From China (Jilin); Republic of Korea (North Korea) and Democratic People's Republic of Korea (South Korea).
In China it occurs in the northeast of the country in the Province of Jilin in the Changbai Shan. This subpopulation straddles the border between China and North Korea.
In North Korea it occurs on the following mountains: Mt. Baekdu (Ryanggang-do); Mt. Sungjeok, Mt. Pinandeok (Jagang-do); Mt. Myohyang (Pyeonganbuk-do/Pyeongannam-do); Mt. Sasu (Hamkyongnam-do/Pyeongannam-do); Mt. Haram (Pyeongannam-do/Hwanghae-do); Mt. Chuae (Hamkyongnam-do/Gangwon-do).
In South Korea it occurs in the Province of Gyeonggi-do on the following mountains: Mt. Hwaak, Mt. Geumgang, Mt. Daeu, Gari Peak; Mt. Seorak, Mt. Jeombong, Mt. Bangtae, Mt. Odae, Mt. Gyebang, Mt. Hambaek, Mt. Jang and Mt. Taebaek.
The estimated area of occupancy (AOO) of the accessible locations in China and South Korea (12 in all) vary between 2km² to ca 100km². The biggest location in South Korea is Soeraksan (Mt. Sorak) which is thought to be ca 100km². Most other locations in South Korea have very small AOO, e.g. 20km² or less (H. Lee pers. comm. 2011). Although there is no specific information on the AOO of the North Korean locations it is expected that these will have a relatively small AOO and be within degraded habitats. The overall AOO of the species is certainly less than 2000km².
In North Korea and China the subpopulations are fragmented. In South Korea there is less fragmentation with most locations clustered along the Taebaek Mountains. It is notable that locations throughout the global population have few or no sexually mature individuals, for example, even the relatively large subpopulation in the Chinese sector of the Changbaishan is estimated to have only 15 mature individuals.
Habitat and Ecology
It mostly forms dense low thickets to 60cm tall but in some locations it will form a tree to 10m tall (Fu & Jin, 1992). Typically it occurs on middle and upper mountain slopes at altitudes between 750–1950m. At the higher elevations it forms low dense thickets on exposed rocky boulder screes, but in more sheltered habitats, such as forests, it can form a small upright tree. It can be associated with a range of other conifer and broad-leaved species. For example, in the Changbaishan, which straddles the boarder between China and North Korea, it is associated with Abies nephrolepis, Betula ermanii, Taxus cuspidata, Acer ukurunduense and Sorbus pohuashanensis. In South Korea it can be associated with Abies nephrolepis, Sorbus commixta, Prunus padus, Betula ermanii, Quercus mongolica and Acer tschonoskii var. rubripes and with Rhododendron schlippenbachii and R. yedoensis (M. Gardner pers. obs.). Other conifer associates include Pinus pumila, Picea koraiensis, P. jezoensis, Pinus koraiensis and P. sibirica. It appears to avoid rocks of volcanic origin and grows most abundantly on exposed, granitic slopes and crags with acidic skeletal soil.
Vulnerable B2ab(ii,iii,iv,v); C2a(i); D1
Although there is little or no information about the conservation status of Thuja koraiensis in North Korea, there is sufficient documentation concerning the critical state of the forests to assume that the species is under threat. Most of the loss of forests in North Korea’s is related to the policy of increasing arable land in mountainous areas (UNEP, 2003; Hayes, 2009). The level of deforestation is suspected to have caused a loss in the area of occupancy (AOO) and had a negative impact on the extent and/or quality of any existing habitats and caused severe fragmentation. In South Korea the species is more secure but the underlying problem here and throughout the global population is the lack of recruitment due to the paucity of mature individuals. Even in the Changbaishan reserve where there is an estimated 2500 individual plants, only 15 individuals are sexually mature. Some locations, for example, Mt. Taebaek (M.Gardner pers. obs. 2010) have no mature individuals. The total number of mature reproducing individuals is uncertain but is estimated to be between 250 and 1000. The AOO is estimated to be between 500 and 2000km² with subpopulations severely fragmented. A recent survey could not locate plants on Mt. Hwaak due to disturbance from military buildings, indicating a decline in the number of locations and probably mature individuals (H. Lee pers. comm. 2011).
The forests where the subpopulation in Changbaishan (China) and the Baekdu-san (North Korea) occur regularly suffer from wind-blow and of particular note is the damage caused in 1987 (Tang, 2010). Although logging is strictly prohibited in the Chinese sector of this Biosphere, there has been a 50% loss of primary forest and 75% loss of the primary forest landscape in the core area up to 2007 in the North Korean sector (Tang, 2010). Such a loss is suspected to have had a detrimental effect on the Thuja although there is no documented evidence for this. There is no specific information about the state of the Thuja habitats in six other locations in North Korea but we have assumed that as a result of a 30.9% loss of forest cover within the last two decades (UNEP, 2003; Hayes, 2009), at least some of these are likely to have been affected. In South Korea some important habitats are in protected areas, but most locations are very small and with few or no mature individuals. A recent survey of these locations failed to observe any sort of regeneration and could not locate plants on Mt. Hwaak due to disturbance from military buildings (H. Lee pers. comm. 2011).
The subpopulation in Changbaishan (China) and the Baekdu-san (North Korea) is protected within a Biosphere Reserve which is contiguous across the country’s border. The former was established in 1979 and the latter in 1989. However, over 50% of this area has deteriorated due to seed harvesting of pines, and systematic logging (Tang, 2010). While much of the logging in the Changbaishan reserve occurred before it became a protected area, however, there is a continuing deterioration in the Baekdu-san reserve (North Korea). Furthermore, there are huge pressures from tourism in both reserves, which has dramatically increased since the early 1980s; the Changbaishan reserve receives almost 1 million visitors per annum and the Baekdu-san reserve has 200,000 visitors. In South Korea, the largest location of Thuja koraiensis is protected in Soeraksan (Mt. Sorak) National Park.