Natural Habitat of Hinoki Asunaro and Abies Sachalinensis

Hinoki Asunaro and Aotodomatsu Native Habitat

On the western coast of the Oshima Peninsula in southern Hokkaido, the small town of Esashi faces the Sea of Japan. Inland from its harbour, the Todogawa River carves a deep valley through the hills, and the upper basin holds roughly 500 hectares of old-growth conifer forest. Two tree species meet here by geographic chance. Hinoki asunaro (Thujopsis dolabrata var. hondae)—locally called hiba—reaches the northern edge of its natural range. Aotodomatsu, a southern form of the Sakhalin fir (Abies sachalinensis), reaches its southern edge. Forests pushing up from Honshu and forests spreading down from northern Hokkaido overlap in this single valley.

The site was designated a National Natural Monument by the Ministry of Home Affairs on 12 October 1922 (Taishō 11). For more than a century since, the forest has been left largely to itself.

A rare overlap of two distributions

Hinoki asunaro is an evergreen conifer of the cypress family, a northern variety of the species Thujopsis dolabrata. In Aomori it is known as hiba, in Ishikawa as ate (Noto hiba). Its range extends from Tochigi Prefecture on the Pacific side and the Noto Peninsula on the Sea of Japan side, north into the Oshima Peninsula. Esashi—together with the neighbouring towns of Kaminokuni and Assabu—marks the northernmost natural occurrence of this tree.

The Sakhalin fir, meanwhile, is the dominant conifer of Hokkaido. It splits taxonomically into two forms based on cone morphology: akatodomatsu, more common in the northeast, and aotodomatsu, more common in the southwest. The distinction is gradational rather than sharp, but the Todogawa basin lies near the southern limit of the aotodomatsu form.

The two species do not normally share the same climatic niche. Located within the cool-temperate beech zone, this site sits at the cold extreme for hinoki asunaro and the warm extreme for the Sakhalin fir. Forests where both grow together are unusual on a national scale, and the resulting plant and animal community is distinctive enough that biologists treat it as an entity in its own right.

Why it was designated

Japan's Law for the Protection of Cultural Properties allows a natural monument to be designated under several criteria. This site qualifies under two of them: "representative virgin forest, or rare forest flora" and "notable limits of plant distribution." One reading values the forest itself; the other values its position at a botanical boundary. Most natural monuments meet only one criterion. The double rationale signals just how unusual this place is.

Recent genetic studies have added another layer of meaning. Hinoki asunaro in Esashi shows the closest affinity to the ancestral populations that survived the last ice age, suggesting a refugial lineage isolated in southern Hokkaido. Genetically, these trees are distinct from the hiba forests of Honshu—a living archive of glacial-era diversity.

On 17 August 1943 (Shōwa 18), the designated boundary was partially revised. The Forestry Agency later strengthened protection by establishing the Todogawa Plant Community Protected Forest on 5 November 1988, which was reorganised in April 2018 as the Todogawa Biotic Community Protected Forest, covering 498.60 hectares. The land also falls under designations as a soil-conservation forest and a special protection zone for wildlife—several layers of legal protection stacked over the same ground.

Reading the forest itself

Forestry Agency surveys describe a canopy dominated by hinoki asunaro and aotodomatsu, with trees averaging about 16 metres in height and 24 to 26 centimetres in trunk diameter at chest height. This is not a forest of giants. Northern conifers tend to be compact rather than colossal, and the stand here has the disciplined, mid-sized profile typical of high-latitude growth. Scattered broadleaf trees—Japanese beech (Fagus crenata), Mongolian oak (Quercus crispula), and Japanese castor aralia (Kalopanax septemlobus)—add seasonal colour.

Beneath the canopy, hinoki asunaro continues to dominate. The shrub layer holds Japanese dogwood and small ashes; the forest floor is dense with ferns. Where a fallen tree has opened a gap in the canopy, you can find seedlings of beech, basswood, hinoki asunaro and Sakhalin fir competing for the new light. The forest is renewing itself slowly, on its own terms.

