Forest fecundity declines as climate shifts

Tree fecundity underpins regeneration, range tracking, and seed supply for assisted migration, yet may decline as climates move beyond reproductive niches. Using 34 years of nationwide harvest records from Poland (40,530 observations across 438 forest districts) for five dominant taxa — oaks (Quercus robur, Q. petraea), European beech (Fagus sylvatica), Scots pine (Pinus sylvestris), and silver fir (Abies alba) — we tested whether sustained climate change has reduced fecundity after accounting for seed demand. Mean viable seed production declined by 32–65% across species (oaks ~65%, pine ~64%, fir ~44%, beech ~32%). Summer warming was the dominant driver, with hotter summers reducing seed output across all species. Growing-season moisture and spring temperature contributed little to long-term trends, although they shaped local responses. Weather effects varied with background climate, indicating divergence between short-term (within-site, transient) and long-term (across-site, equilibrium) sensitivities. This modulation by local climate indicates substantial capacity for local adaptation or acclimation, offering actionable leverage for management. Together, our results show fecundity declines consistent with warming, pushing populations beyond reproductive climatic niches, but also identify potential to mitigate risk by aligning provenance choice and assisted migration with projected site climates.