Forest tree 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. How fecundity has changed under recent climate change remains unclear. Here,  using Polish seed harvest data including 40,530 observations for five species across 438 districts, we show a reduction in viable seed production by 32–65% across 34 years (Quercus robur, & Q. petraea ~65%, Pinus sylvestris ~64%, Abies alba ~44%, Fagus sylvatica ~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 on fecundity varied with site climate, with the diverging within-site (transient) and across-site (equilibrium) responses suggesting local adaptation or acclimation capacity. Together, our results indicate warming-driven fecundity declines, suggesting that climate change has pushed populations beyond the optimum of their reproductive niches, while revealing management potential through informed provenance selection.