Citation: Kosugi, R., M. Shibuya, and S. Ishibashi. 2016. Sixty-year post-windthrow study of stand dynamics in two natural forests differing in pre-disturbance composition. Ecosphere 7(11):e01571. 10.1002/ecs2.1571Abstract. We examined the dynamics of stand structure and composition over 60 years in two natural secondary forest plots in central Hokkaido, Japan, that had been severely disturbed by a stand-replacing windthrow in 1954. We determined temporal trends in tree density, species richness, species diversity, successional composition, and stand developmental stage. One of the plots contained a coniferous stand prior to the disturbance; the second plot contained a mixed stand. Tree density and species richness increased after the windthrow, and peaked in both stands in 1991, 37 years after the windthrow. Based on the dynamics of tree density and species richness, the stand development phase of both plots remained within the stand initiation stage until 1991, and then progressed to the stem exclusion stage by 2014, 60 years after the windthrow. Based on patterns of increasing tree density by species in both plots, vegetative regeneration was presumed to have contributed abundantly to the recovery of the stand after the windthrow. Species diversity increased after the windthrow as species richness increased in both stands; diversities peaked 23 or 32 years after the windthrow and declined slightly thereafter. Although the successional composition of the pre-disturbance coniferous stand regressed to an early stage under the influence of the windthrow, the relative importance of late-successional species approached the pre-disturbance value as the plot recovered. In contrast, the successional composition of the mixed conifer/hardwood plot was unaffected by the windthrow, but gradually changed to an early stage over several decades following the windthrow. The difference in successional composition between plots was likely a result of differences in pre-disturbance stand conditions between stands. Our 60-year permanent plot study determined stand and population dynamics, the duration of the stand initiation stage, and the interrelationships between diversity and species richness. Long-term permanent plot studies contribute crucial data for the interpretation of forest dynamics following disturbance.