Future Forests Start with Seeds: Warming-Driven Disruption of Forest Fecundity

Forest resilience under climate change depends on adequate seed production, yet it remains largely unclear how tree fecundity is changing and how this may constrain current and future range expansion. Using a long-term dataset spanning broad climatic gradients, we show that climate warming is already disrupting reproductive synchrony and fecundity across Europe.In European beech (Fagus sylvatica), we analysed seed production records from 341 sites (mean record length 31.7 years) to test how climate change is altering masting dynamics. We show that increasing frequency of the main reproductive cue strongly erodes year-to-year variability and synchrony in seed production, particularly at colder, high-latitude and high-elevation sites. In these regions, masting variability has declined by up to ~54%, with SSP2.45 projections for 2070 indicating reductions of up to ~83%. With masting underpinning the production of pollinated, unpredated seeds, this challenges the assumption that cold-range margins are refugia from climate impacts and indicates that disruption of reproductive dynamics is likely to become the norm for this species.Extending beyond a single species, we used a temporal attribution framework to analyse three decades of fecundity change in five dominant taxa (40,530 annual observations, 348 sites) for oaks (Quercus robur, Q. petraea), European beech, Scots pine (Pinus sylvestris), and silver fir (Abies alba). Across all species, viable seed production declined by 32–65%, with summer warming emerging as the dominant driver. Growing season drought and spring temperature had comparatively minor effects. Weather effects varied with climate, indicating diverging short-term (within-site) and long-term (across-site) sensitivities, and suggesting potential for local adaptation or acclimation.Together, these results show that reproduction may emerge as a key bottleneck for forest resilience under climate change, as warming drives populations beyond their optimum reproductive niches. Integrating reproductive processes into forest projections and management is needed to avoid overlooking critical transitions in forest dynamics.