Thick and tall oak attracts oak ambrosia beetles
Did you see the leaves of tall trees turning reddish-brown in the middle of this summer? That may be caused by Japanese oak wilt.
The oak wilt occurs in species of konara oak that bear acorns, including deciduous trees such as konara oak and Japanese chestnut oak (Quercus acutissima) and evergreen trees such as Chinese evergreen oak (Quercus myrsinifolia) and Japanese stone oak (Lithocarpus edulis).
In plants, water taken up by roots transpires from the surfaces of the leaves by evaporating from the stomata, but trees infected with oak wilt can no longer take up water for the leaves and wither.
The disease began to spread in western and northern Japan around 2000. The Tokyo metropolitan area remained free from the disease even after that; however, it has been observed since several years ago.
The oak ambrosia beetle is considered a native species because of its genetic diversity. Unlike the chestnut gall wasp, it is not a foreign species. It is said that the reason why the damage spread recently is that human involvement in coppice woodlands has changed.
In the decades from 1955, propane gas has become commonly used in Japan, and firewood and charcoal were not used anymore. Before that change, trees in coppice woodlands were cut when they grew as thick as my arm. If they get thicker than that, they are too thick to be used for firewood or charcoal and need to be chopped. Tall trees such as the konara oak and Japanese chestnut oak were several meters high. Because the use of propane gas spread rapidly, coppice woodlands were left all at once. The height of thicket trees reached 10 meters by 1975 and 20 meters by 2020.
Oak ambrosia beetles have a habit called mass attack, producing aggregation pheromones and gathering on large trees. The oak wilt began raging widely because konara oak and Japanese chestnut oak in coppice woodlands became larger.
The fundamental countermeasure against the oak wilt is to clear-fell and coppice (the act of cutting down a tree and rejuvenating it by growing shoots from the stump) woodlands. This theory was established more than 15 years ago. It is said that the best way for the ecosystem is to clear-fell and coppice a small area such as several hundred square meters. In reality, however, the clear-felling and coppicing of small areas has rarely been performed; all that has been done is work to prevent oak ambrosia beetles from flying away from trees infected with the oak wilt and fell only oak trees that wither owing to the oak wilt.
There is also rich nature created through human involvement
From the end of the Showa period, when people left satoyama areas (undeveloped woodland near a village) and memories of the days when they lived in satoyama began fading away, people started to strongly oppose tree-felling in public places and in places where trees are prominent.
Since the preparation for the last Tokyo Olympic Games, trees of good-looking size have been planted. Then they became overcrowded after a while. The solution to the overcrowding is to cut down some of the trees, but some people complain that they feel pity for the trees. Another way that does not require tree-felling is to prune them drastically, which changes their shape. The cost of pruning is much higher than that of felling. Moreover, felling is required only once, but pruning needs to be done repeatedly.
People also complained that wild cherry trees (Cerasus jamasakura) should not be felled even if it is a part of clear-felling and coppicing of small areas to maintain coppice woodlands in large parks in hilly areas. The tree shape of wild cherry trees is the same as that of konara oak, with the trunk separated from the root. When we look at the shape, we can see that wild cherry trees have been cut down repeatedly as a part of coppice woodlands, and young shoots have been restored. In addition, wild cherry trees have a large canopy, which casts a shadow on rice fields in valley areas, making it difficult to maintain the light environment necessary for agriculture.
I think that is because of the spirit of tree protection, which lost the connection with people’s lives. We have a kind of solidarity with creatures and trees, and deeply respect large trees that have lived for a long time.
The coppice woodland in Musashino was created during the new rice fields development in the early Edo period as a set of farms, farmland, and coppice woodland. Wood materials had been absolutely necessary as firewood and charcoal, and fallen leaves had been essential as fertilizer for farmlands. The coppice woodland has been used as an agricultural forest for more than 300 years. It is thought to have been a grassland before that. Konara oak is believed to have become dominant in the process of converting grasslands into forests.
