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| Prof. Yanase on the symposium day |
By 2005 it was already more than 40 years since humans last tended footpaths for rice fields and thinned jolcham oaks for fuel in Koajiro Forest. When 70ha of the Forest was secured as a nature reserve, the trees on the slope transited as they do in the wet and warm climate of Miura Peninsula. When the oaks planted by humans for fuel survived, they were huge trees it would have never become when people used them for daily life. Otherwise, evergreen broadleaved trees had overridden the deciduous oaks. The Koajiro slopes became dark. Undergrowth could not survive due to lack of sunshine. The soil started to collapse as anywhere else in the neglected forests of Miura Peninsula. The uncontrolled slope vegetation and insufficient flow control of Uranokawa River started failing to provide enough water for wetland downstream. Meanwhile those who wanted to make Koajiro Forest “miracle” worked further to make the area “Special Conservation Zone 近郊緑地特別保全地区” (; my post last week). In 2011 they obtained the title, and in 2012 Miura City and Kanagawa Prefecture decided to open the place as a nature park in 2014. Yet the former rice field was showing aridification. Reeds and bulrush were retreating to the level of near extinction. Instead, sasa bamboos covered the bottom of the valley. The stream itself was roofed by overgrown broadleaved trees and sasa bamboos. They prevented the stream from receiving sunshine. Diatoms could not live in Uranokawa River. Creatures such as aquatic lives needing diatoms for food and refuges had hard time for survival. In short, Koajiro Forest was on the verge of collapsing biodiversity.
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| The
ridge area of Koajiro Forest, February 2025. This is definitely improved version in terms of invasion of sasa bamboos. You may think why? Please read on. |
So, with legally secured and stronger status of Special Conservation Zone, volunteers and local government entered the forest and started to work to stop further transition of the area, aiming opening of the nature park in 2014. Not only that, they intended to reverse the course and to recover the forest with fewer broadleaved evergreens and vibrant wetland and tideland. Big moneys from Keikyu and local government built boardwalk in the forest to stop stomping by the visitors. Prof. Yanase who was a young student engaged in all the process including hard manual toil of mowing and rebuilding the collapsing footpath, he said. Prof. Kishi and experts of hydrology directed to rebalance the flow of Uranokawa River to wet the drying former rice paddies. The entire operation of volunteers was “concrete free.” They cleared the vegetation covering the water system, then using logs harvested from their thinning and mowing to strategically build small weirs along Uranokawa River. When typhoons came, the flood of Uranokawa River paved the water ways along the weirs. River’s water returned to the area of former rice paddies. The “new” water path was covered by boardwalk, and so protected from drying up.
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| Mowed riverbank
and “organic” weirs for Uranokawa River. Yeah, still sasa bamboos are coming, but Polystichum longifrons Kurata, which needs sunshine for photosynthesis, is recovering, thanks to cleaning up. |
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| Uranokawa
River still has the remnants of pipes showing this place was once a part of water supply system for the area. |
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| After
mowing in 2025. It is like this even today. We imagine how it was when the volunteer intervention started … |
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| Looking
the ridge from the beginning of wetland area. Could you notice human world is just there. And the mass of sasa bamboos obscure our sight. |
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| The
beginning of wetland for Koajiro Forest has Japanese alders and Salix eriocarpa, both need continuously boggy soil. In Kanagawa Prefecture, this is one of the rare places where such species can survive. The environment was maintained as such by heavy human interventions. |
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| Could
you notice in this photo there are two streams: obvious one and another at the back of the picture? When volunteers began to work, there was only one river, the one in the back. The one we can see clearly was created by organic weirs situated upstream. |
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| Volunteers
mow the wetland during winter to stop aridification. |
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| Sorry for
this photo being unclear. The ground here is a continuation of former rice terraces when people did rice cultivation. At the end of each terrace, they intentionally planted Chinese Hackberry whose root can act as earth retainer. |
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| This is also a restored water flow. |
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| On March
11, 2011, huge tsunamis overflowed every 10 minutes with the concrete bridge seen in the bottom of this photo. This is the structure built when fishermen used the place for their rice cultivation. Perhaps luckily, the attack of tsunamis quickened the recovery process for the wetland from aridified downstream of Uranokawa River. The bridge is now off limit and no longer used. |
Volunteers‘ labor was paid off. Now Koajiro Forest is a miracle nature in Tokyo’s suburbs. It has a dry ridgeway collecting rainwater as a water source of Uranokawa River. The water flows down safely covered by boardwalk, and steadily provides water to wetland. Moreover, volunteers are still patrolling the forest, engaging in mowing sasa bamboos et al in winter, and checking organic weirs to see if they do job to maintain waterflow. If an old log cannot do sufficiently or water flow changes the course again, humans build new weir with logs to keep water entering the wetland. Yes, Koajiro Forest looks very pristine with minimum artificial structure. But, to make it as such with rich biodiversity, the place definitely needs heavy and clever interventions of humans. The place is a secondary forest per excellence.
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| The
wetland nearest to the mouth of the River is now a site for making flower garden of daylilies. The flower has strong roots so that it is expected to act as earth retainer when the garden is established. The volunteers, now a Non-Profit Organization, acts cleverly this time again to obtain grants from large corporations in Tokyo for this project. |
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| Koajiro
Bay. Perhaps thanks to these efforts upstream for earth retaining, the depth of the bay is stable these days. Moreover, many kinds of sea crabs compartmentalize each of their colony according to the depth of the water and direction of tide. Also, this is a quiet bay ideal for baby fish to nurture themselves. Local fishermen do a nice business outside the bay, beyond the conservation zone, to catch grown up fishes and shrimps coming out from the nursery. |
Koajiro was used lively for about 1000 years before petrol took over for fuel, et al. After petrol takeover, people skipped the task tending Koajiro and the forest was losing biodiversity. But now we have noticed it and engage in operation for recovering biodiversity of the time of rice field. Such place is ubiquitous in “forests” in megalopolis Tokyo Area. Another example? Niiharu Citizen Forest. It is a common knowledge in Japan probably more than 95% of our forest is the secondary forest, once used heavily for food, fuel, and tools production for human life. And this human usage of nature made our forest rich in biodiversity. Otherwise in Kanagawa, the climate lets broadleaved trees dominate and create dark forest where the floor lacks sunshine, and so of poor vegetation. In such places, the creatures dependent on a variety of flora cannot survive. Our task for the 21st century is preserving or recovering biodiversity of such secondary forests which were once used heavily but abandoned. This is the thing we have never done before: we no longer use the forest for fuel or rice cultivation, but keep it as nature reserve. It’s a new way of building relationships with our forest. Koajiro Forest is one of the vanguards for this Anthropocene world of environmental protection, I think.
Greenery Section, Environment Division
Yokosuka-Miura Region Prefectural Administration Center
横須賀三浦地域県政総合センター環境部みどり課
2-9-19 Hinode-machi, Yokosuka 238-0006
〒238-0006 横須賀市日の出町２-９-19
Phone: 046-823-0381
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