## 1 Introduction  
Forest ecosystems have experienced changes owing to climate change and anthropogenic pressure [1; 2], and these changes are particularly rapid in areas with a sensitive ecosystem structure and high population density. In addition, forest degradation, deforestation, and desertification are the main issues that threaten sustainable terrestrial ecosystems, therefore maintaining ecological productivity is important [3]. The northern mid-latitude region (MLR), which is located between 30 and 60\({}^{\circ}\) N latitudes with various land covers and a lot of human activities, including the mid-latitude ecotone (MLE), is a transition zone between forests and drylands [4; 5; 6]. In Eurasia, dense vegetation is distributed along the MLE in regions such as Northeast Asia, Siberia, and Europe, however, drylands also exist in Mongolia, Central Asia, and the Mediterranean region [7; 8]. Forests in the MLE consist of boreal and temperate forests, and form a belt connecting the Far East and Europe. This forest belt has important roles in maintaining ecological sustainability and preventing the expansion of drylands. The Northeast Asian MLE, including Korea, China, Japan, and Mongolia, has faced environmental changes during the past decades and will be affected by future climate change [9]. Through the forests of Mongolia and China, the boreal forests of Russia near Lake Baikal and Siberia are connected to the temperate forests in China and the Korean Peninsula [10]. Along this connection, each nation has different forest management issues and environmental conservation regimes. Temperate forests in South Korea and Japan were restored during the 1960s to 1970s, and now both countries have successful restoration outcomes, with the aim of sustainable forest management [11]. As a case in point, the forest growing stock volume (GSV) in South Korean forests was 10.5 m\({}^{3}\)/ha in 1952, and it reached 100.4 m\({}^{3}\)/ha in 2007 [12]. However, forests in North Korea have suffered from deforestation and forest degradation due to logging for food and fuel production [13; 14]. There are some differences in reports on the extent of deforestation in North Korea; the deforested areas there are estimated to have ranged between 1.2 million ha and 2.5 million ha during the late 1980s to 2000s [15]. These differences are also represented in the forests' net primary productivity (NPP) as a negative carbon budget in North Korea [16]. Temperate forests were changed to boreal forests in China, Northern Mongolia, and Russia near Lake Baikal. North China and Inner-Mongolia are widely known as regions facing ongoing deforestation and land degradation [17], but the Chinese and Mongolian governments strive to recover their forest ecosystems to secure ecological benefits [18; 19; 20], whereas, the forests near Lake Baikal and around the MLE have suffered natural or anthropogenic forest fire [21]. There is great uncertainty in measuring the effect of fire in boreal forests; one estimate revealed that 2.9% of Russian boreal forest loss was caused by forest fires [22]. Therefore, forest management regimes in the Northeast Asian MLE can be summarized as: (1) well-restored and managed areas in South Korea and Japan; (2) land degradation in North Korea; (3) semi-arid and new plantation in China and Mongolia; and (4) cold temperature with forest fire in Russia. The complexity of the management regimes of different countries leads to different forest statuses. NPP and GSV can represent the effects of these differences, and many modeling approaches were applied in the MLE of Northeast Asia using Flux data, Moderate Resolution Imaging Spectroradiometer-Leaf Area Index (MODIS-LAI), Boreal Ecosystem Productivity Simulator (BEPS), Vegetation Integrative Simulator for Trace gases (VISIT), MAPSS-CENTURY1 (MC1), and Biome-BioGeochemistry (Biome-BGC) [15; 16; 23; 24; 25; 26; 27]. Forest fluxes were calculated from data obtained from field surveys, but such data were procured at different times and locations [10; 24].