Graphical analysis of the life history evolution of Oenothera glazioviana Micheli.

Five-year census data of a semelparous Oenothera glazioviana population in a sand dune system were graphically analyzed. Changes in survivorship until seed production and potential fecundity were calculated from the size-dependent growth, survival and reproduction. The net reproductive rates after logarithmic transformation were plotted against the presumed age of reproduction. The optimal age and size at reproduction that maximize the intrinsic rate of natural increase were determined as a gradient of the tangent drawn from the origin. The natural population reproduced a little earlier than the calculated optimal age but at nearly the optimal size. Examination of the effect of environment on the optimal age and size revealed that an environment imposing higher mortality increases both the optimal age and size, while an environment imposing lower fecundity increases the optimal age but not the optimal size. The relatively large critical size for reproduction of O. glazioviana causes delayed reproduction in infertile environments, which helps the species maintain itself in a risky environment such as the sand dunes.