Masting, or mast seeding, refers to the phenomenon in which perennial plants exhibit high variability in fruit production across years. Phosphorus (P) is a critical nutrient for plant reproduction, yet its role in masting has received less attention in temperate forests, where plant growth is typically limited by nitrogen rather than P. In this study, we present evidence that leaf P (and also twig P) contributes to fruit development in Japanese beech (Fagus crenata), a deciduous tree species that shows distinct cycles of fruiting and non-fruiting years. Through the continuous monitoring of a Japanese beech forest on Mount Naeba, we found a decline in leaf and twig P concentrations during fruiting. This decline was absent in non-fruiting individuals or during a non-fruiting year, suggesting that leaves and twigs act as internal P sources supporting fruit production. The reductions of leaf and twig P concentrations were associated primarily with decrease in organic P, indicating that organic P pools were degraded and re-translocated to meet reproductive demand. This trend was especially strong in twigs, whereas the decline in leaf P was partly associated with the reduction of inorganic P. In the talk, we further discuss how these reductions integrate with shoot-level P allocation.