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The contributions of the internal nitrogen (N) cycle and N uptake from soil to growth in mature trees remain poorly understood, especially during reproduction. In order to elucidate how reproduction affects N uptake, allocation and remobilization, we applied pulse 15N labelling to three fruiting (F) and three non-fruiting (NF) Fagus crenata Blume trees after the leaves were fully unfurled. Three-year-old branches were sampled from upper crowns at about 2 week intervals until leaf fall. 15N content per organ dry mass (15Nexcess) and N concentration in all new shoot organs were determined. Fruiting led to greater 15Nexcess uptake from the soil during the first month following application. Cupules absorbed the highest fraction of 15Nexcess initially and nuts contained about half the 15Nexcess at the end of the growing season. Biomass of reproductive organs represented up to 70% of new shoot growth in F trees. This fruit burden led to 34% and 38% reduction in biomass and 15Nexcess, respectively, in mature leaves compared with NF trees. Moreover, the increment of 15Nexcess in new shoots of F relative to NF trees was lower than the increment of biomass between the two. These results indicate that N is a limiting resource during masting in F. crenata. 15Nexcess incorporated into nuts started to increase dramatically once 15Nexcess in leaves, branches and cupules hit seasonal maxima. Similar seasonal biomass growth patterns were also found in these organs, indicating that sink strength drives uptake and allocation of 15Nexcess between new shoot compartments. These results, together with translocation of 15Nexcess from cupules and senescing leaves to nuts (contributing to fruit ripening), suggest that a finely tuned growth phenology alleviated N limitation. Thus, fruiting did not influence the N concentration in leaves or branches. These reproduction-related variations in N uptake and allocation among new shoot compartments have implications for N dynamics in the plant-soil system.

Influence of reproduction on nitrogen uptake and allocation to new organs in Fagus crenata