Nature

Shirakawago: an unexpected place for a lesson in forest insect ecology

I had three things in mind for my Christmas vacation to Shirakawago: the idyllic Christmas view of snow covered Gasshou-tsukuri villages, the sericulture workshops in the attic of the Gasshou-tsukuri houses and maybe (just maybe) the Japanese take on mulled wine. Unfortunately, it didn’t snow over the weekend I was there, the sericulture workshops were no longer operating and there was no mulled wine.

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Photograph of snow covered Gassho-tsukuri houses sourced from Wikipedia

The village didn’t hold my attention for long and I started to wander towards the bushes surrounding the Gasshou-tsukuri houses. Colourful mushrooms of all shapes, sizes and textures caught my eye.  Favourable conditions such as heavy rainfall (jelly fungi become more visible when it rains because they can hold up to sixty times their dry weight in water) and an excess supply of  decaying wood (many mushrooms are the fruiting bodies of saprophytic fungi that grow on dead wood) resulted in a large quantity of mushrooms in the area. The acorns scattered all over the ground suggested that the mushrooms were most likely growing on decaying beech and oak logs.

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Black turkey tail (Trametes versicolor) mushroom

An Amur Honeysuckle (Lonicera maackii) berry lying on a bed of  Turkey tail (Trametes versicolor) mushrooms 

From left to right: Mock Oyster mushroom or キヒラタケ (Phyllotopsis nidulans) and unidentified small gilled mushrooms 

From left to right: Unidentified species of brown jelly fungus, unidentified species of black jelly fungus and Calocera fuscus

Most surprising of all was that the decaying wood had a reach much further than the woods surrounding the village. Many logs were discarded on walkways along big roads. Why were the local lumberjacks so wasteful? Were the logs really of no use?

I met a group of old ladies at the local bathhouse later that day and they told me some stories about the wood industry in Shirakawago after I commented on what I saw during the day, “our parents used to go to the mountains to make charcoal for days on end and we had to cook, bathe and go to school on our own. We became very independent, very quickly. Things have changed so much since – charcoal has been replaced by fossil fuels and demand for oak trees has dwindled”. They went on to explain how there was an insect outbreak that killed many oak trees in the area, “[they] had to cut down so many oak trees because the insects got to them. You can probably see random logs lying around”.

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Image of an adult female Oak Ambrosia Beetle (Platypus quercivorus) sourced from FFPRI

Details of their stories led me to papers describing Oak Wilt Disease in Japan. The keywords sent flashbacks to a forest insect ecology class – Professor Kamata had told us about his research on how and why the symbiotic relationship between the Oak Ambrosia Beetle (Platypus quercivorus) and the Pathogenic Ambrosia Fungus (Raffaelea quercivora) caused high incidence of mortality among oak trees (mainly Quercus serrata and Quercus Mongolica) in Japan. The relationship is truly symbiotic because the fungus spreads with the help of the flying Oak Ambrosia Beetle as a vector and the adult female and Oak Ambrosia Beetle larvae feed on the hyphae of the Pathogenic Ambrosia Fungus while inhabiting  the host tree. The process starts by the male Oak Ambrosia Beetles boring an entrance hole in a healthy host tree. It emerges from the entrance hole when a female Oak Ambrosia Beetle arrives. The pair then mate in a tunnel and the female Oak Ambrosia Beetle bores horizontal galleries, where it lays eggs. The female Oak Ambrosia Beetle carries the Pathogenic Ambrosia Fungus on its mycangia, a special morphological structure that some insects adapt to carry symbiotic fungi spores, and inoculates the spores as it lays eggs in the horizontal galleries. The female Oak Ambrosia Beetle feeds on the hyphae of the fungi as it continues to lay eggs and the larvae feed on the hyphae once they hatch.

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Image of tyloses  in Quercus petraea cells sourced from Wikipedia

As the fungi and larvae colonise the sapwood, the tree intitiates defence mechanisms such as formation of gummy substances in tree cells called tyloses (tyloses block tree cells so fungal hyphae cannot grow through them). Unfortunately, this also stops water conduction and the tree eventually dies as its vascular system is clogged by tyloses, fungal mycelium and spores. Symptoms such as leaf wilting and mycelium growth near the entrance hole start to show within two to three months after infestation and tree mortality can occur within a season. Once trees become weak or die, saprophytic fungi and microorganisms can move in to degrade the wood.

Although not all the mushrooms I saw in Shirakawago were growing on decaying wood of trees that died from Japanese Oak Wilt, I’m sure some were. I am in awe of how everything is interconnected: increases in fossil fuel consumption caused a decrease in the demand for oak wood, relative oversupply of oak trees created a favourable environment for Oak Ambrosia Beetles – which facilitated proliferation of the Pathogenic Ambrosia Fungus, spread of Japanese Oak Wilt led to high incidence of oak tree mortality and the sighting of so many mushrooms was only made possible by high availability of decaying wood.

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