Risk assessment of forest disturbance by typhoons with heavy precipitation in northern Japan

Under future climate regimes, the risk of typhoons accompanied by heavy rains is expected to increase. Although the risk of disturbance to forest stands by strong winds has long been of interest, we have little knowledge of how the process is mediated by storms and precipitation. Using machine learning, we assess the disturbance risk to cool-temperate forests by typhoons that landed in northern Japan in late August 2016 to determine the features of damage caused by typhoons accompanied by heavy precipitation, discuss how the process is mediated by precipitation as inferred from the modelling results, and delineate the effective solutions for forest management to decrease the future risk in silviculture. In the results, we confirmed two types of behaviours in the model: one represents the same process as that of forest disturbance by strong wind, which has been widely studied, and another represents a unique process mediated by storms and precipitation that has not been previously investigated. Specifically, the ridges that received strong wind from the front side had the highest risk of disturbance. Precipitation increased the probability of disturbance in forest stands, and its effect was dependent on the dominant species composition. Our hypothesis regarding treefall mediated by storms and precipitation is that rainwater flows into the gaps around the tree root systems during sway and the introduction of rainwater below the root-soil plate decreases the root anchorage. The species-specific vulnerability to rainfall may depend on the volume of lateral roots. Modelling the disturbance risk helped us to examine the kinds of factors that were related to exposure and vulnerability that should be managed to effectively decrease the risk of disturbance by typhoons during future uncontrollable hazards. It is recommended to avoid silviculture on the ridges of plateaus considering the high risk estimated in this area. In addition, species with dense lateral roots would be suitable for planting because they may have high resistance to typhoons with heavy precipitation.