Satoyama, which people began appreciating as land that we can use sustainably, is a complex of ecosystems consisting mainly of coppice woodland, farmlands, and farms, in addition to Japanese silver grass fields and short grasslands. It was the use of plants by farmers that brought about the sustainability.
For citizens who have not experienced such use in their daily lives, it is natural that they consider tree-felling to be a bad thing. When I asked them to create a coppice woodland during greening work, they pointed to a coppice woodland in a nearby park along the cliff line and said, “No way. A coppice woodland is a virgin forest.” When you simply look at current coppice woodlands, it looks like a virgin forest.
Coppice woodlands with rich biodiversity are bright ones that had existed until about 1965. The reason why satoyama came to be appreciated again is that people started to recognize the rich biodiversity.
A few years ago, a forestry scholar published a book called Shinrin Howa (Forest Saturation). Currently, we are seeing the phenomenon of the number of cobblestone shores decreasing drastically in the middle reaches of rivers. For example, the riverbed in the middle basin of the Tamagawa river, where I am doing fieldwork, was a small cobblestone shore with many stones around 1960. However, during the period of high economic growth, a large number of stones were collected from the riverbeds to be used as aggregate for concrete, and the supply of soil and sand from the upstream drastically reduced. As a result, most of the riverbeds have been changed to forests.
This, of course, changed the ecosystem: plants such as pearly everlasting (Anaphalis margaritacea) and Kawaranogiku aster (Aster kantoensis Kitamura), which bear beautiful flowers, have become rare, and Kawarabatta grasshopper (Eusphingonotus japonicus), which has beautiful blue hind wings, has become an endangered species.
The supply of soil and sand from the upstream stopped because the mountain was covered with forest. In other words, it is the result of our tree planting.
It is effective from the viewpoint of flood control, but on the other hand, it causes Japan to lose nature in its original form.
Enjoying coppice woodlands also means using resources
The point is that not only nature in its original form is important, and it is not good at all that people change nature without considering the impact on the surrounding environment. I think that to strike a balance, we need to consider patterns and areas.
Probably, the backside of the mountain should be a meadow. But further back, it would be better to have a virgin forest. In addition, forests near the city should be coppice woodland that humans are involved in. The government is required to make environmental plans considering the balance between the patterns and areas.
On the other hand, we need to change our assumption that we should not cut down any trees.
We are planning an activity at our Kurokawa Field Science Center where not only students but also the general public can participate in work related to coppice woodlands and gain a hands-on understanding of biodiversity.
Since the carrying capacity is limited, we cannot allow everyone to enter the coppice woodland of the University Farms freely. However, I believe, for example, that it is meaningful for both children and the city of Kawasaki to let children who live close to beaches in Kawasaki City, where Kurokawa Field Science Center is located, know so that they can learn that the city has an environment where people can experience creatures living in satoyama.
I would be grateful if people knew that vegetation management is necessary to maintain such an environment in the future. I think that such understanding will lead to learning about the material cycle and the great cycles of nature.
I also believe that in order to make more people in society accept tree-felling, it is necessary to build a society that can utilize felled trees as a resource.
Of course, it is not practical to use wood as an energy source such as firewood or charcoal as it was in the Edo period, and it is not efficient to split them into chips and burn them to generate electricity, in terms of cost-effectiveness.
So, what I am thinking about now is to have fun with felled trees. I think it is fun to enjoy woodworking with felled trees and participate in felling. In particular, it is an exciting dream to manage growing shoots from tree stumps.
I think it will be a fun experience to rejuvenate coppice woodlands and to come into contact with the flora and fauna that live there. The experience is priceless.
We discover new and interesting things and are surprised by coppice woodlands nearby because there are activities between nature and humans.
I believe that that having fun with thickets will lead to the preservation of nature with human activities such as coppice woodlands.
* The information contained herein is current as of November 2021.
* The contents of articles on Meiji.net are based on the personal ideas and opinions of the author and do not indicate the official opinion of Meiji University.
* I work to achieve SDGs related to the educational and research themes that I am currently engaged in.
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