Dieback of pine forests and the tree population structure in pine wilt disease forests in Higashi-Hiroshima City

We surveyed secondary Pinus densiflora forests, widely distributed in Higashi-Hiroshima City. These forests have changed drastically due to heavy damage from pine wilt disease. The aim of this study was to clarify the state of pine wilt disease forest and their population structure. Vegetation data was obtained by the simplified vegetation survey method at 82 plots, each measuring 10m×10m. Detailed vegetation survey data was obtained by complete enumeration and coring in 4 large plots, each measuring 20m×20m. P. densiflora mortality was high in abandoned south slope forests. P. densiflora existed in 53 plots, almost all of which were in the tree layer. There were only P. densiflora in the tree layers of 27 plots. However, it seems that P. densiflora could not regenerate in pine welt disease forests. If pine welt disease were to progress, the number of immature forests lacking tree layers might increase. Fagaceae (mainly Quercus serrata) were considered to be the second dominant tree species becasue its growth rate was higher than other species except P. densiflora. However, about one third of all plots did not have any Fagaceae in their tree and sub-tree layers. Other species (Ilex macropoda, Symplocos lucida etc.) dominated mainly the sub-tree and shrub layers. Although the ages of these species were old, their growth rates were low. Therefore it seems that pine wilt disease forests become Fagaceae dominant forests. However, if Fagaceae seedlings are in short supply and there are obstructed from taking root, it seems to take longer to become a Fagaceae forest. This is because light reaching Fagaceae is reduced by the canopy of low sub-tree layer species