The ecoregion, catalogued by WWF as IM0136, runs roughly 350 kilometres along the spine of the northern Annamite Mountains across central Laos and Vietnam, up to 150 kilometres wide, covering about 47,137 square kilometres in all. Around three-quarters of it is still forested - some 35,000 square kilometres, mostly closed broadleaf evergreen. Rain falls at between 1,500 and 2,500 millimetres a year, less than the Vietnamese lowlands below and cooler with it, and the monsoon rhythm is extreme: the wet summer months can deliver ten times or more what the winter months manage. Mean elevation is a modest 453 metres, but the range reaches 2,158.

Between 800 and 1,200 metres, lower montane forest builds a dense two-tiered canopy fifteen to twenty-five metres up - myrtles, oaks, Castanopsis, Lithocarpus, laurels, with podocarp conifers mixed in. Orchids and other epiphytes crowd the branches, and many of those orchids grow nowhere else on Earth. Beneath it, undisturbed forest is surprisingly open; the understorey is sparse. Where the canopy has been broken, bamboo moves in fast, thickets of Chimonobambusa and Dendrocalamus filling every gap. Higher up, from 1,200 to 1,800 metres, the air turns cool and wet and the conifers take over: Fujian cypress, Dacrycarpus imbricatus, Cunninghamia lanceolata. Fokienia here can reach forty to fifty metres, and standing beneath one is the moment the scale of these mountains stops being an abstraction.

More than 134 mammal species have been identified in the ecoregion, a striking number of them endemic or threatened. The critically endangered douc langur - a monkey, despite the name, and one of the most spectacularly coloured primates anywhere - lives here alongside the vulnerable gaur, the enormous wild cattle also called Indian bison, and the vulnerable Sumatran serow picking its way across cliff faces. The saola is endemic to this ecoregion and no other. The giant muntjac occurs here and in the Southern Annamites. Roughly 525 bird species have been recorded. The word that recurs in the survey literature for this range is unexplored, which in the twenty-first century is an unusual thing to have to write down.

Just over 33 percent of the ecoregion falls inside protected areas, and the network is genuinely substantial on the map. Pu Mat National Park in north-central Vietnam - the name means high slope - protects 911 square kilometres and has recorded 2,461 plant species. Nam Kading in central Laos covers 1,690 square kilometres of country so rugged it has almost no human population at all. Phou Xang He adds another 1,060 square kilometres of mixed deciduous and dry dipterocarp forest. Nakai-Nam Theun accounts for 3,445 square kilometres of the ecoregion, the limestone country of Phou Hin Poun another 1,801, and Hin Nam No a further 935. Six areas, close to ten thousand square kilometres between them. What that acreage actually gets in the way of staff and funding is another matter entirely.

The most consequential recent change came in 2025, when Hin Nam No in east-central Laos was inscribed on the World Heritage List as an extension of Vietnam's Phong Nha-Ke Bang National Park. The two sit on opposite sides of an international frontier and on the same block of limestone, and the caves running beneath them never cared which country they were in. Recognising the karst as one system, rather than two national inventories facing each other across a line, is the sort of administrative act that sounds bloodless and is not. It commits two governments to the same piece of ground. In a range where the most important species may be down to its final dozens, that kind of commitment is the scarcest resource of all.