Climate warming reduces seed mass in European beech through altered resource dynamics and drought

Michał Bogdziewicz, Cherine C. Jantzen , Jakub Szymkowiak, Jess Farmer, Laura Nescoi, Marcel E. Visser , Andrew Hacket-Pain

Seed mass is a key life-history trait that influences dispersal, seedling establishment, and plant fitness, yet its long-term response to climate change remains poorly understood. We used two long-term datasets of European beech (\textit{Fagus sylvatica}) from the United Kingdom and the Netherlands (1976-2024) to test whether seed mass has changed over time and whether any decline can be attributed to changing climate or altered resource dynamics. Seed mass declined by 13\% over the study period, with a non-linear decrease starting in the mid-2000s. Models of year-to-year variation showed that seeds were smaller in years with lower reconstructed stored resources and in years that were dry during the period of seed filling (May- June). Temporal contribution analysis indicated that the long-term decline in seed mass was driven primarily by declining resource availability, with increasing drought making an additional contribution. Climate warming affects seed size in beech by direct effects of drought on seed development, and indirectly by altering reproductive dynamics and shortening the intervals available for resource recovery between reproductive events. Because seed mass influences dispersal and survival of seeds and seedlings, declines in seed mass are likely to reduce regeneration potential. Our findings identify an indirect pathway by which climate change may weaken forest regeneration.