Resource pulses are rare events with a short duration and high magnitude that drive the dynamics of both plant and animal populations and communities1. Mast seeding is perhaps the most common type of resource pulse that occurs in terrestrial ecosystems2, is characterized by the synchronous and highly variable production of seed crops by a population of perennial plants3,4, is widespread both taxonomically and geographically5, and is often associated with nutrient scarcity6. The rare production of abundant seed crops (mast events) that are orders of magnitude greater than crops during low seed years leads to high reproductive success in seed consumers and has cascading impacts in ecosystems2,7. Although it has been suggested that mast seeding is potentially synchronized at continental scales8, studies are largely constrained to local areas covering tens to hundreds of kilometres. Furthermore, summer temperature, which acts as a cue for mast seeding9, shows patterns at continental scales manifested as a juxtaposition of positive and negative anomalies that have been linked to irruptive movements of boreal seed-eating birds10,11. Here, we show a breakdown in synchrony of mast seeding patterns across space, leading to asynchrony at the continental scale. In an analysis of synchrony for a transcontinental North America tree species spanning distances of greater than 5,200 km, we found that mast seeding patterns were significantly asynchronous at distances of greater than 2,000 km apart (all P < 0.05). Other studies have shown declines in synchrony across distance, but not asynchrony. Spatiotemporal variation in summer temperatures at the continental scale drives patterns of synchrony in mast seeding, and we anticipate that this affects the spatial dynamics of numerous seed-eating communities, from insects to small mammals to the large-scale migration patterns of boreal seed-eating birds.