Long-term dynamics of species diversity in natural secondary forest stands after severe windthrow damage

Species diversity encompasses both richness and evenness, but the interrelationship between these two aspects remains poorly understood. The long-term dynamics of species diversity were examined in three natural secondary forests in Hokkaido, northern Japan, using permanent plot data gathered over 65-66 years after severe windthrow damage. The relationships among species diversity, species richness, and evenness were analyzed in the context of stand dynamics. Temporal trends in species diversity were unimodal-shaped in three permanent plots, consistent with the pattern predicted by the intermediate disturbance hypothesis. Species richness and evenness increased following the windthrow, but decreased 27-37 years later in all three plots. In two plots, species diversity was closely associated with species richness and evenness at the early and late successional stages, respectively; in the other plot, species diversity was significantly related to both species richness and evenness in the later stage. The former two plots and the latter one plots differed markedly in terms of the extents of fatal tree damage caused by windthrow. Thus, windthrow severity affected the relationships among diversity, richness, and evenness as succession proceeded in these secondary forests. The tree density, species richness, evenness, and species diversity in all three plots decreased when the basal areas exceeded 18-24 m(2)/ha, indicating that increased competition among trees affected the temporal trends in these metrics. In conclusion, after a severe disturbance, the relationships among species diversity, richness, and evenness in secondary forests change with succession.