Origin of spatial genetic structure in an expanding oak population.

Spatial genetic structure (SGS) results from the interplay of several demographical processes that are difficult to tease apart. In this study, we explore the specific effects of seed and pollen dispersal and of early postdispersal mortality on the SGS of a seedling cohort (N = 786) recruiting within and around an expanding pedunculate oak (Quercus robur) stand. Using data on dispersal (derived from parentage analysis) and mortality (monitored in the field through two growing seasons), we decompose the overall SGS of the cohort into its components by contrasting the SGS of dispersed (i.e. growing away from their mother tree) vs. nondispersed (i.e. growing beneath their mother tree) and initial vs. surviving seedlings. Patterns differ strongly between nondispersed and dispersed seedlings. Nondispersed seedlings are largely responsible for the positive kinship values observed at short distances in the studied population, whereas dispersed seedlings determine the overall SGS at distances beyond c. 30 m. The paternal alleles of nondispersed seedlings show weak yet significantly positive kinships up to c. 15 m, indicating some limitations in pollen flow that should further promote pedigree structures at short distances. Seedling mortality does not alter SGS, except for a slight increase in the nondispersed group. Field data reveal that mortality in this group is negatively density-dependent, probably because of small-scale variation in light conditions. Finally, we observe a remarkable similarity between the SGS of the dispersed seedlings and that of the adults, which probably reflects dispersal processes during the initial expansion of the population. Overall, this study demonstrates that incorporating individual-level complementary information into analyses can greatly improve the detail and confidence of ecological inferences drawn from SGS.