Growth dynamics of shoot height and foliage structure of a rhizomatous perennial herb, Polygonum cuspidatum

The growth dynamics of shoot populations of Polygonum caspidatum were investigated at the Houei crater (approx. 2380 m above sea level) on the south-eastern slope of Mount Fuji. At this study site, a genetic individual of this species produces a population of shoots in the form of a patch occupying a certain ground area. Generally, genetic individuals are located away from each other and hence there is little interaction between individuals. A large-sized individual occupying 31.2 m2 ground area with shoots, a medium-sized individual (5.6 m2) and a small-sized individual (1.4 m2) were selected for this study. In each individual, growth was investigated at the shoot level. The results were analysed based on the diffusion model. Early in the growing season in 1990, there was little difference in LAI (leaf area index) and shoot density between the individuals. Shoots of the small- and medium-sized individuals showed size-independent height growth, whilst those of the large-sized individual showed size-dependent height growth. Consequently, small-sized shoots of the small- and the medium-sized individuals had greater RGRs of shoot height growth than those of the large individual at the early stage. As a result, in the small- and medium-sized individuals, cv (coefficient of variance) and skewness of shoot height decreased with time. Increases in cv and skewness of shoot height were found in the large-sized individual. The size-independent growth pattern of shoot height in the small- and medium-sized individuals during early growing stages is different from the growth pattern of non-clonal plant species, in which plant height growth is positively size-dependent. The existence of a regulatory mechanism of shoot height growth is suggested for the small- and medium-sized individuals. The foliage structure of the large-sized individual was different from that of the medium- and small-sized individuals. The foliage structure of small- and medium-sized individuals was similar to the theoretical "optimal foliage structure" of plants. In clonal plant species, a genetic individual occupies a certain ground area with its shoots. Therefore, "optimal foliage structure" per unit ground area brings about maximization of photosynthetic rate for a genetic individual, which is consistent with the maximization of fitness at the level of the individual plant.