IMPACTS OF SECONDARY SEED DISPERSAL AND HERBIVORY ON SEEDLING SURVIVAL IN AESCULUS-TURBINATA

Aesculus turbinata is a tree species with large seeds (6.2 g mean dry weight). We studied the demography of its seeds and seedlings in a tem perate deciduous forest in northern Japan to elucidate the ecological significance of large seeds with special reference to herbivory and se condary dispersal. Both seed and seedling stages suffered greatly from herbivores. Seedling herbivory was important judged from experiments with shoot clipping and hypogeal cotyledon removal. However, some seed lings survived through re-sprouting after herbivory. Survival rate and percentage resprouting seedlings were lower than those with remaining cotyledons, though seedling size was not affected. This suggests that stored resources in hypogeal cotyledons are working as a kind of 'ris k hedge' against severe above-ground shoot clipping experienced by A. turbinata. The spatial distribution of seedlings was expanded via seed scatter-hoarding by rodents. Seedling survival rate was higher within canopy gaps than under closed canopy, indicating that canopy gaps are safe sites for establishment, and was negatively correlated with seed ling density. Therefore, secondary seed dispersal in this species seem s to be effective in 'finding' safe sites and in 'escaping' density-de pendent mortality. The large seeds and seedlings of A. turbinata are a ttractive to herbivores, but the high resistance of seedlings to herbi vory due to large reserves and the effective secondary dispersal appea r to mitigate these disadvantages.