Climate warming dampens masting-driven pulsed resources

Pulsed resources arise when environmental forcing synchronizes biological responses. This synchrony generates episodic booms and busts that structure food webs. Mast seeding is a major example, yet climate warming is increasingly disrupting the synchrony that underpins these pulses. Importantly, the ecological consequences of masting depend on which tail is synchronized: spatially coherent seed failures (synchrony in the lower tail) create trophic bottlenecks, whereas coherent mast peaks (upper tail) generate resource pulses that fuel consumer outbreaks. Climate-driven changes in synchrony may be tail-specific, reshaping not only the strength but also the character of resource pulses. Here, we test how warming-driven changes in European beech (\textit{Fagus sylvatica}) masting translate into tail-specific shifts in spatial synchrony and whether these shifts arise from altered coupling between weather cues and reproduction. Using 45 years of individual-tree seed production data from the United Kingdom and 33 years of seed harvest records from Poland, we found that, as predicted, synchrony declined strongly in mast peaks (44\% locally; 50\% regionally). However, synchrony also declined in failure years, though to a lesser extent (35\% locally; 25\% regionally) than in mast peaks. This asymmetry was not explained by increasing heterogeneity in responses to the warm-summer cue preceding flowering. Instead, reproductive dynamics shifted toward dominance of the cold-summer cue two years before seedfall, while sensitivity to the warm-summer cue weakened. This flattened previously nonlinear cue–reproduction relationships: reproduction increasingly occurred under conditions that formerly produced synchronized failure, and amplification during favourable years was reduced. Our findings show that warming alters the cue structures that generate masting-driven pulses, weakening and desynchronizing both failures and peaks, and reducing their predictability.