Size-Dependent Effects in the Analysis of Reproductive Effort in Plants

Size-dependent variation in reproductive effort (RE) appears to be a common phenomenon in many plant species, but researchers have not fully appreciated the factors causing such variation and the potential significance of these patterns to plant ecology and life history. Data from desert winter annuals and the reexamination of published work indicate that many plant populations can be characterized by a significant linear relationship between absolute reproductive biomass and vegetative biomass. The form of this allometric relationship results in variable RE with size (monotonically increasing or decreasing) in many of these species. Both increasing and decreasing RE-with-size patterns are found in both iteroparous and semelparous species, but theoretical expectations for such patterns in different plants remain undeveloped. Some of the conclusions of previous studies of RE in plants may be in error if size-dependent effects were not taken into account. The graphical-regression approach presented here provides a simple means of testing for size-dependent effects within populations and for assessing the significance of differences in size-dependent patterns among populations.