Campus Landscaping

The Mita and Hiyoshi campuses of Keio have a long history and are home to many old trees. Hiyoshi has the ginkgo avenue, which is the "face" of the campus, and the forest spreading into Mamushidani behind it. Mita is famous for the large ginkgo tree in the courtyard.

These trees are indispensable to the campus landscape and hold many memories for alumni, but on the other hand, there are management issues such as fallen trees. Today, I would like to talk with Mr. Hosono, an arborist and a graduate of the Juku.

I have an unusual background; after graduating from the Department of Law in the Faculty of Law at the Juku, I went to Chiba University and became an arborist. The reason would be a long story, but to put it simply, the main reason was that I loved trees.

That's simple.

Actually, I was a student studying for the bar exam, but when I thought about my career path, I realized, "I don't actually like law that much" (laughs).

My grandfather originally loved gardening. As I grew various plants in my garden in my spare time, my desire to grow trees professionally increased, so I went to the graduate school at Chiba University, where they research pruning, management techniques, and landscaping.

At one point, a professor in the lab advised me to "make use of what you've done so far," so I decided to combine trees with legal social issues for my research.

You often see news reports about "trees falling." If one thing goes wrong, a serious accident can occur. Naturally, there are owners or managers, so the question arises as to what their responsibility is. There was no one specializing in that area at all.

It's not that you must always take responsibility if an accident occurs; rather, it becomes important whether the manager could have foreseen the accident. In other words, how the manager was managing the trees on a daily basis is crucial.

Exactly. Last year, during Typhoons No. 19 and 21, an accident occurred where trees on the Hiyoshi Campus damaged the fence and roof of a Yokohama City-certified nursery school to which Keio leases land. The issue of responsibility surfaced then as well, but because we had kept proper management records of regular tree pruning, the other party concluded, "We can't claim money under these circumstances."

When I first came to this department in Hiyoshi, we didn't have much of a budget for tree management. We would only cut them if a problem occurred, and we couldn't really spend money even on the ginkgo avenue.

Also, perhaps because the cherry trees were reaching the end of their lifespan, there were frequent incidents of fallen trees. Therefore, for the past two or three years, we have allocated a larger budget to focus on tree management for dangerous spots on campus. If a fallen tree were to injure a pedestrian or a student, it would result in a huge cost. So, we have finally established a system for daily patrols and emergency response in partnership with a company called Tokyu Green System.

How frequent are the daily patrols?

Staff from Tokyu Green System are stationed here and do it every day, but the management area of Hiyoshi is quite vast if you include Shimoda, so they can't cover everything in one day. We divide the areas and focus on high-risk spots, while ensuring that every location is inspected at least once a month.

Since the Hiyoshi Campus is on a hill with many slopes, we also conduct disaster prevention checks at the same time, such as looking for spring water.

I am currently in the Office of Facilities and Property Management at Mita, and I have become responsible for tree management as one of my duties.

Although the Mita Campus grounds are small, there are a considerable number of trees. It's difficult to prune all the trees in a single year, but even cutting and tidying them up partially allows light to shine into the campus, making it very bright and the view from the outside beautiful. I sometimes feel that the students who commute and spend time there also find it pleasant.

Also, while not as large as Hiyoshi, Mita is also on a hill and has cliff areas that can be dangerous.

In recent years, we have been actively proceeding with surveys and maintenance of the ginkgo avenue in Hiyoshi. When I arrived, the situation was such that the branches hadn't been cut for six or seven years. The lower branches weren't getting enough sunlight, so they were a bit weak and would sometimes break on windy days. The upper branches were interfering with the branches of neighboring trees. So, first, I had an arborist take a look.

When we conducted a tree vigor survey, it seemed that the condition would worsen sharply after pruning, and then it would take three or four years to somehow recover, a cycle that was being repeated. There also seemed to be canker disease, with some bark looking strange and some trees having a high hollow rate.

