RAINFALL AND FLOWERING SYNCHRONY IN A TROPICAL SHRUB - VARIABLE SELECTION ON THE FLOWERING TIME OF ERYTHROXYLUM HAVANENSE

We tested the adaptive significance of flowering synchrony by means of a quantitative analysis of selection and by flowering induction exper iments with the deciduous shrub Erythroxylum havanense. Temporal sched ules of flower and fruit production were determined for a local popula tion (in three sites) in a Mexican seasonal forest for 2 years (1987-1 988). The consequences of natural variation in flowering time (floweri ng initiation day) on maternal reproductive success (fecundity) were e valuated. We observed high levels of inter- and intraindividual flower ing synchrony in 1987, but not in 1988 and this contrast was related t o differences in rainfall patterns between the two years. A significan t proportion (15.4%) of the phenotypic variation in flowering initiati on day was accounted for by environmental variance. The expression of phenotypic variance of flowering time and, consequently, the opportuni ty for selection to act, are controlled by annual variation in rainfal l. Despite the between-year difference in flowering synchrony, we dete cted a relatively intense directional selection on flowering initiatio n day in both years, but selection coefficients were of opposite sign (standardized directional gradients were -0.326 and 0.333 for 1987 and 1988, respectively). For both years there was a significant relations hip between individual relative fitness and the number of neighbouring flowering plants in a given day, suggesting positive frequency-depend ent selection.