In this study, reforestation follows the standards of A/R CDM, and it means reforestation on land that was previously forested and then converted to non-forest use. Land cover change through spatial information is essential to meet the eligibility assessment to be selected as a target for afforestation and reforestation (Yoo et al., 2010). Afforestation targets areas that have been non-forested for the past 50 years; there is a limit to the application of the Korean Peninsula because data prior to the 1970s only exist in document statistics. The Korean Peninsula consists of the ROK and the DPRK; however, since 1980, they have possessed contrasting land use patterns. In particular, the trend in forest area change appears in contrast to each other, which causes a difference in the area for carbon sink. In terms of securing CO₂ sequestration through forest conversion, the expected effect will be distinctly different. It could be classified as a reforestation target based on the A/R CDM standard as major changes in the forest of the Korean Peninsula occurred after 1980. Spatial data for estimating land cover change used the 10-year land cover map (1980s, 1990s, 2000s, and 2010s) of the Korean Peninsula officially published by the ROK's Ministry of Environment (Table 1) (https://egis.me.go.kr/). Each data set was established from the Landsat TM imagery for the Korean Peninsula. The spatial resolution is 30 m, and they had more than 70% accuracy for classification (Kang and Choi, 2014). The resolution of the land cover map was applied in units of 30 × 30 m and was matched with the most recent land cover map in 2010. The land cover change was estimated using a time series matrix for each type (Klein et al., 2012). The type of land cover change defines an area that has been converted to settlements, cropland, wetland, bare ground, or water from a forest. The settlements have limitations for reforestation because economic loss occurs in the removal of urbanized land and infrastructure when it converts into a forest. Therefore, cropland, wetland, and bare ground were considered as possible reforestation patches, while settlements and water were classified as impossible for reforestation.

where δ represents the standard deviation, and μ represents the mean value.

To estimate the effect of carbon sequestration by reforestation in the future, a reforestation scenario was established. The area for reforestation varies depending on when the reference period is established (Teuling et al., 2019). In the land cover change data, the largest reforestation area appears when the reference period is 1980 and the smallest area is derived in 2000 because of the rapid forest decrease in the DPRK. The reference period for the reforestation scenario was 2010 compared with 2020, which shows the smallest area for reforestation in the time series. This is to minimize the burden on food production in the DPRK, and it could support the establishment of a feasible policy that reduces the overestimation. The assumed reforestation area covers the total area of the reforestation derived from each country and applies to the same area annually. The annual reforestation area was derived by dividing the total area of the reforestation target for each country over 70 years from 2031 to 2100, and it was assumed that 5-year-old woody plants were planted for the annual area. The total amount obtained by reforestation was calculated by the planted amount in the current year and the growing amount generated by tree growth in the previous period (Equation 2). A temporal growing stock by tree age is predicted according to the stand harvest table of the Korea Forest Service (Table 2). The stand yield table presents the growth by site index. As this study targets planting in non-forest areas, overestimation was avoided by applying the growth amount that appears at the lowest status index for each species. In cases of growing stock in 10-aged Pinus rigida, Chamaecyparis obtuse and Quercus acutissima are not known in the ROK, so it is assumed there is half of the growing stock for the 15-age.

where V_Total represents the total forest stock each year, V represents the forest stock prediction without management, V_{Reforestation} represents the volume for the new yearly reforestation, and V_t2−t1 represents forest accumulation by growth.

The ROK forest types were reclassified from the forest map made by the Korea Forest Service (https://map.forest.go.kr/forest/) to the dominant species, which consists of pure stands, such as Pinus densiflora, Pinus koraiensis, Larix kaempferi, Pinus rigida, Chamaecyparis obtuse, Quercus acutissima, and Quercus mongolica, while the tree stock was derived by applying the age of actual vegetation (Hong et al., 2022). These plants are the main species of the Korean Peninsula, and they have an advantage in surviving in a wide and barren environment. It was used as a major afforestation species in the forest reforestation process of the Korean Peninsula in the past. As a result, it constitutes the dominant species on the Korean Peninsula. In the case of the DPRK, the amount of tree stock was calculated using the species distribution data announced by FAO (2020). The prediction of the forest stock change without management was assumed as the baseline for a future period until 2100. The carbon dioxide equations (Equations 3 and 4), which were suggested by IPCC guidelines were applied. The country-specific coefficient derived from the National Institute of Forest Science was used to calculate the effect from 2031 to 2100 when the reforestation will be performed (Table 3) (Son et al., 2010). According to the IPCC method, the carbon stock in the forest sector is derived through the sum of biomass and soil, but a country-specific coefficient that could be used in the Korean Peninsula has only been developed for biomass. To analyze the amount of carbon sequestration in the soil by reforestation in a time series, it is essential to estimate the change after the early and middle periods, but there are insufficient data to calculate this. Therefore, the research subject was limited to biomass in this study. The future annual net CO₂ sequestration is calculated by the predicted baseline and the reforestation effect. The change of stock volume shows a high increasing rate during the early stage of growth and causes a positive effect on the annual net carbon sequestration by reforestation (Başkent et al., 2011; Seo et al., 2018).

where C is carbon stock (Mg C ha⁻¹), V is the stand volume (m³ ha⁻¹), D is the basic density of the wood, BEF is called biomass expansion factor, R is the root-to-shoot ratio, CF is the carbon fraction (IPCC factor = 0.5), t_i is the year, j is the tree species, t_i and t_i+1 are the annual change in carbon stock (Mg C ha⁻¹ year⁻¹), and 4412 is the stoichiometric ratio of CO₂ and C.

