high precision timing of the Moran effect, leading to the subcontinental-scale synchrony of beech mast seeding.