Is cutting down trees evil? Let's rejuvenate our old forests
Updated by Hiroshi Komiyama on May 07, 2025, 3:15 PM 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.
Many of you may remember learning that forests absorb carbon dioxide (CO2) and release oxygen. But did you know that forests do not always absorb CO2? There is a "peak" in CO2 absorption capacity, which changes as trees grow. Trees absorb CO2 through photosynthesis to produce nutrients for their own bodies, and CO2 is fixed as carbon in the "biomass" (material of biological origin) such as leaves, trunks, and roots. As long as this continues, CO2 does not return to the atmosphere, but remains stored in the tree.
However, trees do not continue to grow forever. When young trees grow rapidly and photosynthesize actively, they absorb more CO2. However, as the trees reach a certain age and stop growing, the amount of CO2 absorbed gradually decreases. As the tree ages, the amount of CO2 emitted through respiration and the amount absorbed through photosynthesis become almost the same, and the tree approaches a state of "practically no CO2 fixation. A tree in this state may look like a large, magnificent tree, but it is not making much of a contribution in terms of global warming mitigation. Moreover, old trees are at high risk of falling and disease, and once they fall and begin to decompose, the carbon that was previously fixed in the tree will be returned to the atmosphere.
As entire forests age, their capacity to fix CO2 is greatly reduced. Most of Japan's forests are made up of cedar and cypress trees planted after World War II, but many of these trees have now been planted for more than 50 years and are approaching old age. In other words, the overall CO2 absorption capacity of forests is declining. On the other hand, by planting new saplings in the areas where trees have been cut down, young trees will grow again and CO2 absorption will become more active. In this way, the cycle of properly cutting down trees and letting them grow again is the key to sustaining the forest's CO2 fixation capacity.
It cannot be said that "leaving trees" is always good and "cutting them down" is always bad. Rather, if we want forests to carry the function of absorbing CO2, we need to systematically cut down trees to rejuvenate them. Each tree in a forest may look the same. However, there is an age, a stage of growth, and each has a different role to play. The starting point of the concept of recycling and making the most of forests is to pay attention to such "tree time.
When you hear the word "deforestation," many people probably have the image of environmental destruction. In fact, one often hears the argument that "to stop global warming, we must leave the trees alone without cutting them down. However, this view is not necessarily true. Rather, there are situations in which it is necessary to "cut down trees that should be cut down properly" for the sake of the global environment.
The University of TokyoProfessor Asaomi Kumagaiconducted a study to determine "how much carbon is absorbed and stored by forests at any given age," based on data from planted forests across Japan. What they found was that once a tree exceeds a certain age, it saturates in terms of CO2 absorption and fixation. For example, artificial forests of cedar and cypress trees that were planted in large numbers after World War II are now reaching the peak of their maturity, and if left unattended, their absorption will eventually decrease. Moreover, old trees are at increased risk of disease and windfall, and when they fall over and decompose, they may in turn release carbon back into the atmosphere.
Professor Kumagai's research has shown that thinning and logging these old forests and planting new seedlings in their place can sustainably increase the forest's ability to absorb CO2. In other words, "logging" is not destruction, but a strategic means of "sequestering" carbon. Of course, it does not make sense to cut trees unnecessarily. What is important is the perspective of "where, what kind of trees, and how to cut them. Planned logging and appropriate reforestation. By carefully repeating this cycle, forests will be rejuvenated and their absorption capacity will be increased once again.
In fact, this type of thinking has already become mainstream in Europe. In Germany and other Scandinavian countries, the cycle of forest resources, "cutting down, using, planting, and growing," has taken root as a social mechanism. Wood is first used for building materials and furniture for a long period of time, and when it has finished its role, it is used for fuel and other purposes. The forests are then planted to rejuvenate the entire forest, thereby maintaining their ability to absorb carbon dioxide. Thus, the basic concept of a sustainable society is to view forests not only as "objects to be protected" but also as "resources to be utilized and passed on to the next generation. Logging is a planned part of this cycle.
In the "recycling society" that we are aiming for, the essence is not "not to use" but "to circulate while using. Forests are also a resource that should be addressed based on such a concept. (Hiroshi Komiyama, Chairman of Platinum Network)
