Mechanisms of Central Die-back ofReynoutria japonicain the Volcanic Desert on Mt. Fuji. A Stochastic Model Analysis of Rhizome Growth

Reynoutria japonica (Polygonaceae) is a pioneer clonal herb colonising the volcanic desert on Mt. Fuji (height 3776 m), central Japan; establishment of secondary successional species occurs only in the central die-back parts of the clonal stands of R. japonica. Clonal stands were excavated and the morphology and growth pattern of the rhizomes were investigated with special reference to the mechanisms of central die-back. Four morphological parameters, length of mother rhizomes, and number, branching positions and branching angle of daughter rhizomes on respective mother rhizomes were measured or counted, and their roles in rhizome growth were examined employing a stochastic computer simulation model of the whole stand development. The simulations clarified that, of these four parameters, the branching angle was the most influential determinant of the whole pattern of shoot distribution and that the central die-back was produced when the mean branching angle was 40° or smaller. These results strongly infer that the onset of central die-back is caused intrinsically by R. japonica itself by having the mean branching angle of 40°. The adaptive significance of the growth pattern is discussed in relation to resource acquiring and habitat exploitation strategies of this species.