Introduction
The terrain of North Korea consists of a series of medium-sized mountain ranges and large hills, separated by deep, narrow valleys. Natural forests remain in small-protected areas that are located primarily in the high elevations of northern North Korea. North Korean forests have mostly been converted to croplands (Lim et al., 2017). Deforestation implies the long-term or permanent loss of forest cover and its transformation into another land cover. Especially, almost 40 percent of the forests in North Korea had either been converted to fields for food crop production, or cut down for fuel wood. between 1990 and 2015 (Jin et al., 2016), Thus, accelerated deforestation in North Korea in recent caused the famine and floods its people suffered (Kang and Choi, 2014). Therefore, deforestation in North Korea triggered by famine and food shortages needs to be studied with respect to agricultural perspective.
Land use/cover change results in significant consequences in the environment, especially in agricultural production. North Korean government have been made to increase arable land area. Therefore, they have encouraged terrace field cultivation on steep mountainous regions since 1976 (Lee et al., 2017). Under this nationwide policy, forest lands were observed to have continuously deforested.
Several severe floods damaged much of the land used for agriculture in North Korea between 1995 and 1996. The area of agricultural land damaged by inundation and sedimentation was estimated to 1.3 million has (IUFRO, 2007). Therefore, the debris left afterward in lower elevation zones made the land unusable for growing crops, resulting in more clear-cutting of forest to expand the agricultural land (Kim and Chi, 1998; IUFRO, 2007). Estimation to account for the nationwide soil erosion potential in North Korea was made. The North Korean government officially reported that soil erosion was estimated to be 40 to 60 tonnes ha-1 yr-1 on sloping lands, but it exceeded 100 tonnes ha-1 yr-1 in severe cases (MoLEP, 2012).
Since North Korea is one of the closed countries in the world, literature related to land cover change in North Korea is quite sparse. The purpose of this study was to characterize changes of land cover and land uses including deforestation in North Korea through geographical features and deforestation taken place recently.
Geographical features and biogeography of North Korea
North Korea which is characterized by successive, rugged mountain ranges that crisscross the peninsula has a total area of 120,538 km2 that is slightly greater than the total area of South Korea. The terrain of North Korea consists of mountains rising over almost 80% of the total area of North Korea and only 17% of the land is available for agricultural use. North Korea’s steep terrain is a function of differential erosion. The Taebaek Range forms a backbone down the east side of the Korean Peninsula as shown in Fig. 1. The country is divided by the Nangnim Mountains running from north to south through the middle into the eastern and western slopes of the peninsula. The Hamgyong Mountains, extending from the Nangnim Mountains to the northeast, form a steep slope between the Kaema Highlands and the East Sea (CIA, 2016; Jin et al, 2016; Lee et al., 2017; Wikipedia, 2018). Also, the plateaus of very rugged mountains in the north and east regions of the country are above 2000 m. The land around Mount Paektu (2,750 m) includes a basalt lava plateau with elevations of between 1,400 and 2,000 m above sea level. The Kaema Highlands, the topographic roof of the entire Korean peninsula, have an average elevation of 1,000 m above sea level and form.
The territory of North Korea that is composed largely of granite, gneisses and other metamorphic rocks represents denudation remnant of deformed basement rocks and sedimentary successions as well as granitic intrusions and volcanics as seen in Fig. 1 (Chough et al., 2000). The resistant metamorphic rocks generally form ridge lines, while the granite formations generally form valleys and depressions. The combined effect is generally high and steep in the east, low and rolling in the west (Cameron, 1998).
More than three-fifths of the soils are locally derived from the weathering of granitic rocks or various kinds of schists developed from crystalline rocks. Major suborders of soils along the coastal lines of the east and the west in North Korea are cambisols and orthents (lithosols) which are characterized by the absence of a layer of accumulated clay, humus, soluble salts, or iron and aluminum oxides. Acrisols are found in the areas of lower part of South Hwanghae province and Kaesong-si and fluvisols and histosols are found in the north-western part of South Hamgyong province. Orthents found in very steep, mountainous regions are defined as entisols that lack horizon development and are exceedingly shallow soils. Therefore, orthents are not suitable for arable farming because of their extreme shallowness and, usually, steepness and consequent high erosion hazard (FAO, 2006; Wikipedia, 2018). Natural forests remain in the high elevations of northern North Korea. The largest valleys that have been used for agriculture are located along the west coast, although inland and east coast valleys are much smaller (Fig. 2). Some of these hillsides have been replanted with food crops; nut, fruit, or timber tree plantations; or have regrown as grassland (Park, 2001; CIA, 2002; Palka and Galgano, 2002). Throughout the country, lands that exceed 400 m have poor cultivation potential (Chung, 1994) due to elevation, climate relationships, slope angle, and soil conditions. The alpine zone is the landscape higher than 2000 m, an environment of low-growing plants, transitioning to barren ground (Byun, 2001).
