Interpreting reproductive allometry in plants

Background/Question/Methods Size-dependent or allometric relationships between reproductive and vegetative size are extremely common in plant populations. Reproductive allometry where plant size differences are due to environmental variability has been interpreted both as an adaptive strategy of plant growth and allocation, and as the product of fixed developmental constraints. Patterns of development are crucial in defining reproductive allometry. Reproductive allometry can be a product of a minimum size for reproduction and a linear relationship (with constant slope) between reproductive and vegetative size. After reproduction is initiated, relative increases in reproductive size are greater than relative increases in vegetative size - a difference that diminishes with increasing plant (vegetative) size. Since the relationship between plant size and reproductive size within populations is frequently linear with a positive x-intercept, this linear model is a persistent and important mechanism explaining observed allometry in reproductive allocation. However, development is not fixed across individuals. For example, environmental adversity (e.g. resource impoverishment) tends to favor reproduction at relatively small sizes. Delaying the onset of reproduction could result in total reproductive failure if plants are killed before they reproduce, or if environmental adversity prevents a plant from achieving the optimal size for the initiation of reproduction. I examined the importance of variation in the size at reproduction in interpreting reproductive allometry in plant populations.

Results/Conclusions I demonstrate that where individual plants within a population initiate reproduction at different sizes, the absence of population wide reproductive allometry is an appropriate null expectation. Significant reproductive allometry occurs in plant populations where initiating reproduction at small sizes yields relatively high or low reproductive size at final development. Both of these outcomes are common in plant populations. My interpretation of reproductive allometry combines previous adaptive and developmental constraint interpretations, and is the first to successfully explain the range of relationships between reproductive and vegetative size observed in plant populations.