Density regulation during the regeneration of two monocarpic bamboos: self‐thinning or intraclonal regulation?

. The population dynamics of two monocarpic bamboos, Sasa kurilensis and S. tsuboiana , were studied for more than 10 years after establishment following mass flowering. Both species show vigorous rhizomatous vegetative reproduction after growing up to maturity, but horizontal expansion in the seedling stage was much more vigorous in S. tsuboiana than in S. kurilensis. The pattern of changes in culm density in the two species was strikingly similar: culm densities of both species increased until they reached full‐density states, after which they decreased in accordance with seedling growth. However, the mode of regulation in culm density was different. S. kurilensis seedlings were composed of only a few culms and scarcely extended their rhizomes during the observation period. Such poor lateral expansion resulted in asymmetric competition as observed in many non‐clonal plants, and consequently their culm density decreased as a result of the mortality of genets due to self‐thinning. In S. tsuboiana seedlings, the number of culms per genet increased considerably by frequent tillering and sprouting from rhizomes. However, after reaching full density state, the Bud Utility Ratio (BUR), (the proportion of the rhizome nodes with culms to the total number of rhizome nodes), decreased drastically. In this manner, S. tsuboiana regulated culm density intraclonally as is observed in the stable states of many clonal plants. Hence it is important for the understanding of the regeneration process in clonal species to clarify when and how their seedlings extend rhizomes during their growth.