POPULATION DYNAMIC CONSEQUENCES AND EVOLUTION OF SEED TRAITS OF HETEROSPERMA PINNATUM (ASTERACEAE)

We summarize research on variation in achene morphology of Heterosperma pinnatum. Each flowering head has a broad range of achene morphologies. There is no between-year seed bank. There are no ecologically relevant differences in achene size, seedling size, or seedling growth rates of the different morphs. Achenes located centrally in the heads lose dormancy earlier during the period between autumn achene production and the onset (May-July) of the rainy season. This results in relatively more early emergence of central achenes. Awned central achenes also have greater adhesive dispersibility. Emergence, survival, and fecundity were measured for achenes sown into natural populations in central Mexico. When harsh conditions result in few seeds surviving to reproduce, the advantage tends to go to the more “conservative” peripheral achenes. Greenhouse experiments show a strong genetic component to the determination of achene proportions within and among populations and a variety of genetic correlations. Populations with a high proportion of awned achenes tend to be found in vegetation types where they are likely to be ephemeral. A high proportion of central achenes tend to be found in sites with little pre-rainy season precipitation. These patterns are interpreted in terms of selection for dispersal and the within-year timing of germination.