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Joshin’etsukogen NP, Nagano
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Ikenotaira Marsh lies inside the ancient crater of Mt. Sanbōgamine. Rain and snowmelt gathered there over millennia, gradually building the wetland’s peat hummocks.
The crater rim once enclosed the marsh. Its southeast “open window” formed after part of the rim collapsed, but the cause is unsettled: interpretations propose either fault movement or erosion by rain and snowmelt.
Tomi City is home to 3 butterfly species designated natural monuments by Nagano Prefecture. Benihikage still occurs in numbers at Ikenotaira, where visitors may spot this highland butterfly among the flowers.
The marsh’s peat hummocks are more than plant-covered mounds: 2 local snakes, the aodaishō and jimuguri, tunnel into them to lay their eggs.
Look closely near Kagami Pond for round-leaved sundew, a tiny carnivorous plant. Its sticky hairs trap insects, and in August it sends up a slender stem bearing small white flowers.
At about 2,000m, Ikenotaira’s unusual climate and steep terrain bring together plants and animals of lower-elevation foothills and species usually associated with mountains around 3,000m.
More than 100 kinds of flowers bloom here in summer. Around the marsh and crater rim, komakusa appears in early summer, irises in July, then willowherb, scabious and gentians later on.
Bog bilberry bears edible, blueberry-like fruit, and its leaves are often where the moorland clouded yellow butterfly lays its eggs. The shrub is especially abundant near the marsh’s open crater rim.
The crater rim changes character from one side to the other: Japanese larch dominates the northern ridgeline, while the southern slope holds stands of northern Japanese hemlock.
On nearby slopes of Mt. Miharashi and Mt. Sanbōgamine, komakusa grows on gravel where other plants struggle to take hold. Its deep roots and leaves adapted to fog help it manage this demanding habitat.
