Any living things need nutrition to grow bigger, right? For a tree, in general at least, that food comes from the ground, aka soil. So, as a corollary, people of Japanese forestry say we can “measure” the “power of soil” by seeing the height of a tree. Especially in areas where afforestation was done some 70 years ago, “tree” means cedars and cypresses in Japan. And we can find a bit sorry 70 years’ old trees here and there in the forests of Kanagawa Prefecture. Is it the matter of our soil?
In theory, those trees planted during the 1950s to the early 60s should be now tall enough to produce standard materials for housing. Alas, in 1964 Japanese logs lost the battle of international competition after the complete import liberalization. As it did not pay, people have not cared the afforested area sufficiently. Now supposed-to-be good enough construction materials are found in congested mountain slopes. No proper thinning was done, which made them lanky and not tall enough for 60 or 70 years old cedars. Cypresses have lots of scars on their trunks as they have not received appropriate pruning when they were young. Such regretful features make market value of timber from our forest even lower. Yap, it’s a man-made result. Even so, in the first place for Kanagawa, was our land rich enough to nurture trees to grow faster and bigger in 70 years? Did it have a potential to be a good forestry area when that afforestation policy was started 80 years ago? The other day, I had a chance to see how the soil in afforested areas of Tanzawa Mountains 丹沢 is. That was shocking.
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| I’m
afraid to say this forest in Yokohama shows ample signs of neglect. |
Geologically speaking, Kanto Region is generally covered with Kanto Loam, the stratum made of 25-40% silt and clay. The origin of it is volcanos surrounding Kanto Plane. For Kanagawa Prefecture, especially Mt. Fuji 富士山, Mt. Hakone 箱根山, and Mt. Ashitaka 愛鷹山 contributed a lot with their eruptions during Pleistocene era that was some 2.58 million to 10 thousand years ago. They spewed out massive amount of volcanic ash and granule whose size in general is Ø 2 mm or less. They have been winded up countless times for these millions of years, which made their ingredient, iron, oxidized. So, Kanto Loam is often called Red Soil, Akatsuchi 赤土, in Japanese. Loam stratum itself is very heavy clay that was not so good for vegetation to grow. No matter, Japanese flora has tried to live on it for a very long time. They have gradually turned the red cray into black soft humus. (More to it, next week.) This black top soil is named with onomatopoeia as Black Bok Soil, Kuro-boku-do 黒ボク土, as farmers felt its texture “Bok-bok.” So, the Japanese bok-bok black soil started with volcanic ash. It is different from chernozem in the continents. Bok-bok or not, humus is good for agriculture and forests to grow, right? So, the thicker the bok-bok soil, the larger the trees are.
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| Kanto
Loam in Niiharu Forest. As it is heavy in clay, we can use it, for example, for constructing kiln for charcoal baking. |
Peculiar feature of the soil in west Kanagawa is, Kanto Loam is covered heavily with scoria stratum originated from Fuji 1707 tephra. In October 1707, present-day Sizuoka Prefecture 静岡県 was attacked several big earthquakes, including the one of M 8.7. Then, on 16 December, the south-east slope of Mt. Fuji around timberline opened 3 volcanic mouths. As of July 2019, it is the latest eruption of Mt. Fuji. Now there remain craters called Hoei volcano 宝永火口. At that time, Mt. Fuji spewed out for 2 weeks humongous amount of ashes and granule that was estimated to reach 20km high in the atmosphere. People in the downtown Tokyo still find about 2cm of scoria stratum in their soil. Niiharu Forest 新治市民の森 has about 5cm of scoria beneath. Yokohama’s South Forest, like Yokohama Nature Observation Forest 横浜自然観察の森, has 15cm of them. But it was the present-day Gotemba City 御殿場 in Shizuoka, and West Kanagawa that received, literally, tons of it. For western Kanagawa Prefecture, the thickness of scoria reached 80-30cm.
