Stand development and soil C and N accumulation along a chronosequence of Japanese oak (Quercus crispula) plantations in eastern Hokkaido, Japan

Introduction Quercus crispula is a valuable and widely utilized tree species in Japan, yet standardized management practices for its plantations remain undeveloped, and no comprehensive study has examined the entire process from planting to harvesting. Although monoculture plantations may contribute to carbon sequestration, soil carbon dynamics under such systems are poorly understood. To address these gaps, we investigated stand development and soil properties using a chronosequence approach in the Ashoro Research Forest, Kyushu University, where Q. crispula has been planted annually for 50 years. Methods Seven stands aged 8 to 48 years (established in 2014, 2003, 1996, 1991, 1987, 1980, and 1974) were sampled, with one plot per stand. We assessed stand structure through the relationship between mean stem diameter and stem density, and measured soil carbon and nitrogen contents. To evaluate the validity of chronosequence sampling, we compared current data with a regression model derived from 38 sites sampled 12 years earlier and a subset of six sites from that dataset. Results Aboveground biomass increased linearly with stand age and exceeded the predicted values. However, structural development appeared slower than expected when assessed by stem diameter–density relationships, likely due to the absence of thinning. Soil carbon and nitrogen concentrations showed little variation with stand age. However, leaf carbon and nitrogen concentrations and fine soil content differed from earlier measurements and model predictions. Discussion These findings indicate that chronosequence studies, when combined with repeated sampling, can provide valid predictions of aboveground biomass and structural development in Q. crispula plantations. However, discrepancy between predictions derived from chronosequence approaches and the observed changes in leaf traits and soil properties suggests that environmental changes during the observation period can obscure age-related trends inferred from chronosequences.