Introduction / Chapter 9
Forests and Carbon
Forests are places that produce timber, and at the same time places that store water, protect soil and take in carbon dioxide from the air. This chapter checks forest functions in numbers and sorts out how "using" wood relates to carbon dioxide.
Forest functions in money
Following a report by the Science Council of Japan, the monetary value of those forest functions that can be priced was estimated (2001).[1]
| Function | Value per year |
|---|---|
| Preventing surface erosion | ¥28.26 trillion |
| Purifying water | ¥14.64 trillion |
| Storing water (water resources) | ¥8.74 trillion |
| Preventing shallow landslides | ¥8.44 trillion |
| Mitigating floods | ¥6.47 trillion |
| Health and recreation | ¥2.25 trillion |
| Absorbing carbon dioxide | ¥1.24 trillion |
| Substituting for fossil fuels | ¥226.1 billion |
Source: Forestry Agency, "Multiple Functions of Forests"[1]. Each function is valued by a different method, so the figures cannot be added together. These are estimates under specific assumptions.
CO₂ absorption gets much attention, but in monetary terms the functions of protecting soil and regulating water are valued far more highly. This is also why Tokyo's watershed forests have been protected since the Meiji era (Chapter 1).
How much CO₂ do trees take in?
Forests do not only take in CO₂. Respiration by trees and plants and the decomposition of fallen leaves in the soil also return CO₂ to the air.[3] A forest stores carbon on balance only while its trees are growing and their trunks and roots are thickening.
The Tokyo Development Foundation for Agriculture, Forestry and Fisheries explains that as cedars age, their CO₂ absorption falls and the male flowers that release pollen increase.[4] That about 80% of Tama's plantations are 51 years or older (Chapter 2) is considered a problem from this angle too.
What about oxygen?
Through photosynthesis, trees make sugar from carbon dioxide and water and release oxygen, roughly one oxygen molecule for each CO₂ molecule taken in. Conversely, when trees respire, when dead trees and leaves decompose, or when wood is burned, oxygen is consumed and CO₂ is released.
In other words, a forest releases oxygen on balance only while it is storing carbon in trunks and soil. In a mature forest where growth and decay are in balance, oxygen inflow and outflow are roughly balanced too. Calling forests "oxygen factories" is easy to grasp but not accurate. The importance of forests lies in storing carbon and protecting water and soil.
This section explains the general mechanisms of photosynthesis and respiration. That forest respiration and decomposition release CO₂ is confirmed in a Forestry and Forest Products Research Institute report[3].
What changes when we use wood?
When felled trees are used in buildings and furniture, the carbon in the wood stays locked up. According to the Forestry Agency's annual report, a timber house stores about four times as much carbon as a steel prefabricated or reinforced-concrete house, and building it emits far less CO₂.[5] Wood biomass fuel, compared over its whole life from gathering materials to burning, emits far less greenhouse gas than fossil fuels for the same heat.[5]
This is why Tokyo and its forestry foundation promote the forest cycle of "harvest, use, plant and tend" (Chapter 2). Using wood does not destroy forests; it helps rejuvenate plantations that have aged because the cycle stopped. But without replanting and tending after harvest, the cycle cannot work.
Sources
- 林野庁「森林の有する多面的機能について」(日本学術会議答申、平成13年) Link
- 林野庁「森林はどのくらいの量の二酸化炭素を吸収しているの?」 Link
- 森林総合研究所 所報 No.27(2003年6月) PDF
- 公益財団法人東京都農林水産振興財団 説明資料 PDF
- 林野庁『平成28年度 森林・林業白書』第IV章 Link