Frankia communities at revegetating sites in Mt. Ontake, Japan.

In 1984 at Mt. Ontake in Japan, an earthquake caused a devastating landslide, and as a result, the vegetation on the south slope of the mountain was completely eliminated. In higher elevation (2000 m) areas, revegetation has not yet been completed even 30 years after the landslide. Revegetation progress throughout the area was heterogeneous. In the partially revegetated areas, actinorhizal plant species such as Alnus maximowiczii and Alnus matsumurae have been found. In the present study, we investigated the Frankia communities in the higher-elevation area using sequence analysis of the amplified nifH (dinitrogenase reductase) gene from nodule and soil samples collected in the disturbed region, undisturbed forest, and in the boundary between the disturbed region and the undisturbed forest. Phylogenetic analysis of partial nifH sequences revealed the presence of six clusters, each of which consisted of highly similar (> 99%) sequences. Four clusters showed significant sequence similarity to Frankia (three Alnus- and a Casuarina-infecting strains). Diversity in the Frankia community was relatively low-only one or two clusters were detected in a site. At most of the sampling sites, a dominant cluster in a nodule coincided with that in rhizosphere soil, indicating that community structure in the rhizosphere is a primary factor that determines occupancy in a nodule. No significant difference in community structure was observed between plant species. Diversity in the Frankia community varied depending on revegetation progress. Cluster A, which was the most dominant in the disturbed region, was likely to have invaded from undisturbed forest.