Secondary forests are a huge carbon sink in terrestrial ecosystems, but it remains unclear whether growth of secondary forests has accelerated or decelerated due to global environmental changes, especially in East Asia. In this study, we analysed 30 years of data at 26 permanent plots along a secondary succession of temperate forest in central Japan to test whether the aboveground biomass (AGB) growth of secondary forest has accelerated or decelerated. The AGB–age relationship has clearly shifted upwards from the 1980s to the 2010s. The observed annual net AGB change (ΔAGB) was higher than that expected from the AGB–age relationship. These results indicate that ΔAGB has accelerated. However, the observed ΔAGB adjusted for the effects of stand age has gradually decreased from the 1980s to the 2010s. This indicates that ΔAGB of the secondary forest had accelerated prior to the 1980s, and decelerated after the 1980s. The decrease in ΔAGB after the 1980s was due to decreased biomass gain in the 1990s and increased mortality after the 1990s. Climatic factors are not considered the driver of the acceleration of ΔAGB because mean annual temperature and annual precipitation at the study site was relatively stable before 1980. However, mean annual temperature has increased since the 1980s and annual precipitation was low in the 1990s, which would result in increased drought stress and may have contributed to the decrease in biomass gain in the 1990s. Other abiotic and biotic stresses may also have contributed to the increased mortality after the 2000s. This study revealed a long-term transition from an acceleration phase to a deceleration phase of forest growth, indicating that forest growth changes relating anthropogenic environmental changes since mid-20th century are not necessarily unidirectional.