The terrain is steep, and hinoki asunaro favours the moister lower slopes. Many trunks are bent sharply at the base by years of snow-load. Some branches creep along the ground, take root, and grow into independent trunks—a regeneration strategy called layering. This horizontal expansion, alongside seedling regeneration, allows the species to hold its ground in difficult conditions.

What "Hiyama" tells you

The administrative name of this region—Hiyama, written with the character for hinoki (檜) and mountain (山)—is itself a clue. Hinoki is the famous Japanese cypress (Chamaecyparis obtusa), prized for shrine and temple construction in central Japan. True hinoki does not grow naturally in Hokkaido. But the hiba trees of this region resembled hinoki so closely in material and appearance that local people called the hills "Hinoki-yama," hinoki mountains. The name stuck and now identifies the entire sub-prefecture.

Hiba timber is durable, fragrant, and resistant to decay, and Edo-period builders treasured it. In 1596 (Keichō 1), at the end of the Sengoku period, a logging office was set up in nearby Kaminokuni to manage hiba extraction and trade. By the Edo period, logging had expanded into Esashi and Assabu, and the office moved to Esashi. The Matsumae clan, which governed southern Hokkaido, shipped hiba timber to Honshu for use in castles and temples, and the trade fattened domain finances substantially.

The unintended outcome was the near-disappearance of the old hiba forests of southern Hokkaido. The reason this stand in the Todogawa basin still exists is precisely that it was placed under national protection in 1922, before logging could reach it. The Edo and Meiji-era hiba buildings that survive in central Esashi—including the Kyū-Nakamura House, an Important Cultural Property—are essentially relatives, in cut timber, of this living forest.

Before you plan a visit

The heart of the protected forest sits deep in a steep valley, and is not realistically accessible to general visitors. A forest road once led into the area and a viewing route had been maintained, but the road has long since reverted to nature and is no longer passable. Entry is restricted to researchers and forestry officials operating under permit.

Esashi Town has worked with the Forestry Agency on a long-term project called Hiyama Koto-no-Mori (Hiyama Ancient Forest), a "model hiba forest" within the nearby state forest, where hiba seedlings are being cultivated on a four-hundred-year timetable to supply restoration timber for future generations of historic buildings. The model forest was at times open as a walking trail, but as of 2025 access is suspended. Anyone planning a visit should contact the Esashi Town Office or the Hiyama Forest Management Office in advance.

If the forest itself is out of reach, the cultural connection is not. The Inishie Kaidō, the historic merchant street that runs through central Esashi, is lined with houses, warehouses and shrines built largely from local hiba. Walking through the streets gives you a way to encounter the timber—and indirectly, the trees—up close.

What else to do in Esashi

Esashi was one of the earliest Japanese settlements in Hokkaido, growing wealthy in the Edo and early Meiji periods on the herring fishery and the hiba trade. A local saying ran: "Esashi in May has no equal even in Edo." The town is roughly 1.5 hours by car from Hakodate Airport, or about 2 hours by bus from JR Shin-Hakodate-Hokuto Station.

• Kamome-jima Island—A small island just offshore from Esashi harbour, about 2.6 km in circumference and now linked to the mainland by a causeway. A walking path circles the cliff edges and reaches viewpoints famed for sunsets over the Sea of Japan.
• Kyū-Nakamura House—A late-Edo merchant complex built by Ōmi traders and now an Important Cultural Property. Four wooden structures lined up along a slope, mostly in local hiba timber, are open to visitors.
• Kaiyō-Maru Memorial Museum—A full-size replica of the Tokugawa shogunate's flagship warship Kaiyō Maru, which sank off Esashi in 1868. About 3,000 artefacts recovered from the wreck are on display.
• Ubagami Daijingū Shrine—The oldest shrine in Hokkaido. Its annual festival (9–11 August) features thirteen elaborate floats, the most elaborate matsuri parade in the prefecture.
• Esashi Oiwake Kaikan—A centre devoted to Esashi Oiwake, the famous folk ballad of northern Japan, with live performances and historical displays.