It mostly forms dense low thickets to 60 cm tall but in some locations it will form a tree to 10 m tall (Fu and Jin 1992). Typically it occurs on middle and upper mountain slopes at altitudes between 750-1,950 m. 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.

• 1998
  Data Deficient
  (Oldfield et al. 1998)
• 1997
  Rare
  (Walter and Gillett 1998)

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.