Primary succession of the warm-temperate broad-leaved forest on a volcanic island, Miyake-jima, Japan

The primary successional sere of a warm-temperate forest was inferred from an age sequence of four basaltic volcanic substrates (16-, 37- and 125-yr-old lava flows, and volcanic ejecta older than 800 years) at several altitudes (ranging from 30 to 550 m) on Miyake-jima Island. The DCA analysis of species composition indicated that the vegetation is patterned by substrate age and altitude. The vegetation successionally changed fromAlnus sieboldiana shrub, toMachilus thunbergii andPrunus speciosa forest, toCastanopsis sieboldii forest at all altitudes. The rate of succession seems to be slower at higher altitudes. The above-ground biomass was 20 kg/m² (160 m a.s.l.) and 12 kg/m² (300 m a.s.l.) on the 125-yr-old lava flow, and 54 kg/m² (100 m a.s.l.) and 16 kg/m² (350 m a.s.l.) on the old ejecta. However, the large difference of above-ground biomass between low versus high altitudes on the old ejecta might not only reflect climatic differences but also past human disturbances at the higher altitude. In spite of the colder climate of Miyake-jima the rate of above-ground biomass accumulation here was considerably faster than that of a Hawaiian montane tropical evergreen forest on the same basaltic lava substrate. We suggest that the faster above-ground biomass development is due to the facilitation effects of N-fixation byAlnus. The inorganic N soil concentration was extremely high on the 37- and 125-yr-old lava flows whereAlnus was dominant. The foliar N concentration inAlnus was 2% irrespective of substrate age. The deposition of N fromAlnus via litterfall would decrease the soil C/N ratio, which in turn facilitates net soil N mineralization and consequently provides an ample supply of inorganic N to plants. N limitation on vegetation development, which is prevalent during the early stage of succession on volcanic lava flows or similar substrates elsewhere, is thus alleviated.