More than three-fifths of the soils are locally derived from the weathering of granitic rocks or various kinds of schists (crystalline rocks). The soils are generally brownish, abundant in sandy materials, and low in fertility. Most of the soils are infertile and lack organic content, the valleys and coastal plains have relatively rich alluvial soils. Well-developed reddish brown soils derived from limestone are found in North Hwanghae province and the southern part of South Pyeongan province. However, the reddish hue of exposed soil indicates a lack of organic matter, which is vital for farming in North Korea. Podzols (ash-gray forest soil) have developed in the Kaema Highlands as a result of the cold climate and coniferous forest cover there as seen in Fig. 3 (McKenna, 2013).
Michalk and Mueller (2003) reported that the major cause of degradation of soil physical and chemical properties in North Korea was caused by rapid loss of clay particles and organic matter, as well as a failure to replace nutrients after successive cultivation of crops could cause the changes of soil chemical properties in the arable land. Besides these reasons, the evident acidification by large quantities of chemical fertilizers and not having a crop rotation and growing the same crop year after year destroyed the soil physical and chemical properties that reduced base saturation in arable soils (Guenat and Mueller, 1999).
Less than 15% of the land area was due to the rugged terrain of North Korea limiting agricultural cultivation. The decline in soil fertility in North Korea had developed over a relatively short time due to the use of outdated and frequent tillage methods coupled with continuous cultivation of cereals and non-leguminous crops, leading to serious reduction of organic matter levels as well as clearing of the original vegetation (Michalk and Mueller, 2003). This also resulted in a drastic change in soil structure that increased erosion thereby further reducing soil fertility, especially on the light textured (sandy loam) upland soils derived mainly from granite. Therefore, 50% of the soils in the arable was classified as poor (Guenat and Mueller, 1999).
Types and changes of land cover in North Korea
South Korea has an area of approximately 100,000 km2, while the area of North Korea is about 120,000 km2. The agricultural areas of North Korea are more extensive than those of South Korea. 20% of the total area (20,000 km2) in South Korea was used as agricultural land as of 2014, compared to 24% in North Korea (30,000 km2). However, North Korea has displayed a large difference in land use and land cover patterns (MOE, 2014). Farming in North Korea is concentrated in the flatlands of the four west coast provinces, where a longer growing season, level land, adequate rainfall, and good irrigated soil permit the most intensive cultivation of crops. The interior provinces of Chagang and Ryanggang are too mountainous, cold, and dry to allow much farming. The mountains contain the bulk of North Korea's forest reserves while the foothills within and between the major agricultural regions provide lands for livestock grazing and fruit tree cultivation (FAO, 2013).
The percent of agricultural land, referring to the share of land area that is arable, under permanent crops, and under permanent pastures, was 21.84% in 2014 (Table 2). And the percentages of arable land and the high mountain area in the northeast representing 21% of the territory is essentially a forest area with practically no agriculture. Two-thirds of the arable land is found in four western provinces (North Pyeongan, South Pyeongan, North Hwanghae and South Hwanghae), while the three provinces fronting the Yellow Sea contain 60% of the nation’s paddy fields (Fig. 4). Onefourth of the arable land is contained in the three provinces of the eastern region (North Hamgyong, South Hamgyong, and Kangwon). The interior region (the provinces of Chagang and Ryanggang) is mountainous, dry and cold, but rich in forest resources.
The hilly areas which are suitable for the cultivation of crops such as potato, wheat, barley, and vegetables and surround the high mountains in the north and the central chain of mountains with large areas under forest represent 40 percent of the territory (Fig. 4). The east coast, representing 22% of the territory, is sub-mountainous or hilly, but it also includes a narrow strip of lowlands along the east coast and low hills in inland away from the coast where rice is cultivated. In addition to forest and pasture, there are slopes suitable for maize and vegetable cultivation. As seen in Fig. 6,large river-valley plains that represent 17 percent of the territory and merge along the narrow, irregular coastal plain on the west coast have developed between the western mountains. These wide coastal plains in the western plains mainly devoted to rice cultivation (CIA, 2009; FAO, 2012). The western lowlands were originally covered by temperate mixed forests, but continuous deforestation has left only remote patches of the original forests. Most of the lowlands are now cultivated, except for some of the hills that are covered with small pine groves mixed with oaks, lindens, maples, and birches. Along streams that are subject to flooding or where the ground is too stony for cultivation. Therefore, only about 22% of the land is arable (CIA, 2016; Wikipedia, 2018).
North Korea used to have abundant forest stocks but underwent substantial deforestation and degradation of forest in recent decades (Kang and Choi, 2014). The UNFCCC (2003) defines deforestation as the direct human-induced conversion of forested land to non-forested land, while the FAO (2006) defines deforestation as the conversion of forest to another land use or the long-term reduction of the tree canopy cover below the minimum 10% threshold.
Forest cover continued to decrease in North Korea (Engler et al., 2014). Deforestation including the long-term or permanent loss of forest cover and its transformation into another land cover is one of the most influential factor of land cover change. The major reasons of deforestation in North Korea were famine and floods. Between 1990 and 2015, almost 40 percent of the forests in North Korea have either been converted to fields for food crop production, or cut down for fuel wood (Zheng et al., 1997; Bhatia and Thorne-Lyman, 2002). However, literature related to land cover change in North Korea is quite sparse, and even fewer peer-reviewed studies exist in specific relation to deforestation in North Korea. Lee and Kim (2000) said that there had been virtually no reliable information regarding the size and condition of forests in the country until recently. The deforestation in North Korea was estimated at 20% of the forest between 1997 and 2014 because of reflective of improper land use practices on steep slopes, leading to severe soil erosion (Fig. 5).
