A Weapon Falls from Above… The Shock of Tokyo’s “13,900 Trees Showing Abnormalities”! The Irony of “Fallen Leaf Complaints” That Lead to Fallen Trees

Fungus eating through the trunk… 600 trees immediately felled

Around 10:00 a.m. on June 16, staff at the World Heritage Site Shimogamo Shrine (Sakyo Ward, Kyoto City) discovered that a sacred Japanese chinquapin tree—standing 30 meters tall, with a trunk circumference of approximately 3 meters, and estimated to be about 450 years old—had snapped at the base and fallen.In Tokyo as well, in March, a cherry tree fell at Kinuta Park in Setagaya Ward, trapping a woman in her 70s beneath it. The very next day, a Himalayan cedar next to the parking lot at the same Kinuta Park fell, damaging two cars.

In response to these successive incidents of fallen trees, the Tokyo Metropolitan Government conducted emergency inspections of approximately 800,000 trees taller than 3 meters located at metropolitan facilities such as parks and schools. The results revealed that “approximately 13,900 trees (2% of the total) showed abnormalities.”

“I was surprised. I thought, ‘Are there really that many?’ At the same time, I felt that this must be an accurate figure,”

These are the words of Hiroyuki Wada, a tree doctor who conducts surveys and works on restoring the vitality of trees, including the Yamataka Jindai Cherry, a nationally designated natural monument.

But what exactly does “abnormalities” refer to?

“This survey was based on the Reiwa 3 edition of the ‘Street Tree Diagnosis Manual’ compiled by the Tokyo Metropolitan Government.According to that manual, ‘abnormalities’ include partial trunk decay, the presence of hollows, the presence of decay-causing fungi such as Bekkoutake or Kofukitake at the base of the tree, poor branch growth, and the presence of dead branches.

For trees where rot or hollows inside the trunk are suspected, we use diagnostic equipment to examine the interior, and those in severe condition will be felled. If it is determined that felling is not necessary, the trees will be re-examined in one to three years.”

Of the approximately 13,900 trees, about 600 have already been felled. Without this comprehensive survey, the situation could have been dangerous.

“Trees at Tokyo Metropolitan Government facilities are supposed to be inspected on a rotating schedule, but to be honest, I think there were some that were overlooked.”  

Scorching Heat and Excessive Pruning… The Tragedy Striking Trees

It has been 60 years since Kinuta Park opened. Some speculate that the trees planted back then may be reaching the end of their “lifespan.”

“Since plant cells continuously divide and regenerate, there is no such thing as a ‘lifespan.’ However, as a tree ages and its roots reach their full extent, the branches and leaves may no longer grow as vigorously as they did when the tree was young, making it appear to be in decline.

The longer a tree lives, the more opportunities it has to sustain injuries, which increases the likelihood that fungal spores will take hold there, potentially leading to decay or hollowing out.”

According to Mr. Wada, mushrooms feed on the tissue inside the trunk and multiply rapidly. He explains that mushrooms are the cause of the decay and hollowing out.

However, fungi aren’t the only factor weakening trees. The narrow spacing between trees is also a contributing factor.

“The spacing between the cherry trees that fell at Kinuta Park was also quite narrow. It’s presumed that this prevented the roots from spreading sufficiently. Furthermore, when the spacing is narrow, the number of leaves becomes insufficient relative to the trunk’s thickness and the spread of the branches, causing the trees to decline.”

For example, Somei-Yoshino cherry trees require 15 meters of spacing between them. However, in reality, they are often planted 7 to 8 meters apart, and in some narrow areas, as close as 5 meters apart. According to Mr. Wada, when the spacing is too narrow, the branches of neighboring trees cross over each other, making it difficult for sunlight to reach the lower leaves and reducing photosynthesis.Since plants perform photosynthesis through their leaves to produce the energy they need to survive, a smaller number of leaves causes the tree to weaken.

Although it is often said that Somei-Yoshino cherry trees—being clones with identical genes—have a lifespan of 60 years,

“It is not true that Somei-Yoshino cherry trees have a lifespan of 60 years. They are naturally robust trees; within 20 years of planting, they exceed 10 meters in height and reach a trunk diameter of nearly 50 cm. In fact, there are many Somei-Yoshino cherry trees across the country—including those in Hirosaki Park in Aomori Prefecture—that are over 100 years old.”

