Oak masting under climate change: when spatially heterogeneous shifts in flowering phenology reshape pollen limitation.

Masting, the irregular, synchronised production of seeds within tree populations, drives forest regeneration and ecosystem dynamics. Its response to climate change may depend on phenological shifts in key reproductive stages, yet this link remains poorly understood. Using 30 yr of daily airborne pollen data from 20 temperate oak populations (Quercus petraea and Q. robur), we examined the relationship between shifts in pollen phenology and pollen limitation (estimated from annual pollen concentration). We analysed how these changes were driven by site-specific weather trends during the forcing and diffusion periods. Pollen phenology advanced overall, while pollen limitation decreased. On average, pollen concentration nearly doubled in 30 yr (+93%) yet with striking contrasts among populations. During the study period, late-flowering populations experienced significant warming during the preseason period, leading to earlier phenology but stable pollen concentration. Conversely, early-flowering populations saw little change in preseason temperatures or phenology but experienced sharp warming during the diffusion period, resulting in a strong increase in pollen concentration. These findings demonstrate that heterogeneous local climate trends induce divergent pollen phenology and pollen limitation trajectories, reshaping reproductive conditions differently among populations. Consequently, oak masting and forest regeneration success may respond differently across populations under continued climate change.