Beyond seed counts: divergent climatic windows shape seed mass and viability in European beech

Forest regeneration depends not only on how many seeds trees produce, but on the physiological quality of those seeds. Yet while climate-driven shifts in seed quantity and masting have received sustained attention, the parallel question of whether climate change degrades seed quality remains poorly resolved. Using a nationwide dataset of seed mass and viability in European beech (Fagus sylvatica L.) collected between 1996 and 2024 (13,349 seed lots from 381 forest districts across Poland), with climate-quality analyses focused on 5,374 freshly harvested seed lots from 353 districts (2004-2023), we asked whether the two components of seed quality respond to different seasonal climatic windows, and whether harvest-year climate also shapes seed performance during long-term cold storage. Seed mass and seed viability were only weakly correlated (Spearmans {rho} = 0.15), acting as two independent dimensions of seed quality. Both revealed substantial temporal variation over the study period, but along distinct trajectories. Seed mass declined markedly between segmented-regression breakpoints in 2009 and 2019, more steeply at higher latitudes, coinciding spatially and temporally with the masting breakdown reported at the species northeastern range margin. Climatic associations were correspondingly divergent. Viability was positively associated with previous summer temperature, consistent with temperature-cued flower initiation, and negatively with spring temperature in the harvest year, plausibly reflecting thermal disruption of early embryogenesis. Seed mass showed no significant association with any seasonal climatic predictor, indicating control by slower or unmeasured processes. Storage duration progressively reduced viability, and this decline was further modulated by climate during seed development, with seeds developing under climatically suboptimal conditions losing viability faster. These results expose a hidden decoupling between seed quantity and seed quality under contemporary climate change, with direct consequences for forest regeneration and for ex situ conservation strategies that assume mast-year seeds will remain viable for decades.