Since the method of cutting seemed to have a large impact, we decided to have the person said to be the master of masters within Tokyu Green System do the cutting. We went to the trouble of doing "model pruning," where we chose one tree to cut first, waited a year to confirm there were no ill effects, and then proceeded to cut the other trees.

You watched the model pruning for a year? That was quite a thorough approach.

Yes. The ginkgo avenue is the face of the campus, so we felt it should be cherished, and the impact if a tree fell would be quite significant. Also, since it's a symbol, it seems quite difficult to cut them beautifully into a conical shape.

Ginkgo trees do not naturally have a conical shape. The shape of the ginkgo trees at Mita is different, isn't it? A generous, rounded shape is the original form of a ginkgo tree. However, the ginkgo trees in front of the Meiji Memorial Picture Gallery at Jingu Gaien became famous, and everyone has come to think that's how ginkgo trees should look.

However, since the ginkgo avenue at Hiyoshi also serves to create a vista (a clear view) toward the Hiyoshi Commemorative Hall, I think the conical shape is good.

When we investigated the ginkgo trees at Hiyoshi, about 3 out of 117 had a high hollow rate. We have secured these trees with wires to prevent them from falling, but if the hollow rate continues to increase, I think we will eventually need to replant them at some point.

We have no plan yet for replanting, and I think it will be very difficult. It's impossible to replace all 117 ginkgo trees at once, and of course, we can't create a situation where the avenue disappears entirely.

That said, replacing them half at a time would also be difficult. So, I'm thinking we'll have to consider whether to replace them by thinning them out in the future.

Where are the dangerous trees located?

One is a tree near the entrance, and this is the one I'm most concerned about. In the past, it was small, so the soil area around the trunk could be small, but as it grew larger, the roots eventually went toward the asphalt. I think hollows are occurring for various reasons, such as people stepping on those areas and fungi getting in.

Probably near the entrance or on the right-hand side, there's the Health Management Center and roads, so that's likely an area that has been modified in various ways. I suspect the roots were damaged because of that, and fungi are entering from below.

There are many ginkgo avenues all over Japan. Are there any places that are doing replanting?

I think many local governments are struggling because roadside trees planted long ago have grown too large.

In the case of ginkgo trees, they are still resilient to pruning, so they can endure even if you cut them back significantly to reduce their size. But with something like a zelkova, which has a fan-shaped structure, if you cut back the thick primary branches that grow directly from the trunk, the branches will wither all at once, the tree shape will collapse, and the tree will lose its vigor. The roots will also fail, leading to a risk of falling.

The ginkgo avenue at Hiyoshi is cherished by everyone. If you ask whether people want to cherish each individual tree or the avenue itself, I think it's the avenue. The question is how to maintain the avenue to preserve that landscape from Hiyoshi Station toward the Hiyoshi Commemorative Hall.

For example, an avenue that is being managed carefully is the Washingtonia palm avenue in Miyazaki. That is a plan to replant them little by little from one end over several decades. But in the case of Hiyoshi, they aren't so damaged that you need to consider a total renewal of the avenue, are they?

That's true. I'm not an expert, but I have a sense that the so-called growth rate has stopped. I'm not sure if they've reached a ceiling because there's no longer much environment for roots to spread, or if they've lost the vigor to grow and stopped, but for the past 10 years or so, there seems to be almost no growth in the trunk circumference.

However, based on what I saw the year before last, I think they are still okay. Even the ones said to be dangerous due to high hollow rates are only about three trees.

How many years has it been since this avenue was planted?

They were planted in 1935, the year after the Hiyoshi Campus was established.

I hear that some ginkgo trees have survived for over 1,000 years.

Yes. The ginkgo tree at Tsurugaoka Hachimangu, for example, fell a few years ago, but it had been there since the Kamakura period.

The one where Kugyo is said to have hidden when he assassinated Minamoto no Sanetomo...

It's highly likely that later people created that as a legend, but it wouldn't be strange for a tree to live that long.

Regarding the cherry trees mentioned earlier, I don't think the "60-year lifespan theory for Somei-yoshino" is true either.

Is that so? Does it depend on the environment?

