Oki Japanese Cedar and the Former Oki Peninsula
There are many giant Japanese cedar trees on Dōgo Island. The most well-known are the three giant Japanese cedar trees: Yao-sugi, Chichi-sugi and Kabura-sugi. In addition to these trees, there are many natural Japanese cedar forests with trees that are hundreds of years old, such as at the base of Washigamine Ridge. These trees are found in such a variety of shapes that you might not believe they are the same species.
In fact there is some truth in this. While not different species as such, Oki features a mix of various types of Japanese cedar. Japanese cedars are broadly divided into three different types. ‘Omote-sugi’, which is naturally found around the pacific-side of Japan, ‘Ura-sugi’ which is found around the Sea of Japan-side, and ‘Yaku-sugi’ which is found in Kyushu. In Oki there are artificial planted forests of Omote-sugi, as well as natural forests of Ura-sugi. However, in addition to these two types is a Japanese cedar that is unique to Oki. It has a mix of features from both Omote-sugi and Ura-sugi.
Genetic research has revealed that Ura-sugi actually spread from Oki to the mainland. During the most recent glacial period, which occurred 20,000 years ago, the sea level declined, causing the Oki Islands to become connected to the mainland Shimane Peninsula. The decline in temperature forced Japanese cedar to migrate from the now cold and dry Honshu inland to Oki, which protruded out into the sea and was thus comparatively warmer. The species was able to survive in Oki during the glacial age. It is thought that once post-glacial warming commenced again the species spread back to the mainland via the Oki Strait, which was still connected to the mainland. Considering that Ura-sugi survived the glacial age in the Oki Islands and later spread from there to the rest of Honshu, perhaps it should be called ‘Oki-sugi’ instead.