Environmental Effects on Growth and Fruit Production in Phlox Drummondii

(1) To determine how differences in size and reproduction arise in natural stands of Phlox drummondii, 368 plants were tagged at time of seedling emergence. A variety of morphological and life-history traits were measured on each plant during the growing season. Fruit production was used as a measure of female reproductive success. The immediate biotic environment of individual plants was assessed in several ways. (2) Fruit production was closely related to final plant size, and final size was influenced by plant size and environmental variables at earlier points in development. (3) The magnitude and direction of environmental effects on plant growth changed during the growing season. The relationship between environment, development and fruit production differed between the two sites. (4) No single stage of development (e.g. seedling establishment) was critical in establishing differences in reproductive success among individual plants. (5) These results suggest that causal models for the origin of variation in size and reproduction in plant populations must consider multiple environmental and developmental factors, and that relationships among these factors will often vary from site to site and from one season to the next.