This blog post covers two articles about sacred forests from Jinja Shinpō, because these two drifted a bit from the core topic in talking about the history and etymology of sacred forests.
The article in the January 31st issue was mainly about sacred forests in other cultures; they were found in ancient Egypt, and in India, and in China from the fifth or sixth century. It did talk briefly about Japan. There are remains of structures from the mid-Yayoi period onwards (from around 100 BCE) that appear to be ritual structures, but there is no evidence of forests around them. Similarly, there is no sign that forests were originally established around the kofun burial mounds created from the third to seventh centuries CE. However, by the early eighth century, there are explicit references to sacred forests in surviving documents. The implication may be supposed to be that sacred forests were a relatively late import, but the evidence presented is utterly inadequate to showing that. The evidence that we have for Shinto-like rituals anywhere is later than the Yayoi period and in different locations from the kofun tombs, as discussed in my essay about early Shinto, so I found this article a bit mystifying.
The article in the February 7th issue was about the use of “mori”, the word for “forest”, and the kanji used for it. In the poems of the Man’yōshū, which mostly date from the eighth century although some are probably a bit earlier, the kanji used for “jinja” today are often read “mori”. “Jinja” is written with two kanji, the one for “kami”, and one that is read “yashiro” or, historically, “mori”. In the Man’yōshū, the “yashiro” reading is abstract, talking about the dwellings of kami in general, while “mori” is used when talking about a particular place. “Mori” is most often written with the second character alone, but sometimes in the same way that “jinja” is written today.
It seems that one folklorist, Kamata Hisako (I think), has suggested that “mori” originally also meant the activity of a spiritual power, particularly power that protects or creates. Thus, the forests on the mountains gave birth to the waters and rivers that made rice agriculture possible, and so “mori” was associated with both the spirit of the rice, and with human fertility.
If this is all correct, then it suggests that there was a very strong connection between wooded areas and the residence of kami by the eighth century at the latest. Given that our evidence for the situation of Shinto before that date is tenuous at best, it seems that the importance of sacred forests goes as far back in Shinto as anything does.