Japan’s oak trees are confronting a pandemic. The crisis began long before COVID-19, and the threat comes in the form of a fungus carried by insect pests rather than a virus.
Known as nara-gare (oak wilt disease), oak trees are withering and dying en masse nationwide, which has put the health of satoyama — an intermediate, partially human-made zone between primeval forests and arable flat land — at risk, experts say.
Kobe University researchers, in collaboration with businesses, are now trying to reverse the tide by digitally cataloging the trees and facilitating their use, either as material for furniture or for green tourism. With these methods, extensive oak wilt can be prevented, the researchers say.
“Oak wilt is an old disease”, according to Wakana Azuma, Assistant Professor at Kobe University’s graduate school of agricultural science, which is involved in the project. “But the problem was somewhat contained because, for a long time, people kept cutting young trees to use the wood for fuel before the disease spread widely.”
That changed in the 1950s as oil and gas became the major sources of energy in Japan. People stopped cutting trees and left them unattended, which made their trunks grow wide and created room for the pest in question — oak ambrosia beetles — to create holes in the trees, invade them and then proliferate inside on a massive scale.
What actually kills the trees is not the bugs themselves but a fungus they carry with them. The more the beetles move about inside the trees, the more fungus they spread. The trees then try to defend themselves by exuding fluids containing phenolic substances. However, the fluids themselves are toxic and clog up the trees’ vessels, blocking the movement of water from the roots to stems and leaves, causing the trees to wither.
Oak wilt, which has intensified since the 1990s, was confirmed in 42 of the nation’s 47 prefectures in 2021 — all the way from Aomori in the north to Kagoshima in the southernmost region of Kyushu, according to the Agriculture, Forestry and Fisheries Ministry. The extent of damage fluctuates year by year, but it is estimated to have totaled 153,000 cubic meters in fiscal 2021, the ministry says.
Since satoyama forests are close to where people live, oak wilt can wreak havoc by increasing the risk of trees falling on hikers, cars or houses, experts say. The loss of oak trees, which are tall and long-living, can also increase the risk of mudslides, especially in forests inhabited by Japanese deer that eat away young trees and expose bare ground.
So what’s the solution? It may sound paradoxical, but the best way to keep a satoyama forest in good condition is to constantly cut down healthy trees while they are relatively young, or practice “coppicing,” so the trees put out new shoots from stumps or roots and rejuvenate on their own. Of course, the process needs to be sustainable for it to continue.
Most of the broad-leaved trees used today are made into wood chips for biomass fuel. Such products are extremely cheap, fetching only several thousand yen per kilogram, so they do not appeal to most private-sector forest owners.
The researchers, led by Professor Azuma theorize that this can change because most oak and other broad-leaved trees that are used for furniture and flooring materials in Japan are currently imported from overseas.
Earlier this year, the start of the Russia-Ukraine war and a resulting export ban by Russia on wood chips, logs and veneer sheets sent lumber prices skyrocketing. Azuma’s lab received numerous inquiries from lumber manufacturers in Japan — a sign, she says, that information about trees’ availability in the country is not being shared widely. In fact, even the owners of private forests, many of whom are local farmers, don’t know what kind of trees their forests are made up of.
“If we could replace the imported lumber with that from Japan, at the same price we are paying for imports, that would help with forestry management,” Azuma says.
That’s where the idea of tagging comes in. The researchers are currently doing a pilot study in six locations across the nation, including two in Nagano and one each in Hokkaido, Shiga, Kyoto and Hyogo, attaching tags to 50 to 100 trees in each area. They have developed a smartphone app to upload information on each tree, including its species, size and height, and any other details that might interest users. Photos can also be attached.
A critical player in the project is Keiko Kuroda, a professor emeritus at the university who recently established a forestry management consultancy. Wood product coordinators and Aichi Prefecture-based Karimoku Furniture are also involved.
The researchers hope the system will be used by a wide range of people, including not only forestry owners, forestry associations, lumber mills and housing and furniture companies but also wood-working artists, park managers and academics. It is anticipated that those working in these fields will be able to access the database and reach deals on the trees by paying membership fees.
Ultimately, Azuma wants to help promote the understanding of and engagement with satoyoma forests by citizens, not just by industry.
“Green-woodworking workshops, where salaried workers drop by after work to carve spoons out of unseasoned wood, are catching on in Japan, apparently as a stress release,” she says. “We hope the project will spark more people’s interest in the environment near them, and lead to the creation of tourism and other services that cater to such interest.”
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