It depends on the environment and also on the management. Trees are naturally much longer-lived than humans. I don't know who started saying 60 years, but in the case of Somei-yoshino, they are planted in places where many people gather, so they are very susceptible to environmental pressure. This is because people constantly tread on and compact the soil.

There are Somei-yoshino trees with a natural shape in Kinuta Park; if left alone, they are trees whose lower branches spread out horizontally like an overturned bowl. But if you do that, they hit passing vehicles or obstruct the view, so management often involves cutting off thick branches once they reach a certain stage.

When you cut a thick branch, it naturally leaves a very large cross-sectional area. Trees have a defense mechanism through their bark to withstand bacteria and insects from the outside, but once exposed, they start to rot from there.

It's often said that cherry trees rot easily, but even if you cut branches about the thickness of a thumb, they won't rot at all. It's cutting off the thick branches that is bad. In that sense, I would like to increase the frequency of pruning.

I see, that makes sense.

The more you maintain them, the more you can control the tree's vigor and health. In short, if you pluck the leaves and branches, the total amount of photosynthetic products decreases, so the tree's growth is suppressed. The more carefully you maintain them, the less work they gradually require.

If left alone, many branches will grow and the tree will undergo rapid radial growth. In other words, it gets thicker faster. If you leave that and then suddenly cut off thick branches, the condition will worsen sharply again.

There were 35 Somei-yoshino trees on the Mita Campus, but 5 fell due to heavy rain and strong winds, and in a recent survey, 13 were diagnosed as "problematic."

There was also an instance where a cherry tree standing on a cliff near Maboroshi no Mon fell from its base toward the bottom of the cliff. This isn't due to its lifespan either, is it?

Rather than lifespan, I think it's that the decay of the root crown is progressing.

We made the decision to cut down the weakened Somei-yoshino on the cliff. It's dangerous because the other side of the cliff is a neighbor's property.

Among the remaining Somei-yoshino, we reduced the size of the trees diagnosed as "problematic," putting safety first. On the other hand, until about five years ago, the cherry blossoms were very beautiful and everyone was taking pictures. It's a shame that now we only see sparse blossoms.

I think we are at a time when we should consider how cherry tree management should be in the future.

Cherry trees grow quickly, so rather than leaving the damaged trees as they are, I think it's better to replant them early. In 10 years, the flowers will bloom beautifully.

When replanting, should we avoid choosing Somei-yoshino?

Somei-yoshino is susceptible to a disease called witch's broom. Witch's broom is caused by a filamentous fungus (mold), which makes the branches look messy and prevents flowers from blooming. It's compared to a tengu's nest. It's a disease that eventually leads to the tree withering.

The Japan Flower Association, which is famous for creating cherry blossom spots, has also stopped distributing Somei-yoshino. It seems they have started distributing a variety called Jindai-akebono instead. It blooms around the same time and has a similar atmosphere. Since the branches don't spread as much as Somei-yoshino, it has been attracting attention recently.

On the Mita Campus, Kanzakura (winter cherry) is the next most commonly planted after Somei-yoshino. Fewer Kanzakura trees fall.

Kanzakura doesn't grow that large, does it? So, I think even if they fall, they don't cause particularly major damage.

That's right. The season for graduation and enrollment overlaps with the season when cherry blossoms are in full bloom, but it's very lonely when there are few cherry blossoms at that time. As a facility manager, I want to do something about it.

On the other hand, cherry trees are prone to insects and are difficult to manage.

But they are natural things, after all. The trees themselves don't mind even if they get insects (laughs).

This is entirely a human problem; normally the tree itself doesn't weaken because of it, and since only the leaves are eaten, it will just grow them back. Things like the black-and-red, somewhat unpleasant-looking Mon-kuro-shachihoko (cherry caterpillar) can swarm around the end of summer.

Also, I think the situation is the same in Hiyoshi, but in early spring, crows build nests in the trees and sometimes attack pedestrians, which is a problem.

