The end of boom-and-bust? Observed and projected declines in European beech masting

Reproduction is vital for forest resilience to climate change, as tree populations depend on adequate seed production to recover from disturbances and migrate to more suitable sites. Neglecting reproduction in projections of habitat suitability and range shifts risks overestimating forest resilience to climate change. For many tree species, including European beech (Fagus sylvatica), viable seed production depends on year-to-year variability in seed output (CVp), known as masting. Analysing data from 328 sites (average record length: 32.7 years), we find that rising summer temperatures in Central Europe are associated with declines in masting. Using the identified relationship between masting and summer temperatures, we project changes in masting across the species range under contemporary and future climates. The risk of masting decline is predicted to be widespread across the species' range, due to ubiquitous summer warming, but the risk is highest in the central belt of the distribution range (~40\% decline in CVp). Masting disruption is projected to occur across all populations under future climate scenarios. To mitigate the impacts of disruptions in masting patterns, it is essential to monitor recruitment in these high-risk zones while testing forest management strategies that may mitigate the effects of masting decline. With masting crucial for regeneration, seed consumer population dynamics, and behaviour, its disruption under climate change could have far-reaching ecological impacts.