Effects of caprock on vegetation distribution in areas underlain by granite on the lower slopes of dissected lava plateaus on Teshima Island in Kagawa Prefecture, western Japan

Abstract

We investigated the effects of caprock on vegetation in areas underlain by granite on the lower slopes of dissected lava plateaus on Teshima Island in western Japan. Caprock is a geological term that refers to a harder or more resistant rock type overlying a weaker or less resistant rock type. Teshima Island was divided into eastern and western regions based on the distribution of sanukitic andesite caprock; the east of the island has extensive areas of andesite caprock overlaying granite, while the west of the island has no andesite caprock overlaying the granite areas. Catchment area indexes calculated using a digital elevation model showed that the catchments were larger on the eastern side of the island. Grain size analysis revealed that the andesitic surface soils were mainly composed of fine particles, with the surface soils in eastern areas of granite with caprock tending to be more affected by andesite on the upper slopes. We compared differences in vegetation distribution between the eastern and western areas by superimposing a vegetation map based on a phytosociological survey conducted in 2002 onto a vegetation map published in 1982. The results showed that by 2002, broad-leaved deciduous forests dominated by Quercus serrata and Q. variabilis had expanded considerably eastward, displacing the pine forests (mainly Pinus densiflora) that dominated the area in 1982. Indeed, 19% and 6% of the forests in the eastern and western areas, respectively, were broad-leaved deciduous forests. These findings showed that pine forests have been more resilient in the western areas in the 20-year period between surveys. In addition, it appears that the andesite caprock, which promotes soil water retention and is resistant to weathering and erosion, was associated with the larger catchments, and conducive to the development of pine forests in areas overlain by granite on the lower slopes of the island.