Trade-offs between reproduction and growth influence time of reproduction in Oenothera biennis

Abstract

Size at which reproduction occurs was examined in two populations of Oenothera biennis occupying contrasting habitats: (1) a gravel embankment without complete canopy closure, and (2) a second-year successional field with a closed canopy. One hundred plants were randomly harvested from each site and weighed. A greater proportion of plants reproduced in the closed versus open canopy site (83 versus 61%). There was no difference in mean size at reproduction between the two sites; however, variation in size at reproduction was greater in the closed site where both the smallest and largest plants were found. In the smallest size category, more plants reproduced in the closed than in the open site, indicating that the minimum size required for reproduction is smaller in the closed site. We suggest that plants reproduce at a smaller size in the closed site because of the marked stem elongation associated with reproduction in this species, which increases the ability of plants to compete for light. This question was addressed in a garden experiment in which seedlings were transplanted into either weeded plots (i.e., an open canopy) or a successional field (i.e., a closed canopy). Half the plants at each site were left as vegetative controls and the other half treated with gibberellic acid to induce bolting. Bolting had no effect on growth in the open canopy but increased growth in the closed canopy as predicted. Key words: time of reproduction, Oenothera biennis, life history variation, stem elongation, bolting, size at reproduction, phenotypic plasticity.