Does mast seeding rely on stored nitrogen reserves in Fagus crenata trees?

Background/Question/Methods

According to the resource budget model, the production of large numbers of seeds by masting trees probably depletes stored resources, and these are likely to take more than one year to restore. This is generally accepted to be the proximate mechanism of mast seeding. However, direct evidence of resource depletion in masting trees is very rare. Our previous work has demonstrated that carbon in fruits of three deciduous trees is exclusively built from current-year photoassimilates, indicating that resources other than carbon may be the trigger of masting event if internal reserves should be the physiological mechanism of mast seeding. Here, we estimate to what extent seed production relies on nitrogen (N) storage and/or on current-year uptake from soil by tracing seasonal variation of ¹⁵N isotope labeled N in both fruiting and non-fruiting mature trees of Fagus crenata Blume. The ¹⁵N (NH4Cl) containing water was sprayed carefully to the soil surface around six trees on 31 May, 20 days after bud burst. The fertilized N amount was equivalent to 10% of soil ammonium N.

Results/Conclusions

Nitrogen concentration in ¹⁵N labeled trees did not differ to adjacent non-labeled trees, indicating that the effect of ¹⁵N isotopic labeling on N dynamics was negligible. N concentration per unit dry mass did not differ between fruiting- and non-fruiting trees if the same organ such as leaves, current-year shoots and old trigs (1-3 year old) were compared. However, ¹⁵N abundance differed significantly between the two trees. In current-year shoots, ¹⁵N in fruiting trees was significantly higher early in the growing season than that in non-fruiting trees. This result implies additional uptake from the soil in response to stronger sink strength due to additional burden of fruits. In older twigs, ¹⁵N signal in non-fruiting trees was higher than in fruiting trees, suggesting prioritized allocation of current-year absorbed N to seeds in fruiting trees. These results indicate that compensatory N uptake from the soil contributed greatly to mast seeding. Together with our previous finding that nitrogen storage does not deplete after masting, resource storage may not be the dominant limiting factor controlling masting in F. crenata.