The synchronous production of large seed crops by a population of plants1,2, known as mast-seeding, and synchronous tree-ring growth3,4 within sites are well known phenomena among trees in the temperate zone. But information about the geographic or taxonomic extent of such synchronous growth or reproduction, or about the geographic extent of switching between them, is sparse. We have detected synchrony in growth and reproduction, both within and among genera of Northern Hemisphere boreal trees, across geographical areas almost the size of a continent. Furthermore, we found a significant negative correlation between seed production and tree-ring growth at sites up to 1,000 kilometres apart, implying that there are trade-offs between them. This discovery suggests that mast-seeding is an evolved strategy that occurs on a geographic scale far larger than previously suspected5.