Zao ‘Snow Monsters’ Thinning Rapidly Due to Insect, Snow Damage; Research Shows Ice Tree Formations Have Shrunk Greatly in Recent Years

By Jo Nakato / Yomiuri Shimbun Staff Writer

13:00 JST, September 24, 2025

YAMAGATA — The ice formations on trees that create spectacular winter views in the Zao mountain range spanning Yamagata and Miyagi prefectures have been thinning rapidly since 2019, according to research by Yamagata University Prof. Emeritus Fumitaka Yanagisawa and others.

The thinning is attributed to the death from pest damage of many Maries’ fir trees — the conifers on which ice tree formations, also known as snow monsters, develop — causing branches to break under the weight of snow. The Yamagata prefectural government and other groups are planting trees to revitalize this important tourist resource.

Maries’ firs naturally grow in the Zao mountain range at altitudes between 1,300 and 1,700 meters above sea level; snow monsters form on those situated at heights at and above 1,500 meters. They are created when snow and moisture in the air are blown onto these trees. According to the company that operates the Zao Ropeway, 140,000 people visited to see the ice trees during the season from December 2024 to March 2025.

Juhyo-no-kai (The ice tree association), a society of researchers of which Yanagisawa is a member, analyzed thousands of old photographs of ice tree formations that developed around the ropeway’s Jizo Sancho Station, at an altitude of 1,661 meters, to compare their sizes over time.

The research found that formations with diameters of 5 to 6 meters were common in the 1930s to 1950s. Even after that, ones with diameters of 2 to 3 meters were seen until the 2010s. In recent years, however, the majority have measured only about 0.2 to 1 meter in diameter.

Massive outbreaks of moth larvae and other pests from around 2013-2016 caused the deaths of about 23,000 Maries’ firs across an area of 16 hectares around the station. These accounted for nearly 20% of the roughly 126,000 trees then growing in clusters on the Yamagata side of the mountain range.

Yanagisawa said the rapid thinning of the ice trees began around 2019, when more and more starved or weakened branches started breaking under the weight of accumulated snow. Trees with few branches or leaves cannot support large volumes of snow and ice. Even during this year’s heavy snowfall, most ice trees in areas above 1,600 meters above sea level remained only about 0.5 meters to 1 meter thick.

“We have seen many trees topple over under the weight of the snow,” Yanagisawa said. “We are afraid that the number of ice trees may decrease.”

The prefecture’s division in charge of nature conservation said the Forestry Agency began planting trees in 2019 and has planted 330 saplings around the station to date. In 2023, the agency, the prefecture, tourism groups and experts formed the Prefectural Council for the Restoration of Ice Trees in hopes of restoring Zao’s unusual natural landscape.