As for crows, Hiyoshi seems to have become a major gathering place. Whether it's the roof of the Hiyoshi Commemorative Hall or the Hiyoshi forest. They all return to their respective roosts at once around 4:30. So, the gardeners say things like, "The crows have disbanded, so I guess it's time for us to head home too."

Exactly (laughs). Crows don't harm the trees themselves, but they get quite aggressive during nesting season and sometimes peck at people, so tree managers are often concerned about them. When branches become overgrown and dense, it creates an environment where crows and starlings gather easily, so it becomes important to thin out the branches.

The worst way to do it is to wait five or six years and then suddenly cut them back severely. If you do this, a large number of branches will sprout out like a broom after the cut. This naturally ruins the shape of the tree, and from a bird's perspective, it creates a very safe roosting spot.

Instead, you should thin out the branches at the points where they fork. There is a traditional Japanese method for garden trees called 'sukashi-sentei' (thinning pruning), and if you do it carefully while paying attention to that, it makes a huge difference.

The shape of the Great Ginkgo at Mita has a thick, heavy trunk, and the branches don't spread out until they reach a considerable height, at which point they suddenly fan out. Is this characteristic shape considered a good form for a ginkgo tree?

I took a look at it earlier, and it doesn't look like the lower branches were removed by cutting off thick limbs at some stage. I think it has been maintained in that shape for a long time.

The lower branches are the thick ones growing from the very bottom, and it might have been better to leave a few more of those. The proportions are slightly off. If it were up to me, I'd want to increase the tree height a bit more. However, it certainly makes you feel the history.

Yes. Especially the view of the Great Ginkgo when you come up the stairs from the South School Building is very majestic and picturesque.

Without that tree, it wouldn't be the Mita Campus. Of course, there are many old buildings, but I think the fact that the trees have lived for a long time and marked that growth is what gives the campus its character. Regarding the buildings at Mita, perhaps because the entire site is narrow, they are quite cramped, giving the impression that new buildings are being created like a mosaic. The buildings are quite large relative to the walkways, so you end up feeling like you're looking up at everything. Each individual building is wonderful, but when you look at the space as a whole, they feel disjointed, which is a shame.

I think if you use the landscaping carefully, you can create interesting ways of presenting the space. Landscaping can connect spaces. By planting large trees, you can create a composition where you hide or cover a building before entering the next space.

Japanese stroll gardens (kaiyu-shiki) have that kind of composition. The Mita Campus was originally an Edo-period daimyo's residence and naturally would have had a garden, so I think it would be good to consider planting new greenery or designing based on that heritage.

That is very helpful. It is true that compared to Hiyoshi, the Mita Campus site is narrow and classrooms are often in short supply, so we are forced to build to the limit.

When the South School Building was rebuilt, we apparently listened to many opinions from students. For example, Hosei University and Meiji University built tower-style school buildings of over 30 stories. If we did the same, it would be expensive, but we could reduce the number of buildings and create many classrooms. On the other hand, tall buildings create a sense of pressure. Many students voiced that they didn't want the courtyard to be in the shade, and partly because of that, we have not made the buildings high-rises.

When I graduated, the building at the Maboroshi no Mon (East Building) wasn't there yet, so I feel like it's gradually becoming like a fortress. Once you go inside, though, there is still the central plaza.

The surrounding buildings act as sound barriers, so the courtyard of the Mita Campus is actually surprisingly quiet.

It seems like places like Fukuzawa Park and the garden in front of the South Building have unfortunately become disappointing landscaped spaces...

Fukuzawa Park was developed to create an outdoor space for students when the South School Building was rebuilt.

At that time, we received advice from the Forestry Mita-kai that if we kept the deciduous broad-leaved trees, they would act as a natural roof in summer to block direct sunlight, and in winter the leaves would fall to provide warmth. We laid permeable tiles in Fukuzawa Park and cut some of the thicket to let in a bit of light, transforming it into a roof of deciduous broad-leaved trees.

Since the Mita Campus is an urban campus, we wanted to make effective use of the limited space and create a place where students could sit on benches and gather. Since then, some of the roots have grown and reached areas where pedestrians walk. Whether to prioritize root protection or other uses is a difficult problem. There are various ideas on how people want to use the Mita Campus. Within that, there is the difficulty of continuing to ensure the best environment for the plants.

That's true. It's not about putting trees first, but rather how to use the space in the most comfortable way for humans.

However, if you damage the trees, the space will eventually deteriorate as a result, so we need to think about that carefully. In that regard, I think things would change significantly just by having a plant expert involved.

Landscape is one thing, but with ginkgo trees, there is also the battle with cleaning. With a row of ginkgo trees like that, we end up with sixty 100-liter bags in a single day. Arborists say that if you leave the fallen leaves at the base of the tree, it protects the tree and retains water, but the moment the wind blows, they scatter all over the road (laughs).

That's true. Ginkgo leaves have a lot of oil, so they can be slippery on slopes, so I wouldn't recommend leaving them there.

However, I want to say this: strong pruning is often done to reduce fallen leaves, but even if you think the leaves will decrease by cutting them back severely, the total weight of the leaves that come out the following year doesn't actually change compared to when branches are carefully thinned out.

When branches and leaves decrease, the tree produces fewer photosynthetic products and less energy, so it works hard to sprout leaves. In a year when strong pruning is applied, the leaves look very large from the outside, but in fact, many small leaves are also sprouting. When you measure the dry weight together, there is no significant difference from pruning by thinning branches.

In that case, it is better to control the tree's vigor by carefully thinning out branches. It is possible to manage trees by controlling them with human skill.

You're saying we should give them frequent haircuts, right?

Exactly. Also, we must not forget to consider "why they were planted in the first place." There is a reason from the human side why it's good to have a tree here, and we want to value that.

As for the benefits of landscaping, the first is "green shade." In an urban environment where summers are becoming this hot, spreading branches and leaves to provide green shade is a huge function. Moreover, trees are cool because water flows through them.

Next is "disaster prevention and mitigation." Having a cluster of trees can prevent fires from spreading. They act as a firebreak.

Also, experiments have shown that looking at the green of plants reduces stress psychologically and physiologically. At a university, everyone is studying while under various stresses, so a space that can alleviate that is absolutely necessary.

And regarding rows of trees, there is the vista (viewline) I mentioned earlier. They play a major role in strongly impressing the character and dignity of the Hiyoshi Campus upon visitors. At Mita, the Great Ginkgo would be the same.

Furthermore, there is a community-building function where people of various ages gather around the greenery. There are students at the university who are interested in the environment, and I think we could work together with them to manage the trees on campus or work on new designs.

Hiyoshi is designated as a scenic zone, so if we cut down a tree, we have to plant one. There is very rich nature in the back, so if we decide to fell a tree because it's at risk of falling, the question becomes what to plant after felling it.

We have Himalayan cedars and dawn redwoods, but those trees keep growing upwards. Even if we plant a row of dawn redwoods because they look good at first, once they grow tall, they become unmanageable to cut.

It's always difficult to decide on a policy for what to plant and how. We receive opinions from biology professors at Hiyoshi from the perspective of biodiversity.

I think an overall plan should be made by dividing the areas. If it's inside a forest, I think we should plant tree species that contribute to biodiversity. However, planting something that wasn't originally there can actually cause disturbance, so there must be someone who can think about that carefully.

Also, for trees that face the road or are planted like street trees, species that are commonly planted as street trees in the world would be good. Such trees have been selected since the Meiji era because they are easy to manage or resistant to urban environments.

There is a place called the Shimoda district on the opposite side of Hiyoshi Station where the Athletic Association's baseball field and other facilities are located, and this area had been left completely untouched. Residents have various opinions; some want it left as nature, while others want it cut clean for crime prevention, so opinions don't align.

But this year, we felled all the trees that were at risk of falling. Especially the cherry trees, which were so weakened they had stopped blooming. Then, there were various opinions about what to plant in their place... I'm hoping we can plant something under a theme that makes everyone happy.

In that case, it would be great if you could hold a workshop. I think that's a strength of a university.

