Central Die-back of Monoclonal Stands of Reynoutria japonicain an Early Stage of Primary Succession on Mount Fuji

Abstract

Reynoutria japonicais a common perennial pioneer species on Japanese volcanoes. In a volcanic desert (1500m above sea level) on Mount Fuji (3776m), central Japan, this species forms circular stands (patches). As a patch develops, shoot density decreases in its centre (‘central die-back’). The central die-back has been considered a key process in the early stages of primary succession, though its mechanism has been unknown. The pattern of patch development, population dynamics of aerial shoots, and growth patterns of below-ground organs were analysed in order to investigate the mechanism of die-back, and the following traits are clarified: (1) central die-back areas occur in most small patches (approx. 1m 2) without later successional species; (2) shoot characteristics are dependent both on their position within a patch and on patch size; (3) despite the large differences in shoot density, neither time course of shoot growth nor their mortality differs between the centre and periphery of patches; and (4) rhizomes of R. japonicagrow outwards with regular sympodial branching. From these results, it is concluded that neither interspecific nor intraspecific competition is likely to be a primary cause of the die-back phenomenon, but that central die-back is brought about intrinsically by the growth pattern of the rhizome systems. We also discuss the importance of the central die-back in facilitating establishment of later successional species in the early stages of primary succession.