[Ko Gyu-hong's Trees and People] Trees Along the Road Connecting Human Lives

<20> Trees That Make It Possible to Live in Gray Cities

A zelkova tree standing tall by the roadside, serving as a resting place for passersby. Junggyebon-dong, Seoul. Photo by Ko Kyuhong

As the summer weather becomes increasingly harsh and difficult to endure, both body and mind grow weary. Whenever one has no choice but to step outside, the sunlight is scorching, and there is no way to avoid the humid breeze that touches the skin. One can't help but wonder how people survive if there aren't even trees along the roadside.

Even the brief moments spent standing at a crosswalk waiting for the traffic light to change become exhausting during this season. Some places have created artificial shade with parasols next to crosswalks to shield from the sun, but that refreshing feeling can never compare to the shade of trees. The shade cast by street trees is different from any shade created by man-made structures. One inevitably feels immense gratitude toward street trees.

Recently, an interesting experiment was publicly conducted on a broadcast news program. It took place right in the middle of a road in Jongno, Seoul, where there was no shade at all. A young woman was the participant. Before the experiment began, her facial temperature was 35 degrees Celsius. After standing under the blazing sun without any street trees for 10 minutes, her facial temperature rose to 36.6 degrees Celsius, an increase of 1.6 degrees. Then, she was moved to the shade of street trees along the roadside and stood there for about 10 minutes. Her facial temperature dropped from 36.6 degrees to 34.8 degrees, a decrease of 1.8 degrees. But that was not the end.

The next stage of the experiment was even more surprising. The participant was moved again to continue the experiment. The next location was a shaded area where tall street trees grew densely, and beneath them were low-height trees forming what could be called a flower bed. To put it simply, this was a place with an ‘understory forest.’ After standing there for 10 minutes, her facial temperature was measured at 31.6 degrees Celsius, a drop of a full 5 degrees. The sunlight from the sky was a problem, but the understory forest blocked the radiant heat that bounced back from the ground. A 5-degree difference in facial temperature would feel even greater in reality. This alone is enough to appreciate the effect of street trees.

An old zelkova tree standing on a traffic island in the middle of the road, living while being drenched in the exhaust fumes of passing cars. Siheung-dong, Seoul. Photo by Ko Kyuhong

In fact, while shade that blocks sunlight is very effective, there is another equally important reason. That is the effect of transpiration, which occurs continuously in living trees. Trees draw water up from their roots and perform photosynthesis in their leaves. During this process, the leaves constantly release moisture in the form of vapor, called ‘transpiration.’ This is why the coolness felt under tree shade is more than just the effect of blocking sunlight. The effect of transpiration is something everyone experiences when entering a dense forest. The shade effect is enhanced by the effect of transpiration.

Upon reflection, the need for trees in cities is not just for this season. When the weather is hazy due to fine dust and yellow dust, people also think of trees. Although various methods to effectively remove fine dust are discussed, nothing absorbs fine dust as continuously and effectively as trees.

The principle by which trees absorb fine dust and exhaust fumes is simple. Plants breathe just like humans. Trees continuously inhale and exhale air through tiny pores on their leaves called ‘stomata.’ During the day, when they receive sunlight, trees absorb carbon dioxide for photosynthesis and release oxygen, while at night, they do the opposite, absorbing oxygen and releasing carbon dioxide like animals. This process continues throughout the day. Fine dust and exhaust fumes in the city are drawn into the stomata on the leaves and stick there. Ultimately, trees with more stomata absorb more fine dust.

A willow tree standing at the edge of a quiet village road, serving as a resting place for passersby. Seochon-dong, Cheongju. Photo by Go Kyuhong

To absorb more fine dust, trees with many tiny stomata are effective. That is why people plant trees with large leaf surface areas as street trees in cities. The plane tree, commonly called ‘Platanus,’ is a representative example. Although it does not bloom beautiful flowers like cherry or Ippopan trees, and its bark is mottled like a face with blemishes, it is widely planted as a street tree for clear reasons.

The broad leaves of the plane tree naturally have more stomata than other trees, and the leaf surface is densely covered with very small, fine hairs that are barely distinguishable at a glance. These tiny hairs are excellent at adsorbing fine dust and air pollution. So, when the tree inhales, it draws in fine dust, and although exhaling might release some dust back, the fine hairs on the plane tree’s leaves hold onto the dust for a long time. Moreover, it is a hardy tree that can withstand the polluted urban environment. There is no better tree for purifying the bad air in cities. This is why it is widely planted as a street tree worldwide. It is even said that plane tree species were planted as street trees in Greece around the 5th century BCE, when pollution was presumably not a major concern.

Among recent research results from the Korea Forest Research Institute, a study identified tree species suitable as urban street trees. According to this research, the ‘Hwaback’ tree has a better fine dust absorption capacity than the plane tree. The study found that although Hwaback’s leaves are slender, their surfaces have many fine wrinkles. When these wrinkles are unfolded, the leaf surface area is larger than that of the plane tree’s leaves, and naturally, the surface contains stomata. Furthermore, since plane trees lose their leaves in autumn and remain leafless until spring, their air purification ability stops during that period. In contrast, Hwaback is an evergreen tree that continuously absorbs fine dust even in winter, resulting in a higher annual fine dust absorption amount.

An old elm tree standing tall at the roadside bus stop. Hyangsan-ri, Danyang. Photo by Ko Kyuhong

There is another reason for planting trees in cities. Beyond environmental purification, there is the so-called ‘beautification’ effect that makes the urban environment more beautiful. For example, cherry and Ippopan trees, which bloom beautiful flowers in spring, are planted primarily to beautify the city during that season. Likewise, ginkgo trees, which bear countless fluorescent yellow leaves in autumn and brighten the city, are also planted to enhance the urban environment.

Urban trees are planted and nurtured by city dwellers to live in the dirtiest environments to purify the living environment. Trees are used to improve the living conditions for people. However, people often find street trees as much of a nuisance as a necessity. For instance, when roadside trees grow too large, they can block traffic signs and pose a risk of branches breaking over the road. This creates reasons to prune or cut them down to reduce accidents. Businesses along the road may complain that trees block their carefully made signs. Since trees are planted for practical reasons, practical reasons are also sought to justify cutting them down.

There are even frequent cases of suppressing the nature of trees. Last spring, a local government reportedly removed new flower buds from ginkgo trees to prevent the smell of ginkgo fruits. This cruel measure aims to suppress or even eradicate the reproductive instinct of ginkgo trees. The 《I Ching》 (Book of Changes) contains the concept of ‘Daedae (待對),’ which is a vital life principle emphasizing the harmony of yin and yang. It states that all things in the world must face their counterparts. For ginkgo trees, which have separate male and female trees, the male and female trees must wait for and face each other, which aligns with the life principle in the 《I Ching》.

However, urban ginkgo trees are being deprived of their life principle by people. When recalling the foul smell of ginkgo fruits, one tends to overlook the cooling shade, fine dust absorption effect, and the aesthetic pleasure of yellow autumn leaves. Comparing the time ginkgo trees bring joy to people with the time they emit unpleasant odors is a foolish calculation. The foul smell lasting just about ten days in autumn is a nuisance that tramples on the instinct of ginkgo trees as living beings.

To fully benefit from trees, we must protect them as living beings. The foundation for trees to make our world a better place to live for a long time is to preserve their instincts. This is the right way to live together with trees.

Ko Gyu-hong, Tree Columnist