DEMOGRAPHIC GENETICS OF A PIONEER TROPICAL TREE SPECIES - PATCH DYNAMICS, SEED DISPERSAL, AND SEED BANKS

We consider whether changes in population-genetic structure through th e life cycle of Cecropia obtusifolia, a tropical pioneer tree, reflect its gap-dependent demography and the role of evolutionary processes t hat are important for this species. We asked whether the spatial scale at which population-genetic subdivision occurs corresponds to the sca le of habitat patchiness created by gap dynamics; whether patterns of seed dispersal and storage in the soil affect spatial genetic patterns : and whether spatial genetic patterns change through the species life cycle. We estimated Wright's F-statistics for six successive life-his tory stages for individuals grouped into subpopulations according to o ccurrence in natural gaps, physical proximity, or occurrence within la rge quadrats. For each life stage, F-ST-statistics were significantly higher when individuals were grouped by gaps, although concordant patt erns across life stages for the three grouping methods were obtained. This supports the hypothesis that patchy recruitment in gaps or among- gap heterogeneity influences the species' genetic structure. F-statist ics of seeds collected from females before dispersal (tree seeds), see d-rain seeds, soil seeds, seedlings, juveniles, and adults grouped by gaps, were, respectively: F-IT = 0.004, 0.160, 0.121, 0.091, -0.0002, -0.081; F-IS = -0.032, 0.124: 0.118, 0.029, -0.016, -0.083; and F-ST = 0.035, 0.041, 0.003, 0.063, 0.015, 0.002. Spatial genetic differentia tion in rain seeds was not significantly lower than that of tree seeds . The loss of genetic structure in the soil seed bank, relative to tha t found in the seed rain may be due to sampling artifacts, but alterna tive explanations, such as microsite selection or temporal Wahlund eff ect are also discussed. If structure among soil seeds is unbiased, the peak in seedling F-ST may be due to microsite selection. F-IS of seed s in the rain and soil were significantly greater than zero. A Wahlund effect is the most likely cause of these positive F-IS values. Such f ine-scale substructuring could be caused by correlated seed deposition by frugivores. The decrease in F-IS from seedlings to adults could re sult from loss of fine-scale genetic structure during stand thinning o r from selection.