Why are Japan's Autumn Leaves the Most Beautiful in the World?
Updated by Hiroshi Komiyama on April 01, 2025, 11:22 AM JST
Hiroshi KOMIYAMA
（Platinum Initiative Network, Inc.
After serving as Professor of Chemical Engineering at the University of Tokyo, Dean of the Graduate School of Engineering and Dean of the Faculty of Engineering, Director and Vice President of the University of Tokyo, and President of the University of Tokyo (28th), he became Chairman of Mitsubishi Research Institute, Inc. in 2009, and Chairman of the Platinum Initiative Network in 2010 (to be incorporated as a general incorporated association in 2022). He has also served as President of the Society of Chemical Engineers (2002), President of the Japan Association of National Universities (2007), President of the Association for Further Education, President of the United Nations University Cooperation Foundation, and Chairman of the Ichimura Global Environmental Science Prize Review Committee.
It is said that Japanese autumn leaves are the most beautiful in the world. The reason for this is the richness of the colors. Red, yellow, orange, and some leaves still green, the entire mountain is dyed like a fabric, a scene unique to autumn in Japan. Why is it possible to see such a wide variety of autumn foliage in Japan? In a word, it is because there are so many kinds of trees. The color of autumn leaves differs depending on the type of tree. The maple trees are red, ginkgo trees are yellow, cherry trees are brown, and so on.
So why are there so many different types of trees in Japan in the first place? In fact, this has to do with the topography and history of the Japanese archipelago. For example, there was an ice age about 20,000 years ago. The entire earth became colder and many plants became extinct. However, the Japanese archipelago has a complex topography with a long north-south axis, many mountains, and is surrounded by the sea. Because of this, there were many "refugia" where plants could escape the cold, and more species were able to survive than in other regions.
As a result of the continuous succession of such diverse plants, Japanese forests are rich in "tree species diversity. This is very rare even from a global perspective, and this diversity is reflected in the colors of the forests in autumn. When we look at the beauty of autumn leaves in this way, we may feel the "richness of forests" without realizing it. The diversity of trees living together and changing their appearance according to the seasons - the depth of the forest is blended into our daily landscape.
Now, the reason why I started by talking about autumn leaves is because behind them is the change in the life activity that plants are undergoing. When the leaves lose their green color and begin to show reds and yellows, it is a sign that the tree has stopped photosynthesizing and is getting ready for winter. How do trees live, how do they take in carbon dioxide from the air, and how do they store it? In order to reconsider the forest as a "carrier of cycles," the starting point is to understand the basic structure of the forest.
Plants appear to us as almost motionless entities. However, in their leaves, chemical reactions are constantly taking place to produce energy for life. The two most common mechanisms are "photosynthesis" and "respiration. Both are essential for plants to survive. Photosynthesis is a process by which plants use light energy to produce carbohydrates (nutrients) such as starch from carbon dioxide in the air and water drawn up from the soil. The nutrients produced become materials for the plant's own growth. During this reaction process, oxygen is also released at the same time.
Plant leaves are green because of a pigment called chlorophyll. Chlorophyll absorbs light and is used as an energy source for photosynthesis. In other words, green leaves are small "energy factories. On the other hand, plants, like animals, breathe. In respiration, oxygen is taken in and nutrients in the body are broken down and used as energy. At this time, carbon dioxide is discharged. In other words, plants not only "breathe in carbon dioxide and give out oxygen" but also "breathe in oxygen and give out carbon dioxide.
Photosynthesis and respiration may seem to work in opposite directions, but both are essential for plants. During the day, plants use sunlight to actively carry out photosynthesis, while at night they continue only with respiration. Plants in the sun grow well because they spend more time photosynthesizing and can produce more nutrients. In this way, plants not only build their own bodies, but also take in carbon dioxide from the atmosphere and supply it with oxygen. This steady work of forests is behind the stabilization of the Earth's climate and air composition.
It is important to note, however, that the absorption of carbon dioxide through photosynthesis is closely related to the growth of the tree. Young, vigorous trees grow through active photosynthesis, absorbing large amounts of carbon dioxide. However, as the tree grows older and slows down, it absorbs less carbon dioxide, and the amount of carbon dioxide it absorbs is roughly balanced by the amount of carbon dioxide it releases through respiration. In other words, it is not a simple picture of "trees in the forest = always absorbing CO2. What kind of trees are in the forest, how old they are, and how they live - the amount of carbon dioxide absorbed will vary greatly depending on the balance of these factors.
Trees are constantly moving air in unseen places, nurturing themselves while also influencing the composition of the air. In order to see forests as a "mechanism of circulation," it is an important first step to understand the existence of this invisible metabolism. (Hiroshi Komiyama, Chairman of Platinum Initiative Network)
