Clonal fertility variation and its effects on the effective population size in the seed orchard of dioecious species, Fraxinus rhynchophylla

The numbers of female and male flowers were assessed in a clonal seed orchard of Fraxinus rhynchophylla for four consecutive years from 2004 through 2007. The female and male flowers of the grafts from 48 clones were counted individually over the crown. Fertility variation was calculated by sibling coefficient that is related to the coefficient of variation in parental flower assessment. Estimated maternal and paternal fertilities were not constant but varied from year to year, and they were also found to be weakly correlated. The effective numbers of clones serving as male and female parents were calculated from the sibling coefficients of maternity and paternity. We applied two theoretical approaches to estimate the clonal effective population sizes based on 1) fertility variation (i.e., effective number of parents, N^(c)) and 2) gender balance (effective population size, N_e^e) among parental genotypes. The values of N_e^(c) were higher in the years with abundant flowering and seed production, while sibling coefficients were lower. On average (pooled), the maternal, paternal and clonal values of N_e^(c) were calculated to be 17.3, 21.0 and 38.1 respectively. The N_e^(c) for maternal gamete gene pool was ranged from 9.8 to 16.7. The values of N_e for paternal gamete gene pool were generally higher (ranged from 14.9 to 21.9) than those for maternal. When pooled, the N_e was 47.2 and the values of N_e were higher when the gender ratio was more balanced. An option of equal seed harvest might be good to mitigate fertility variation and to improve gene diversity of seeds.