Sobo, Katamuki and Okue Biosphere Reserve

The landforms of the Biosphere Reserve are the heavily modified result of volcanic activity over the last 14 million years. A massive eruption about 13.7 million years ago involved the Okueyama Volcano-plutonic Complex,[9] and measured 8.0 on the Volcanic Explosivity Index.[10] This formed the almost rectangular Okueyama Caldera with the eruption of an estimated dense rock equivalent (DRE) volume of 1,029 km³ (247 cu mi) leaving a caldera orientated to NW to SE that was 33 km (21 mi) by 23 km (14 mi) in size. Further eruptions over only a few million years formed the smaller more elliptical Sobo Caldera which was 18 km (11 mi) by 13 km (8.1 mi) in size on a NNE to SSW axis with a DRE erupted volume of 177 km³ (42 cu mi) and the Katamukiyama Caldera again on a NW to SE axis which was 11 km (6.8 mi) by 6 km (3.7 mi) in size with a DRE erupted volume of 50 km³ (12 cu mi).[10] Later, most recently about 90,000 years ago, mainly the lower lying land was modified by pyroclastic deposits from the Aso Caldera to the west whose eruption was 384 km³ (92 cu mi).[11]

In the lower core and buffer areas the cover is by old-growth forest including Japanese beech and Tsuga. Rhododendron kiusianum, Japanese gentian and the Japanese maple grow in great numbers. At higher levels, the vegetation changes from evergreen (glossy-leaved) forest to conifer forest and then to Suzu-take and beech closer to mountain summits. A rare plant Angelica ubatakensis is found in the Sobo mountain range.

The southern range limit of the Special Natural monument (特別天然記念物, tokubetsu tennen kinen-butsu), the Japanese serow lies in the Sobo mountain range. The Sobo salamander, a new species, was found here in 2014.[1] The Japanese national butterfly, the great purple emperor, an endangered species, has been seen.[1] Rare sightings have been made of the Asian black bear. Japanese dormice and sika deer are much more commonly seen.