I see. Professor, I'll be counting on you next time (laughs).

Shimoda was famous for its cherry blossoms, but they really weakened. It felt like they all went bad at once.

Cherry trees are quite hard to cut down. Although it might be better in the long run, when I think that what will be planted afterward will be much smaller than what's there now...

Felling cherry trees is difficult. They are trees that many people watch. Especially if they are street trees, there are people who pass by them all the time. Among those, especially during the spring season, there are entrance and graduation ceremonies, and the trees become strongly linked to people's memories.

When it comes to cutting these down, some people naturally feel as if a part of them has been torn away. It's important to proceed while taking those feelings into account. I think we really need to hold a funeral for cherry trees.

At the university, there are many things besides cherry trees that people involved have strong feelings for. There are donated trees and commemorative trees.

When the South School Building was rebuilt, we transplanted the large southern magnolia and olive trees that were in front of the old building, as well as part of a Machilus thunbergii, to the front of the South Building. All of them were commemorative trees, and since the southern magnolia was particularly large, we laid thick iron plates to avoid damaging the asphalt of the courtyard and transported it through the campus on a trailer truck.

Also, to cut the large southern magnolia, we spent two years preparing and did 'nemawashi' (root pruning). This 'nemawashi' is said to be the origin of the term 'nemawashi' (laying the groundwork) that we often use; we cut the roots about two years prior, and after doing nemawashi for all three, we finally moved only the one tree with the strongest vitality. We were able to please the donors and those related to it.

Since it's part of the Juku, I feel like the Hiyoshi forest needs some kind of narrative (a clear policy).

There is an opinion that it's better to leave it as nature without management. On the other hand, the place where the Hiyoshi Campus is located is a plateau, and it's said that during the Jomon period, the area below was all sea. That's why shell mounds and dwelling sites are found. If so, the Hiyoshi forest has an aspect of having been cleared and managed by human hands since that time. You could say it's nature that has been managed in a sense.

I want a narrative where everyone can talk to the point of considering that history, and say, 'Ah, I see, so that's why we're doing this kind of management.'

Staff members change frequently due to personnel rotations, so if there is such a narrative, even if the person in charge changes, it can serve as a major point of reference for continuing management. I hope we can maintain the Hiyoshi forest in that way.

As you say, quite a few shell mounds are found around there, and it's an area where there has been human activity since ancient times, so it's likely a secondary forest and wouldn't have been a forest where people didn't enter. A secondary forest is a forest centered on deciduous trees, so in the Kanto region, it was probably centered on sawtooth oak (kunugi) and konara oak.

The reason it became that kind of forest was to use it as a fuel forest for firewood and charcoal. Because they are deciduous trees, it becomes a very bright space. A bright forest is also a very important perspective for students to live safely on campus. I think it's good to continue thinking of it as a secondary forest centered on deciduous trees.

Beyond that, there are management goals, plans, and then the target tree shape for each individual tree. I want us to firmly establish things like, 'If this tree is planted in this location, let's keep it at about this size.'

Currently, in many large cities like those in the US, all tree information is linked via GIS (Geographic Information System) and made public on websites. If you zoom in on the map, a photo of the tree appears, and it shows everything about what kind of species it is. Some campuses are also taking such initiatives. That's how they get citizens to feel a sense of familiarity.

So there are initiatives like that.

Even though I had been looking at the ginkgo trees at Hiyoshi since high school, I didn't realize they were so good. When I visited again after studying various things, I was moved anew, thinking that I had attended such an amazing place.

There are many things you can't see unless you know about them. If we can take initiatives to let everyone know information and knowledge about the campus's green landscape, exchanges and interesting activities through greenery might also be born.

Mita is the same, of course, but at Hiyoshi, people from the town also pass through that row of ginkgo trees on a daily basis. In that sense, the row of trees at Hiyoshi has a very strong role as a public good, to the extent of receiving the Yokohama City Streetscape Award. Since the forest of the Hiyoshi Campus is also extremely precious, I want to continue to cherish it.

Thank you very much for today.