Population Dynamics of Oenothera Glazioviana in a Sand-Dune System with Special Reference to the Adaptive Significance of Size-Dependent Reproduction

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Abstract
(1) Oenothera glazioviana (a facultative biennial) behaves as a short-lived monocarpic perennial in an infertile sand-dune system at Azigaura, Japan. A 5-year census of population dynamics of the species in permanent sites on the semi-fixed dunes revealed that both annual mortality and relative growth rate were negatively correlated with plant size. (2) A stochastic simulation model was developed to describe the population dynamics of a facultative biennial. Life-history variables were estimated from demographic data on O. glazioviana growing in the dune system. In the model, yearly survivorship, time of reproduction, and fecundity of an individual were determined as functions of plant size. (3) Plant size showed a positively skewed distribution with a large variance. The instantaneous death rate after seedling establishment was fairly constant at about 0.5 year-1. The reproductive value steadily increased with age, reflecting increasing fertility with no increasing mortality. The intrinsic rate of natural increase (r) was 0.04 year-1. (4) Another set of simulations, in which timing of flowering was based on the critical plant size, showed that there is an optimal size for reproduction which maximizes r. The actual size at reproduction in the dune population of O. glazioviana was close to this size. The optimal size increased as the seedling emergence rate decreased. This result agrees with the prediction that in an enviroment imposing their juvenile mortality, delayed reproduction should be favored. (5) Comparisons of a size-specific model with an age-specific model revealed that size-specific reproduction always resulted in higher r. This advantage increases in a changing environment, because in size-specific reproduction, plants reproduce at different ages and use the changing environment as fine-grained, while in age-specific reproduction, plants reproduce simultaneously and use the changing environment as coarse-grained.