On the southeastern edge of the Daisetsuzan National Park (大雪山国立公園, Daisetsuzan kokuritsu-kouen) sits Lake Shikaribetsu-ko (然別湖, Shikaribetsu-ko); and on the southern shores of that lake is Mt. Hakuun-zan (白雲山, Hakuun-zan). Sitting adjacent to it is Mt. Tenbou-zan (天望山, Tenbou-zan), or, as the locals call it, Kuchibiru-yama (唇山, lit. ‘Mt. Lips’). Although a plan had been hatched in recent years to build a 'Shihoro Alpine Grasslands Road' (士幌高原道路, Shihoro kougen-douro) on the Mt. Hakuun-zan’s southern face, public opposition forced its cancellation.
This hike is quite short, but the trail through the evergreens over the shining waters of the volcanic lake below should make it a pleasant one.
The Toumabetsu-gawa River (トウマベツ川, Toumabetsu-gawa) flows into Lake Shikaribetsu-ko in the southwest corner of the lake, and the trailhead sits just next to a bridge crossing it. Just after setting off onto the trail, you’ll come to a junction where the trail splits into one which circles the lake and one which leads to the summit of Mt. Hakuun-zan.
Heading towards the summit, the trail cuts up a steep slope covered in Glehn’s spruce, Ezo spruce, and Sakhalin fir. Along the sides of the trail you’ll see flowers like Canadian dwarf cornel, wood sorrel, foam flower, may lily, and Monotropastrum humile (for which I'm not sure there's actually a name, but which are nevertheless beautiful) growing in the sun.
At the top of the steep climb, you’ll arrive on a flat ridgeline heading towards the base of a rocky outcrop. Looking out over the tops of the Erman’s birch, you’ll be able to see the shape of Higashi- nupukaushinupuri (東ヌプカウシヌプリ); in the fall the view of the forest changing color is a beautiful one. The rocky ground along here is where the Shihoro Alpine Grasslands Road was planned to be built. It’s said that this area is one of Japan’s fastest wind gaps, so hold onto your hat if you're visiting on a particularly blustery day. It’s also home to the tiny pika and the cranberry blue butterfly, and you’ll probably see komakusa bleeding-heart growing among the rocks.
As a side note, building the road through here would have only saved 10 or 20 minutes of driving in the end. The huge development costs and the fear of damaging a fragile mountain ecosystem nature fortunately shut down the project.
Shortly you’ll reach a junction where the Shihoro Kogen Trail (士幌高原コース, Shihoro-kougen kousu) splits off toward Mt. Ganseki-yama (岩石山, Ganseki-yama), and you’ll start a climb over huge boulders to the summit. The groves of Erman’s birch will fall back and the view will open up as you reach the summit of Mt. Hakuun-zan, the quiet lake spread out below you. The mountain you’ll see across the lake is Mt. Upepesanke-yama (ウぺペサンケ山, Upepesanke-yama). If you’re climbing in early summer, sun on the leftover snow will make the forest absolutely gleam. The volcanic lake below, surrounded by mountains, seems welcoming no matter what the season.
To descend, you’ll continue past the summit towards Mt. Tenbou-zan and drop down a steep slope. At the col between the two mountains, the trail will split: one direction leads up to Mt. Tenbou-zan, the other down to the lakeside.
You’ll continue down the slope to the Tenbou-zan trailhead (天望山登山口, Tenbou-zan tozan-guchi) and reenter the evergreen forest. From there you’ll head along the lakeside trail, the lake glimmering off to your right, until you reach the trailhead.
In late May and early June, and Dahurian rhododendron will be flowering near the rocky outcrops. In August you’ll see Japanese lady bell, and the fruits of the kurotsuri will start to ripen. The leaves change in late September; the hiking season dies down in the later end of the fall so the trail will be quiet.