Fruiting phenology uncoupled from seasonal soil nitrogen supply in masting Fagus crenata trees

Masting is a widespread reproductive strategy in perennial plants. Its mechanism remains largely uncertain, although it is generally accepted that internal nutrient dynamics play a role. We aimed to examine whether soil nitrogen (N) supply influences plant N acquisition and drives masting.

We measured spatio-temporal variations in soil N mineralization rate and plant N uptake by roots using plant root simulator probes on the periphery of six fruiting and four non-fruiting trees in a mature Fagus crenata Blume forest. In parallel, N concentrations and biomass growth in leaves, current-year branches, cupules, nuts and winter buds were analysed for the same trees.

Soil N mineralization rates exhibited significant seasonal variations in both nitrate- and ammonium-N, and varied spatially up to 38-fold between spots sampled on the same date and 39-fold for the same tree in a growing season. Fruiting did not enhance N uptake by roots, and a dramatic decrease in soil N mineralization rate was observed in autumn during nut ripening. Translocation from cupules supplemented this N shortfall in mature nuts. Consequently, the amount of N amount per whole shoot, but not in ripe nuts per shoot, was correlated with the cumulative soil inorganic N.

Masting is influenced ultimately by internal resource dynamics, which are decoupled from soil N dynamics. This may be a result of evolutionary selection to insulate trees from the high variation in local soil nutrients to ensure reproductive success.