Photo/Illutration Frost-covered “ice monsters” on Mount Zao in Yamagata in February 2023 (Asahi Shimbun file photo)

YAMAGATA—Insect infestations are reducing the famed “ice monsters” of the Zao mountain range to mere shadows of their former hulking selves, a survey by Yamagata University and its research partners showed.

Known as “juhyo” in Japanese, the ice or snow monsters formed on frost-laden firs have created a surreal, frozen sculpture park straddling the border of Yamagata and Miyagi prefectures.

Researchers trace the decline to relentless insect infestations.

Since around 2013, caterpillars have gnawed away at the branches and needles of the firs, eroding the very foundation of the haunting snow figures.

The ice monsters are Maries’ firs (Abies mariesii), evergreen conifers sculpted by frigid Siberian winds and heavy snowfall into towering, ghostly apparitions.

The creation of these snow monsters is a delicate art of nature, requiring a specific combination of weather and vegetation.

These frosted giants, with their kaiju-like silhouettes, have long captivated visitors from Japan and around the world.

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 main 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 centimeters.

Driving this decline is a cascade of ecological damage.

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.

According to the society, the rapid loss of foliage and branches since 2019 has weakened the firs’ ability to hold snow and ice as before, leading to the dramatic “slimming” of the formations.

The last winter was bitter cold with heavy snowfall, and large ice monsters emerged at altitudes between 1,500 and 1,600 meters, where the trees remain largely healthy.

Higher up, above 1,600 meters, the picture was starkly different: Vast forest stands dead, many trees snapped under the snow’s weight, and the landscape was dotted with skeletal trunks stripped bare.

“We now have quantitative proof of just how much the ice monsters have shrunk,” Yanagisawa said. “If warming and die-off continue, the day may come when these iconic figures vanish altogether. That would be a cause for deep concern.”

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