Forest fragmentation modifies genetic structure of the population by inbreeding and random genetic drift. The effects of inbreeding depression occur successively in different developmental stages, suggesting reduced levels of inbreeding in later developmental stages due to enhanced mortality caused by inbreeding depression. To test the above suggestion, we examined effects of forest fragmentation on the levels of inbreeding (Fis ) and Allelic richness (AR ) as an indicator of genetic drift, in seedlings and adult trees of Machilus thunbergii, together with pollen dispersal. M. thunbergii, once widely distributed in Japanese warm-temperate evergreen forests particularly in coastlands, has been fragmented because of human activities.

For the seedlings, Fis was higher in more fragmented forests, but the opposite tendency was detected for the adult trees. In contrast, AR was consistently smaller in more fragmented forests regardless of the developmental stages, similarly with pollen flow distance.

These results agreed with our prediction that forest fragmentation increases values of Fis particularly in the seedlings, but does not in the adult trees as a result of inbreeding depression.