The impact of climate variation and disturbances on the carbon balance of forests in Hokkaido, Japan

Abstract. We evaluated the long-term (52-year) effect of climate, disturbance, and subsequent recovery on the carbon balance of cool temperate forests by using the process-based ecosystem model VISIT. The study sites were artificial larch forests planted after clear-cutting of mixed forest in Hokkaido, Japan. The model was validated, scenarios were computed, and a sensitivity analysis was performed. First, we performed a baseline simulation of carbon dynamics and compared these values with those observed across a wide range of stand ages (old mixed forest and young and middle-aged larch forests). Second, we ran scenarios to investigate how disturbance and several climate factors affect long-term carbon fluxes. Third, we analyzed the sensitivity of carbon balance to the amount of disturbance-generated tree biomass residues. By taking into account seasonal variation in the understory leaf area index, which played an important role, especially in the initial stage of recovery, the simulated net ecosystem production (NEP), gross primary production, ecosystem respiration, and biomass for the three types of forest were consistent with observed values (mean ± SD of R² of monthly NEP, GPP and RE for the three types of forest were 0.63 ± 0.26, 0.93 ± 0.07, 0.94 ± 0.2, respectively). The effect of disturbances such as clear-cutting, land-use conversion, and thinning on the long-term trend of NEP was larger than that of climate variation, even 50 years after clear-cutting. In contrast, interannual variation in the carbon balance was primarily driven by climate variation. These findings indicate that disturbance controlled the long-term trend of the carbon balance, whereas climate factors controlled yearly variation in the carbon balance. Among the meteorological factors considered, temperature and precipitation were the main ones that affected NEP and its interannual variation. The carbon balance in the initial post-disturbance period, which is strongly affected by the amount of residues, influenced the subsequent long-term carbon budget, implying the importance of residue management. Consequently, carbon release just after disturbance and the length of the recovery period required to balance the carbon budget are controlled by the amount of residues.