Key message <p><i>Enhanced seed production in a mast year suppressed nitrogen allocation to fine roots and leaves of beech trees by decreasing dry matter production and nitrogen concentration, respectively.</i></p> Abstract <p>It is well known that masting influences the productivity of aboveground vegetative organs, potentially altering carbon (C) and nitrogen (N) dynamics in forests. To examine whether masting affects C and N allocation to belowground organs, we investigated the production and concentrations of C and N of fine roots, along with those of leaves and seeds, in a Japanese beech stand during the period 2015–2017, with 2015 being a masting year. The results showed that C and N allocations to fine roots were comparable to or greater than those to leaves, reflecting the productivity of fine roots and leaves (ca. 260–490 vs. 310–340&#xa0;g m^− 2 year^− 1), while C and N allocations to reproductive organs were about 40–50% of those to fine roots and leaves in the masting year. Although C allocation to leaves did not change significantly across the three years, both C and N allocation to fine roots and N allocation to leaves were greater in 2016 compared to the other two years. Yearly variation in fine roots was attributed to fluctuation in fine-root production (FRP), whereas variation in leaves was explained by changing N concentrations. The higher leaf N concentration in 2016 might be a result of enhanced N uptake supported by increased FRP in this post-masting year. These results suggest that resource allocation to fine roots and leaves is negatively affected by masting, but is likely to recover in the post-masting year, although the data were obtained from only a 3-year observation under effects of yearly variations of environmental conditions.</p>