Lönnåsen nature reserve covers 115 hectares, about 10 kilometres west-southwest of Bergsjö in Nordanstig municipality. It includes forest on the southern and eastern parts of Lönnåsen mountain, as well as flatter ground to the west. There are no easy-to-walk trails, so visitors can roam freely through the area.

The forest is mainly older spruce and mixed conifers, with old birches and aspens in places. Some of the pines are more than 250 years old. The woodland grew naturally after a forest fire, probably about 130 years ago, and older trees still show signs of the fire. Lönnåsen is also home to red-listed lichens and fungi, including species that grow on old aspens and fallen spruce trees.

The reserve lies in a forest area with several old summer-farm sites, known in Swedish as fäbodar. An overgrown former path between Högsvallen and Brännåsvallen passes through the reserve. In the western area, near a fork in the forest road, there are old charcoal-burning platforms and remains of a charcoal burner’s hut.