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| Yokohama
Nature Observation Forest in January. OK, the place was almost denuded in the 1960s for a development of golf course. (Oh, yes.) So, their trees of oaks like Quercus serrata are relatively young. Yet the place has certainly thicker scoria stratum than in Niiharu, and it shows in its vegetation, I think. |
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| Niiharu
Forest in May. Er, I tend to be partial when it comes to Niiharu Forest. But I really think the depth and biodiversity here is richer than in the larger South Forest of Yokohama. |
Scoria is made of porous pumice. It cannot retain water or nutrition. If you’ve ever been to Mt. Fuji, especially from Gotemba side, you notice its slope is a “desert.” It is the area where Hoei scoria is piled up more than 1m. The thicker the scoria, the more difficult for soil to establish bok-bok soil is. So, we still have hear-say that after the 1707 eruption those ancestors of landlords in Hadano City 秦野市, the main entrance to Tanzawa mountaineering routes, dug up their bok-bok soil covered by 50cm scoria in order to continue their farming. It is said that many people migrated to Edo, aka Tokyo, as refugees to survive. The downtown was also covered with volcanic ashes. They starved to death. Famine continued for many years. According to the statistics recorded by the feudal lord of Odawara 小田原, their territory could not recover the production level of agriculture until the early 19th century. If farm land with human intervention was in such condition, forests in deep mountains must have fared worse.
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| Observing
Hadamo Basin from Take-no-dai 岳ノ台 (ASL 899m). Could you imagine people dug up this entire area to recover the black soil below the 50cm of scoria? |
Recently, we forest instructors had a training session to learn methodology recording soil conditions in forests. We entered a prefectural forest in Tanzawa Mountains, and vertically dug 1m the slope to create a cross section of the soil.
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| Digging. Could you figure out red below? It’s Akatsuchi. And black sandy thing we’ve dug out is scoria. |
The result was this stratum we’ve dug out.
The border
between red and black is somehow clear, and we can figure out a layer of larger
pumice on it. Those are volcanic bombs reached first here before scoria ashes
fell over slowly. Also, the top part of red soil is a bit brownish. It is a
former top soil before the eruption. The current top soil can also be divided
in two. The deeper part is a mixture of more scoria and less fermented organic
matter. The soil just beneath the surface has more organic matter approaching to
bok-bok.
“Wow, this part has only 15cm of black soil.” “Yeah, below is the scoria … let me see, hmmmmmmmm, it has 40cm of the width.” “And finally, there comes the Kanto Loam.” “Could you see almost no root end in the scoria?” “Yeah, plants must go further deep to Kanto Loam in order to have some water and minerals …” “Otherwise, they remain in the 15cm of top soil.” Oh my Buddha. More than 300 years later, the hummus of the area is only 15cm deep. It takes really LONG LONG time for volcanic ashes to be good enough for flora to survive ...
I was a sort of convinced why those afforested areas in Yadoriki Water Source Forest 寄水源林 show apparent slowness in the growth of trees, compared with Doshi Water Source Forest 道志水源林. The northern limit of Fuji 1707 tephra to reach is the Northshore of Lake Yamanaka 山中湖. Doshi Forests situate in the north of the lake. The depth of scoria in Doshi village was less than in downtown Tokyo, if any. They were spared. It is so amazing to imagine why they are different. Tanzawa and Doshi are less than 20km apart. Maybe the wind? Maybe the form of ridges of Tanzawa Mountains? It’s like a very thin boundary between life and death when Mother Nature completely dominates the scene. Awesome … Then, lately I also had a chance to check the process of creating top soil. I’ll tell you my adventure for this next week. It was also a thought-provoking experience, I tell you.
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| Typical 50-60 years old trees in Doshi Village |
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| About
50 years old trees in Yadoriki. Could you see the difference from the above trees? |
If you find an environmental issues
in Kanagawa Prefecture, please make a contact with Kanagawa Natural Environment Conservation Center 神奈川県自然環境保全センター
657 Nanasawa, Atsugi City, 243-0121 〒243－0121 厚木市七沢657
Phone: 046-248-0323
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