Reconstruction of Forests buried by Fuji 1707 Tephra on the Southeastern Slope of Fuji Volcano, Central Japan

Abstract

The eastern and southeastern slopes of Fuji Volcano which are covered with Fuji 1707 Tephra lack subalpine coniferous forests. At three sites on the southeastern slope, geological and paleobotanical studies were made on the fossil forests buried under this tephra. This pyroclastic fall deposit ejected from the Hoei Craters of December 16, 1707 A.D. consists of a lower pumice bed (Ho-Ia) and upper scoria beds (Ho-Ib) in this area. The wood fossils and other plant macrofossils are buried in or beneath the Ho-Ia, which consists of many bombs and coarse pumice fragments, with a maximum thickness of 70cm. The occurrence of wood fossils and other plant macrofossils indicate that leaves, branches and stems were buried in that order in the lower pumice bed very rapidly. Most of the wood fossils were carbonized by the heat of the pumice fragments.
The forests which existed until December 16, 1707 A.D. were reconstructed. At site MK-4 (1745-1785m in alt.), there were stands composed of subalpine conifers such as Abies veitchii, Tsuga diversifolia and Picea jezoensis, one stand of Larix and one small stand of deciduous broad-leaved trees, that were distributed in patches. At site MK-2 (1680m in alt.), there was a forest of Abies, Tsuga diversifolia, and Picea jezoensis with a few broadleaved trees. At site MK-5 (1630m in alt.), there was a forest of Abies, Picea jezoensis, and Picea cf. maximowiczii with a few montane-zone elements such as Pterocarya or Pourthiaea.
These facts indicate that, prior to the eruption, subalpine coniferous forests and montane ones existed on the southeastern slope of Fuji Volcano, and that the boundary between the subalpine zone and the montane zone was at about 1650m in alt. around the three sites. The elevation of this boundary is lower than that of the present one on the southern slope of Fuji Volcano. Since then, the forests have been replaced by a Larix forest or a Quercus and Betula forest at sites MK-2 and MK-5 where the surfaces have become relatively stable; whereas, a volcanic desert still exists around site MK-4 situated in the course of an avalanche chute.