Assessing Forest Carbon Stocks Transported by Landslides Associated with the 2018 Hokkaido Eastern Iburi Earthquake

Abstract

This study was conducted in a watershed (357 ha) where landslides occurred after the Hokkaido Eastern Iburi earthquake (M_w 6.6) on September 6, 2018. To assess the occurrence of landslides and their impact on carbon stocks, we examined the amount of aboveground and belowground carbon stocks in different forest types to estimate watershed-scale carbon stocks affected by earthquake-induced landslides. Estimated watershed carbon stocks were 6×10⁴ t-C aboveground and 13×10⁴ t-C belowground. GIS analysis identified 428 landslides with a total landslide area of 74 ha and a landslide area ratio of 20.6％. On the other hand, landslide area ratios differed among forest types, with the highest ratio of 31.7％ in old deciduous forests (81 years and older), followed by 39-45 years old larch forests with 29.3％. Alternating layers of volcanic deposits (tephra) and andisol soils were observed to a depth of 2-3 m, with high carbon accumulation in the andisol and associated loam layers. The carbon stock affected by landslides was estimated to be 4×10⁴ t-C, assuming an average failure plane depth of 1.5 m. The carbon stock in the older forest was high, and consequently the carbon runoff from landslides was also high. The amount of carbon runoff in the study watershed was 21.7％ of the total carbon stock, which tended to be high compared to the ratio of landslide area. Due to widespread landslides, changes in the above- and below-ground forest conditions induce long-term effects on carbon stocks as a forest ecosystem service.