For decades, forest ecology has focused mainly on plant growth and survival. Variation in seed production was largely ignored, and forest models often assumed that seeds were available even in the absence of adults. However, the long-term stability of forests depends on tree reproduction, which produces the next generation of plants and determines the composition of future communities. In addition, the seed production of many tree species varies greatly from year to year. In some years, tree species reproduce abundantly and synchronously, creating a bounty that reverberates through the ecosystem for years. Such a year of extremely high yield is called a ‘mast year’. Mast seeding is very often synchronized over large areas, up to thousands of kilometers. This variation introduces pulses of resources into ecosystems that are amplified through food webs and have important consequences, both ecological (e.g. dramatic changes in the abundance of granivores) and socio-economic (e.g. affecting the risk of Lyme disease in humans by influencing the number of disease-carrying ticks). The phenomenon is attracting considerable scientific attention, but fundamental questions about the nature of masting remain unresolved. The main aim of my current project is to identify additional drivers of seed production and its variation. To achieve this, I am using mostly a compilation of databases of seed production representing a large diversity of climatic conditions and a variety of species traits. Our work provides a step change in the identification of processes responsible for global variation in forest reproductive patterns, both between and within species, and will provide future insights into the diversity of 21st-century forests.