Nature Mount Zao’s snow monsters shrinking after insect attacks

A new survey by Yamagata University has revealed that Zao's celebrated "snow monsters" are slowly losing their majestic form, eaten away by insect infestations that have spread across the mountain range. These otherworldly figures, known in Japanese as juhyō (樹氷), appear each winter when freezing winds from Siberia and deep mountain snows coat the Maries' firs (Abies mariesii), turning them into ghostly sculptures of ice. For decades, they have stood as a symbol of the region, drawing travellers to the border of Yamagata and Miyagi prefectures.

That spectacle is now under threat. Since around 2013, caterpillars have been feeding on the branches and needles of the firs, stripping away the dense greenery that is needed for the icy forms to cling to and grow. Without foliage, the snow monsters cannot take shape.

To measure their decline, Juhyo no Kai (society for the study of frost-covered trees), a research group led by Fumitaka Yanagisawa, professor emeritus of environmental science at Yamagata University, examined photographs of Mount Jizo, one of the prominent peaks of the Zao range, taken from identical vantage points as far back as 1933. The team assessed the ice formations using a six-point scale of thickness. According to the findings, the ice monsters often stretched 5 to 6 meters across before World War II. As postwar warming took hold, they dwindled to 2 to 3 meters, and sometimes to only a single meter thick. Since 2019, the dominant forms have withered further to just 0.5 to 1 meter, and in some cases, a spindly 20 to 30 centimetres. Driving this decline is a cascade of ecological damage.

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'Inbound' insects, I bet

They seem to be indigenous critters:

The subalpine Maries' firs in northern Honshu came under siege around 2013 from caterpillars of the tortrix moth (Epinotia piceae). In unusually high numbers, the insects stripped the trees of their needles, crippling their ability to photosynthesize. A second assault soon followed, by bark beetles (Polygraphus proximus) sweeping through the Zao mountains from about 2015. Preferring weakened hosts, the beetles bored into the already stressed firs, triggering widespread die-off and driving the tree line into retreat. On the Yamagata side of the range, an estimated 23,000 trees--about one-fifth of the population--have already died.

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Reminds me of issues we have in Colorado. pine beetles which were always here usually reduced in numbers greatly every winter due to a hard freeze. Around 15 years or so ago it stopped getting quite as cold so we had a lot more of them ending up in many dead trees which is one of the reasons for increased fires we have.

The BBC on the efforts to save Mount Zao's famous "snow monsters."

Juhyō (snow monsters) form only when a very narrow set of weather conditions comes together. Strong winter winds have to drive supercooled droplets against the Aomori todomatsu evergreens, where the moisture freezes instantly and gradually builds up into thick layers of rime ice. On Mount Zao, this happens under steady westerly winds that can reach 26 metres per second, with surface temperatures hovering around –6 °C to 0 °C, and with an unusually high amount of liquid water suspended in the cloud. When those factors align, the ice accumulates on the wind-facing side of the trees in curved ridges often referred to as "shrimp tails", and the merging of these shapes produces the tall juhyō figures that people recognise.

A researcher at Yamagata University, Professor Fumitaka Yanagisawa, has noted that the combination of climate and tree cover needed for this process occurs in only a handful of places, which is why Zao's so-called snow monsters are regarded as something distinctive to northern Japan. These formations are now the main winter attraction in the Zao range, which straddles Yamagata and Miyagi prefectures, drawing large numbers of visitors each year.

"In the 1930s, we saw juhyō five to six metres across," Yanagisawa says. "By the postwar decades, they were often two to three metres. Since 2019, many are half a metre or less. Some are barely columns." The cause is twofold, says Yanagisawa: a warming climate and a forest under attack. The host tree, Aomori todomatsu, suffered a moth outbreak in 2013 that stripped its needles. Bark beetles followed in 2015, boring into weakened trunks. Yamagata officials report that around 23,000 firs, about a fifth of the prefectural side's stands, have died. With fewer branches and leaves, there is little surface for snow and ice to cling to. Another 2019 study found that in nearby Yamagata City, average temperatures from December to March have risen by about 2 °C over the past 120 years. The lower altitude limit of the juhyō formation has shifted upward in step with this warming, it found, while the juhyō also lasts for fewer days of the year.

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Great juhyō imagery here: