Heat-driven declines in Scots pine cone and seed quality contrast with long-term climate adjustment

Sebastian Bury , Marcin K. Dyderski, Jessie Foest, Maciej Barczyk, Witold Wachowiak, Szymon Jastrzębowski, Adam Guziejko, Andrew Hacket-Pain, Michał Bogdziewicz

Climate change can alter forest regeneration through effects on seed production, seed quality, and the structures that protect seeds, yet most evidence focuses on seed output alone. We used 15 years of cone and seed-quality data from 184 Scots pine (Pinus sylvestris) populations to test whether climate effects extend beyond fecundity to reproductive traits that determine viable propagule supply. We separated among-population differences along long-term climate gradients (spatial effects) from within-population responses to seasonal weather anomalies (temporal effects). Across space, warmer climates were associated with heavier seeds (by ~8%) but lighter cones (by ~15%), whereas wetter climates were associated with heavier seeds (by ~10%) and cones (by ~20%). Temporal analyses revealed additional constraints: at warmer sites, hotter-than-average years reduced seed mass (by ~8%), cone mass (by ~14%), and the number of seeds per cone (by ~23%). Thus, heat can constrain Scots pine regeneration before germination by reducing the number of filled seeds, their provisioning, and investment in surrounding cone tissue. Spatial gradients are consistent with long-term adjustment of Scots pine reproductive traits to local climate, potentially through acclimation, genetic adaptation, or both, whereas temporal anomalies reveal near-term constraints that may intensify under rapid warming. Because Scots pine is one of Europe’s most important forest-forming species, these results point to near-term reproductive bottlenecks caused by heat and call for studies testing whether reduced seed number, seed size, and cone protection translate into lower regeneration under climate change.