Oak masting remains stable despite climate warming

Michał Bogdziewicz, Jessie Foest, Jakub Szymkowiak, Marcin Dyderski, Szymon Jastrzębowski, Andrew Hacket-Pain, Valentin Journe

Climate warming alters tree reproduction, but long-term tests of how it affects the large, synchronised interannual fluctuations in seed production known as masting remain rare. Theory predicts that climate-driven declines in masting should be most likely when species combine high reproductive sensitivity to weather cues with rapid climatic change during the corresponding cue windows. We tested this prediction using seed-production records of pedunculate and sessile oaks (\textit{Quercus robur}, \textit{Q. petraea}) from 389 sites in Poland spanning 1988–2022, and compared oak responses with sympatric European beech (\textit{Fagus sylvatica}), where warming-driven masting decline has been documented. Oak seed production responded to spring temperatures, but cue sensitivity was about 60\% lower than in beech. In addition, warming during the oak spring cue windows was weaker than warming during the main beech summer cue window, with March temperatures warming 36\% more slowly and April–May temperatures 68\% more slowly. Consistent with this weaker combination of cue sensitivity and climatic forcing, interannual variability in oak seed production did not decline over time. However, oak variability was lower in decades with warmer March temperatures, indicating that warmer cue regimes can still reduce masting variability when climatic shifts are sufficiently large. In contrast, European beech combined stronger cue sensitivity with faster warming in its main cue window and showed a clear decline in masting variability. These results support the hypothesis that climate-change effects on masting depend on the interaction between cue sensitivity and climate velocity. Because masting governs regeneration pulses and episodic resource supply for seed consumers, species differences in cue sensitivity may determine which forest regeneration processes and food webs are most altered by climate change.