Evolution of life histories of semelparous herbaceous plants

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1. We investigated genetic basis of size-dependent reproduction of semelparous herbaceous plants based on demographic analyses.2. From pot culture experiment, we found that size and age of reproduction of Aster kantoensis are determined by the family and nutrient conditions ass well as their interactions.3. A demographic study on the population of Lobelia boninensis in Bonin Islands revealed that the yearly variations in the population dynamics are most affected by the survivorship of rosette plants. In spite of large year-to-year variations in the population size, the numbers of seeds produced by the whole population were relatively constant. Although the population size increased by ca. 8 times during the two years, the population growth rates estimated by a matrix model predicted rapid decline of the population, mainly due to the high mortality among small sized rosettes.4. We analyzed spatial patterns of the mortality of a biennial, Lysimachia rubida endemic to the Bonin islands. Competition among neighbor plants caused suppression and eventual death of individuals in areas of high local densities. The spatial heterogeneity of soil surface conditions caused density-independent and site-dependent mortality and leaded to clumped distribution of plants.