The March 7th issue of Jinja Shinpō had another article in the series on sacred forests, and this one returned to the core topic, looking at the question of how they should be maintained. It was written by a lecturer at Tokyo Agricultural University, who presumably knows what he is talking about.
It starts by observing that the image of sacred forests is of trees, certainly, but also other plants, and a wide variety of animals, living in an area that people neither enter nor interfere with. On the other hand, most actual sacred forests are maintained to some extent, to keep the sacred path to the sanctuaries clear and clean, and to tidy up around the edges. They are nevertheless thought of as being examples of the natural state of the ecosystem in that area.
He quickly recaps the idea that ecosystems naturally change over time, as bare ground is first colonised by quickly growing plants, and then by trees, and then the types of trees shift over time. There is a final state that is stable for as long as the climate is, and the estimate is that, in Japan, it would take about 500 to 700 years to reach that state if you started from bare ground. However, some locations cannot reach that state, because of local geographical features, and they reach a slightly different equilibrium. Because Japan’s climate is generally favourable to the growth of plants, almost all areas of the country naturally tend to become forest.
He discusses the sacred forest at Meiji Jingū as something that is generally regarded as a successful attempt to create an “artificially natural” forest, with trees planted so that it would pass through four stages, before reaching a mature and steady state in which the forest needed no human maintenance after about 150 years. (It is currently about 100 years old.) It actually reached the third stage earlier than planned, in part, it seems, because an insect pest killed off many of the pine trees before they were expected to die naturally. Overall, however, the plan seems to have worked quite well.
However, in most cases things are not so simple. First, a mature forest tends to be less biodiverse than a forest that is still in the process of changing, and has a relatively simple structure. Thus, there might be reasons to try to freeze the process at an earlier stage. In addition, since new trees cannot get started in a mature forest, because too little light reaches the forest floor, they naturally regenerate only when trees die and fall over. That only works if the forest is large enough that you can just let trees fall over, which is fine at Meiji Jingū because the forest is huge, but is not possible at most jinja. There, you might well need to cut trees down from time to time to maintain the forest.
There is also the problem of changes to the ecosystem. Invasive species are one issue, where plants and insects brought from overseas can start killing off particular species in the forest, or vines can smother the trees. Another issue is, of course, climate change, which will alter the species that can naturally grow in an area.
What this means in the end is that you can’t just leave a sacred forest to nature if you want to make sure that you always have a sacred forest. Priests and parishioners need to have a management plan to make sure that the forest is maintained in a desirable state.