Biomass and net primary production of a <Emphasis Type="Italic">Pinus densiflora</Emphasis> forest established on a lava flow of Mt. Fuji in central Japan

We assessed the above- and below-ground biomass and net primary production (NPP) in a mature (85-year-old) Pinus densiflora forest established on a lava surface of Mt. Fuji in central Japan. The nitrogen (N) concentration of the forest soil was low (1.25%), and the mean soil carbon/nitrogen (C/N) ratio was 34.2; therefore, both plants and microorganisms would compete for N in our research forest. The total biomass was 192.62Mgha^–1, of which 67.28% was in the stems and 25.71% was in the roots. The fine-root biomass was 1.12% of the total biomass. The total NPP of the forest reached 11.89Mgha^–1 year^–1, which fell within the values reported for other cool temperate P. densiflora forests established on non-volcanic-related substrata. The below-ground production was about 39% of the total NPP; the value was relatively small under the conditions of low total N concentration and high soil C/N ratio. Our study suggested that P. densiflora could recruit and grow on geologically new substrata without increasing the allocation of its annual carbon budget to below-ground organs (i.e., roots).     

Nitrogen use by <Emphasis Type="Italic">Pinus densiflora</Emphasis> trees growing on a Mt. Fuji lava flow

To quantify the nitrogen (N) use by Pinus densiflora trees growing on an infertile lava surface, N pools, N requirement and N uptake through fine roots and N deposition from the atmosphere were estimated. The N requirement and the N uptake of fine roots were 55.5kgNha^–1year^–1 and 39.7kgNha^–1, respectively. Thus, the ratio of N uptake to N requirement of the fine roots was 71.5%. Including fine-root contribution, the total N requirement of the P. densiflora trees was 98.6kgNha^–1year^–1, and the total N uptake was 64.2kgNha^–1year^–1. Thus, the N uptake of the P. densiflora trees was 64.1% of the N requirement, indicating that P. densiflora trees growing on an infertile lava surface obtain some of their N from below-ground organic material layers every year and the contribution of N storage in trees for their growth is not any higher than indicated in previous reports that excluded fine-roots contribution. The wet N deposition of our research forest was only 5.8% of the N requirement of the P. densiflora trees and only 8.9% of the N uptake. Movement of the below-ground organic material layer N concentrations in the F- and L-layers coincides with needle development and fine-root growth, suggesting the possibility that P. densiflora trees extract N from the organic N of those layers for growth.