Tail-dependence of masting synchrony results in continent-wide seed scarcity

Abstract Spatial synchrony is the tendency of spatially separated populations to display similar temporal fluctuations. Spatial synchrony may be tail-dependent, meaning it is stronger for peaks rather than troughs, or vice versa. High interannual variation in seed production in perennial plants, called masting, can be synchronized at subcontinental scales. Resulting resource pulses and periods of seed scarcity have important but distinct ecosystem consequences that are amplified by their scale of synchrony. In this study, we used data from 99 populations of European beech (Fagus sylvatica) sampled across the species range to examine whether masting synchrony differs between mast peaks and years of seed scarcity. The association between masting and the weather cue driving regional synchrony was stronger in the upper tails (corresponding to large values of the driver), translating into synchrony of mast peaks reaching 1000 km. However, the synchrony of seed scarcity was even higher, exceeding 1800 km. Consistently low seed production across a large range of low cue values homogenizes seed production across extensive spatial scales. Extensive synchrony in the masting lower tail means that famines caused by beech seed scarcity are amplified by their extensive spatial synchrony, with diverse consequences for food web functioning and climate change biology.