Estimation of stand biomass and species-specific biomass in Japanese northern mixed forests in 1920-1930s: Understanding environmental factors affecting carbon sequestration before recent climate change

Forests are major carbon sinks, but tree species composition and forest biomass have changed significantly under climate change. However, the extent to which forests have changed with climate change is unclear due to a lack of information before the recent climate change. In this study, we used forest inventory and environmental data (480 plots, 467 ha in total) from 1929 to 1938 in northern mixed forests in Japan (1) to estimate the biomass of the forest at that time, (2) to determine the environmental factors (temperature, precipitation, topographic wetness index, solar radiation, geology) affecting the AGB and species-specific biomass and (3) to estimate the relationship between AGB and diversity, structural equation models were used to determine whether the relationships are significant. The results showed that the average AGB of the forest was 61 +/- 22.4 (MgC/ha), which was not much higher than that of the present forest after human disturbances. Regarding species-specific biomass, biomass for subboreal conifers tended to increase in areas with less precipitation, and biomass for cool temperate deciduous broadleaved trees tended to increase in areas with more precipitation. Environmental factors indirectly affected AGB via forest structural factors, and the relationship between diversity and AGB was insignificant. This study revealed the feature of the forest ecosystem before recent climate change, and showed the potential for a detailed understanding of the effects of environmental factors related to biomass by analyzing species-specific response.