Masting affected intra- and inter- annual variations in stored nitrogen reserves in Fagus crenata trees

Background/Question/Methods

Mast seeding potentially depletes plant-internal reserves of one or more resources and a plant might need several seasons to accumulate enough reserves to support the next mast event. A very recent study 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 interval reserves should be the physiological mechanism of mast seeding. A major candidate for such a limiting resource is nitrogen (N) for the following reasons: close correlation between leaf N and photosynthesis, dependence of leaf flushing on internal stored N, the large proportion of N at individual level allocated to reproductive organs in masting years and shortened interval of masting events by periodic fertilization. We tested this hypothesis in mature F. crenata trees by tracing seasonal and inter-annual variations in N in branches, stem and roots for five years including three masting events.

Results/Conclusions

N concentration per unit dry mass in branches decreased during the spring flush and restored afterwards. In contrast, N concentration had less seasonal variation in stem and exhibited different patterns of seasonal change in roots. There was some degree of inter-annual fluctuation when comparing N after leaf-fall in the same tissues, which was related to mast seeding in branches and stems but not in roots. The mobile N pool, which was calculated from the difference in N concentration between N after leaf fall and N during the leaf flushing of the same shoot in the following spring, was affected by mast seeding. These results indicate that mast seeding caused the decrease in stored N reserve in F. crenata and further research should be carried out in order to elucidate if nitrogen is the trigger resource of mast seeding.