Forest fecundity declines as climate shifts

Jessie Josepha Foest, Jakub Szymkowiak, Marcin Dyderski, Dave Kelly, Georges Kunstler, Szymon Jastrzębowski, Michał Bogdziewicz

Tree fecundity underpins regeneration and range tracking, yet may decline when climates exceed reproductive niches. Using 34 years of Polish harvests (40,530 observations across 438 districts) spanning oaks (Quercus robur, Q. petraea), European beech (Fagus sylvatica), Scots pine (Pinus sylvestris), and silver fir (Abies alba), we tested whether climate change has changed fecundity. 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 fecundity across species. Growing-season moisture and spring temperature contributed little beyond local fecundity effects. Weather effects varied with climate, indicating diverging within-site (transient) and across-site (equilibrium) sensitivities. This suggests local adaptation or acclimation capacity, offering actionable management leverage. Together, our results show warming-driven fecundity declines, pushing populations beyond optimal ranges of their reproductive niches, and suggest potential scope for mitigation through informed provenance selection