In addition, cutting their roots during underground construction or pruning them too severely can weaken the trees.

“While mushrooms like the Bekkō-take and Kofuki-take grow at the base of the tree, spores from various mushrooms float through the air and settle on the tree’s cut surfaces. Deep pruning creates larger cut surfaces, making it easier for spores to attach and increasing the likelihood of decay-causing fungi invading the tree.”

I’ve seen something applied to the cut surfaces—is that a treatment?

“That’s a wound-sealing agent to prevent new fungal invasion. When decay has already set in, we need to revitalize the tree to stop the decay from spreading further. Once the tree regains its vigor, it creates its own barrier within the tissues of the trunk and branches to prevent the fungus from spreading.”

To restore the tree’s vitality, you can apply fertilizer, loosen the soil, or even thin out areas where the trees are too densely packed.

There’s another factor weakening the trees: recent weather conditions.

“The environment has been changing visibly lately. Summer temperatures can reach nearly 40°C, and last year, there was almost no rain during the rainy season. From December through February, there was a stretch of over 60 days without rain.

Not only is there a lack of water, but high temperatures also reduce photosynthesis, causing the trees to stop drawing up water. I believe that the intermittent occurrence of high temperatures and low rainfall over long periods is causing significant damage to the trees.”

Tokyo’s Trees Cut Down for Human Convenience

When walking around, you often see street trees with very little space between them. You also see trees that have been heavily pruned, almost to the point of being completely bare. If this continues, it will only weaken the trees, and I worry that they might start falling like dominoes. What can be done?

“Ideally, we should consider the surrounding environment to determine the appropriate size for the tree, then prune it every two years, apply fertilizer, and occasionally expand the space around the base as part of our maintenance routine,”

However, reality rarely matches this ideal. I’ve heard that, due to budget constraints, pruning is often reduced to once every five years. If pruning is done every two years, branches can be cut before they become too thick; but if it’s only once every five years, the branches grow thicker, and the cut surfaces become larger.

“The optimal time to prune deciduous trees is between the time they lose their leaves and the time they begin to bud.During this period, there are fewer fungal spores that cause wood rot, so pruning at the right time makes it less likely for rot-causing fungi to invade. However, as calls to ‘do something about the fallen leaves—they’re such a hassle’ grow louder, pruning before the leaves fall is also becoming common. When that happens, it becomes much easier for rot-causing fungi to invade.”

It’s true that cleaning up fallen leaves is a lot of work, but it’s sad to walk down a tree-lined street where the trees have been completely stripped bare.

“While aesthetics are a factor, trees also play a major role in preventing the urban heat island effect, which is why there’s a global movement to increase tree canopy cover.”

“Canopy cover” is a globally recognized indicator that measures the extent to which a tree’s branches and leaves (canopy) cover the ground.According to a study by a research group led by Professor Toru Terada at the University of Tokyo Graduate School, over the nine-year period from 2013 to 2022, the tree canopy cover rate within Tokyo’s 23 wards decreased from 9.2% to 7.3%.It is reported that approximately 20 percent of the tree canopy cover has been lost. While tree removal due to redevelopment projects, such as at the Jingu Gaien, is considered one factor,

“I think heavy pruning may also be a factor. I understand that people living near tree-lined streets find the falling leaves a nuisance. I believe we need to deepen our understanding of the vital role trees play going forward. I also hope that local governments will manage trees systematically, rather than cutting them down simply out of fear of falling trees.With the climate becoming harsher, I’m concerned about the future of our trees if things continue this way.”

▼ Hiroyuki Wada, Arborist. Director of the Japan Arborists Association (General Incorporated Association). He also serves as Chairman of the NPO Midori Environmental Network! and as a Director of the NPO Tokyo Arborist Project.To date, he has been involved in surveys and vitality restoration efforts for the Yamata-Kojidai Cherry Tree (Yamanashi Prefecture), a nationally designated natural monument; the Great Zelkova Tree at Negoya Shrine (Yamanashi Prefecture); and the Oshima Cherry Tree Stump (Tokyo). In addition, he is engaged in regional planning and design for creating famous cherry blossom spots and “flower towns,” as well as providing guidance to residents and serving as a lecturer at public seminars.