Beyond seed counts: divergent climatic windows shape seed mass and viability in European beech
Mast seeding research has documented climate-driven changes in seed quantity, but tells us little about whether the seeds being counted are physiologically capable of germinating. The widespread practice of treating filled seeds as viable seeds has rarely been tested at scales adequate to detect divergent responses. Using a nationwide, 20-year dataset of 13,349 European beech (Fagus sylvatica L.) seed lots subjected to laboratory viability testing, we show that among filled seeds, mass and viability are only weakly correlated (Spearmans {rho} = 0.15) and respond to entirely different seasonal climatic windows. Viability tracked previous summer and harvest-year spring temperatures, while mass tracked neither yet declined markedly between 2009 and 2019, most steeply at northern latitudes. Pre-harvest climate also left lasting legacy effects on viability during storage. Together, these results reveal two independent dimensions of reproductive quality eroding under contemporary environmental change, invisible to count- or mass-based monitoring.