Restricted seed dispersal is one of the most prevalent determinants of spatial genetic structure (SGS) at a fine spatial scale within a plant population. Rhododendron kaempferi and R. semibarbatum are common and coexistent Ericaceous species in the shrub layer of secondary deciduous broad-leaved forests in the northern Kanto District, central Japan. The two species have entomophilous flowers and are thought to have similar pollination styles. However, R. kaempferi produces threefold heavier seeds than R. semibarbatum. Therefore, we tested the hypothesis that the intensity of SGS was stronger in R. kaempferi than in R. semibarbatum in a forest stand. We comparatively examined the SGS for 73 individuals of R. kaempferi and 36 individuals of R. semibarbatum by using highly variable nuclear microsatellite loci. The analysis revealed significant SGS in both species at the shortest distance (<3 m); a measure to quantify SGS showed a counterintuitive result: R. semibarbatum exhibited stronger SGS than R. kaempferi. This result might be explained by the ecological consequences of R. semibarbatum producing lighter seeds, which might have greater dispersal efficacy, but its safe sites could be more restricted than those of R. kaempferi; in contrast, R. kaempferi producing heavier seeds might have more limited seed dispersal, but its safe sites for seedling establishment could be more prevalent than those for R. semibarbatum. The different strategies for the trade-off between seed weight and site selection of the two Rhododendron species might be reflected in the difference in the intensity of SGS in this study plot.
