Tuesday, August 7, 2007

PS 31-156: Time to reproduce: A phylogenetic comparative study

Kjell Bolmgren and Peter D. Cowan. University of Calfornia at Berkeley

Parents face a timing problem as to when they should begin devoting resources from their own growth and survival to mating and offspring development. Seed mass and number, as well as maternal survival via plant size, are dependent on time for development. Flowering time may thus represent the outcome of such a time partitioning problem. Furthermore, the optimal flowering time is expected to differ between plant life and growth forms, as flowering time is a seasonal trait while resources for growth and reproduction may be stored between years. We analyzed correlations between flowering time, seed mass, and plant height in a north-temperate flora, using both cross-species comparisons and phylogenetic comparative methods. Among perennial herbs, flowering time was negatively correlated with seed mass and positively correlated with plant height, while no such correlation was found among woody plants. Among annual plants, flowering time was positively correlated with seed mass. Cross-species and phylogenetically informed analyses largely agreed, except that flowering time was also positively correlated with plant height among annuals in the cross-species analysis. The different signs of the correlations between flowering time and seed mass (compar.gee regression coefficient = -7.8) and flowering time and plant height (compar.gee regression coefficient = +30.2) for perennial herbs, indicate that the duration of the growth season may underlie a trade-off between maternal size and offspring size in perennial herbs.