Spatial Distribution of the Dieback of Castanopsis sieboldii on Hachijojima, Izu Islands, Japan

Abstract

The aim of this study was to examine the relationship between the spatial pattern of the dieback of Castanopsis sieboldii in 2010 and topography on Mt. Mihara, Hachijo Island, in order to clarify the probability of tree mortality and to mitigate the damage caused by oak-wilt disease. The analysis using a geographical information system and the general linear model (GLM) indicated that the total amount of radiation, slope degree, and altitude were significant factors, and the probabilities of the dieback of C. sieboldii were higher on the ridges of the southwest-facing slopes of Mt. Mihara. We also compared the forest structure and composition before forest dieback, but no clear difference in the dominance and frequency of larger-diameter C. sieboldii trees was detected between the insect-damaged plot on the southwest-facing slope and less-damaged plot on the northeast-facing slope of Mt. Mihara. The observational data showed that the period of no rainfall lasted for 19 days in July 2010; furthermore, the total rainfall in August was 40% less than the normal value. The summer dry conditions in 2010 could have triggered the dieback of C. sieboldii because the mycorrhizal fungus carried by the bark beetle could have affected the tree water relations. Consequently, the dieback of C. sieboldii caused by the bark beetle in 2010 mainly occurred on the drier ridges of the southwest-facing slopes of Mt. Mihara.