As a result of the land cover changes in the Korean Peninsula, a gradual change has occurred during the 30 years since 1980 (Figure 1). Moreover, the forest area decreased from 168,009 km² in the late 1980s to 162,027 km² in the late 2010s, while the settlements increased from 3,630 km² to 7,656 km². The change in the land cover of nations showed different patterns between the ROK and the DPRK (Table 4). In the ROK, the area of forest and settlements has been continuously increasing since 1980, while the cropland is decreasing (Choi et al., 2021). The most distinctive characteristic of land cover change in the ROK is that the area of settlements increased by +3,372 km², resulting in rapid urban expansion and a decrease in agricultural land over the past 30 years. The forest area increased by +1,545 km² from 68,264 km² in the 1980s to 69,809 km² in the late 2010s, indicating that long-term forest management was well implemented. In contrast, there was a clear decrease in forest area and an increase in cropland in the DPRK (Kang and Choi, 2014). The forest decreased by 7,527 km², accounting for 5.68% of the entire land area of the DPRK, and cropland increased by 5,222 km² which is 5.12%. This confirmed, spatially, that the cropland conversion in the DPRK has mainly occurred from the forest. Moreover, spatial distribution demonstrated that the increased cropland gradually widens around the periphery of the forest, meaning that farming in the DPRK has performed at a steeper slope and a higher elevation than in the ROK (Yu et al., 2018). The difference between the countries becomes more pronounced when the land cover is classified into forest and non-forest regions (Figure 2).

As a result of the reforestation target area, in the past period compared with the current land cover (2020), it was found that the target area decreased as the reference period got closer to the present (Figure 3). The area of reforestation in the ROK was 10,117 km² in the 1980s, 9,812 km² in the 1990s, and 8,880 km² in the 2000s, and there was no significant difference, whereas the reforestation area in the DPRK was 16,586 km² in the 1980s, 14,539 km² in the 1990s, and 11,670 km² in the 2000s. For the purpose of expanding the carbon sink, the largest acquisition would be expected in 1980. However, considering the economic situation of the DPRK, the rapid reforestation of cropland could cause new problems for the food supply. An area, which has been agricultural land for a long time, could easily be degraded as the trees are harvested for fuel (Byeon, 2014; Crespo Cuaresma et al., 2020). In this respect, it is important to minimize the impact of external factors by selecting a target period close to the present. Considering the ROK, there was no critical difference regardless of the standard reforestation period, and a difference of 4,916 km² was found when comparing the reforestation area between the late 1980s and 2010s in the DPRK. This is an area where about 1.8 billion trees could be planted assuming that 3,000 trees are planted per ha, which is 3.71% of the DPRK's land area.

The spatial distribution of reforestation in the Korean Peninsula shows a different pattern between the ROK and the DPRK, especially when the types of land cover changes were divided into reforestation targets and settlements (Figure 4). If the late 2000s is selected as the reference period, 7,104 km² (79.80%) of the land that could be reforested and 2,248 km² (20.20%) of the land that could not be reforested are found in the ROK. On the other hand, 11,670 km² (92.97%) of the area is possible and 882 km² (7.03%) of the area is impossible for reforestation in the DPRK. Therefore, the ROK is facing spatial limitations in which reforestation could not be carried out even though forests have been converted to other land covers. In contrast, the DPRK appears to have the conditions for reforestation to be applied in most of the converted areas since the late 1980s.

The annual reforestation area of ROK is 126.8 km², and the DPRK is analyzed to be 166.7 km², which is 1.3 times larger when the reference period set to the late 2000s and the reforestation scenario was applied. As a result of assuming the forest accumulation according to the growth prediction from actual vegetation as a baseline, it is estimated that forest accumulation in the ROK by reforestation calculated at 17,233,460m³ in 2050, 57,848,704m³ in 2070, and 146,087,769m³ in 2100. An increase of ~24.17% is expected through the implementation of the reforestation scenario compared to that of the current forest accumulation of the ROK is estimated to be 604,395,000m³. In the case of the DPRK, it was analyzed that a net acquisition of 22,646,764m³ in 2050, 76,019,899m³ in 2070, and 191,976,252m³ in 2100 was possible. Compared to the DPRK's current forest accumulation of 682,966,400m³, it was analyzed that the maximum potential by reforestation is possible to increase the stand volume by 28.11% compared to the late 2010s (Figure 5).

A net annual CO₂ sequestration of 10,833,600 Mg CO₂ year⁻¹ is expected in 2050, with 20,919,200 Mg CO₂ year⁻¹ expected in 2070 in the ROK through the reforestation scenario (Figure 6). Moreover, the DPRK forest was estimated to have a potential net annual carbon sequestration of 14,236,800 Mg CO₂ year⁻¹ in 2050 and 27,490,400 Mg CO₂ year⁻¹ in 2070. The annual net carbon sequestration would be changed according to the area and period for reforestation because the carbon sequestration effect by reforestation was calculated by including both the increase for tree planting in the current year and the growth of the trees converted to the forest in the previous period. Therefore, it was determined that the potential for carbon sink expansion through reforestation was higher in the DPRK than in the ROK.