The spatial changes related to the land cover had a greater impact on the change of agricultural environment across the three time periods from 1980s to 2000s. Since the 1980s, approximately 8% of the forested land was converted to other categories of land cover as a result of deforestation in North Korea. Approximately 4% of forests were converted between the 1980s and the 1990s and another 4% were deforested between the 1990s and the 2000s (Lim et al., 2017). Fig. 6 shows the changes of land cover between 1980s and 2000s. Compared with the area of cropland in 1980s, the area of cropland in 2000s was significantly increased on the west coastal area in 2000s. Also the increase of cropland was distinctive on the west coast than that of the east coast.
Lim et al. (2017) also mentioned that the data related to soil erosion by water in North Korea could not be obtained from field surveys or satellite images because it was difficult to use for estimating the soil erosion value. Therefore, the time series change of water erosion in North Korea was not clear. Generally, the cropland expansion vulnerable to meteorological events causes changes to soil erosion due by deforestation of the forest in North Korea for the worse, resulting in that the runoff of converted croplands from forest shows an increase of more than 1.5 times. The increase in cropland runoff augments the flood damage during localized torrential downpours and becomes a contributor to the aggravation of vulnerability to climate change, such as the reduction in the flow rate during periods without precipitation (Olichwer and Tarka, 2015).
Lim et al. (2017) verified soil erosion by water using qualitative methods such as the results of the soil erosion estimations in South Korea and the United States using the Revised Universal Soil Loss Equation. The erosion rates were increased with increasing altitudes and times in both upland and paddy soils as shown in Fig. 7. The erosion from croplands cultivating crops such as maize and barley was slightly greater than that of rice paddies. And severe erosion greater than 36 tonnes per ha was occurred from the mountainous area in the central chain of mountains such as the interior region of South Pyeongan province and the west region of South Hamgyong province for both the upland and the paddy soils while the severe erosion greater than 36 tonnes per ha was occurred from the only upland in the provinces of Jagang, Yanggang, and North Hamgyong because there were little paddy soils. Compared with the erosion areas between the upland and the paddy soils during the three decades from 1980s to 2000s, the erosion extent of the upland was extended from the southern part to northeastern part of North Korea. Especially, the erosion rate greater than 36 tonnes per ha for the upland was significantly increased in the provinces of Chagang, Yanggang, and North Hamgyong province in 2000s.
A significant decrease in forest area was occurred between the 1980s and 1990s. The decrease was only 6%, but the areal extent is more than 5000 km2. During the same period, developed that included built-up areas such as residential, commercial, industrial, transportation increased by 44%, agriculture 12%, and grassland 24%. Agricultural land was increased by more than 2600 km2, which was more than half the size of deforestation. However, forest was barely changed in its areal extent between the 1990s and 2000s, but agriculture increased by 27% and grassland decreased by 91%. It is interesting to note that developed decreased by 5% during the 1990s - 2000s. Also there was a report that 1.3 million ha of agricultural land was damaged and lost due to inundation and sedimentation between 1995 and 1996 (IUFRO, 2007). The extent of deforested areas for each of the two decades overlaid by provincial boundaries. Deforestation during the 1990s - 2000s concentrated in western provinces including Jagang-province, North Pyeongan province, South Pyeongan province, and Pyeongyang-si. Deforestation was also quite visible in and outside of Pyeongyang between the 1990s and 2000s. Deforestation in Jagang-province, a very remote and mountainous province, was quite intensive in the middle of the province during the same period. Thus, deforestation was fairly widespread across the country in both decades, except in very inland areas of Yanggang-province which were mostly high mountains and plateaus (Choi et al., 2017).
According to Choi et al. (2017), the area of forest-to-agriculture conversion throughout North Korea was approximately 7,196 km2 between the 1990s and 2000s, that represented 9% increase than that between the 1980s and 1990s. The agriculture-to-forest conversion and the grassland-to-forest conversion were 3462 km2 and 3575 km2, respectively. The grassland-to-agriculture and forest-to-agriculture conversions occurred a lot as well, over areas of 2205 and 7196 km2 (Table 3). Thus, deforestation in North Korea influenced the land cover change.
Regarding to the areas of the land cover by forest conversion, cropland was found to be the most extensive (~ 5800 km2), followed by grassland (~ 1400 km2). This meant that 75% of the area converted from forest used for croplands, indicating that croplands expanded considerably during the past 30 years of deforestation (Table 4).
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Table 3. Land cover changes from non-forest to Forest during 1980s - 2000sz (MOE, 2014). 
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zThe change from forest to forest indicates that forest became non-forest and became forest again. |
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Table 4. Land cover changes from forest to other land cover in North Korea during two periods of 1980s - 1990s and 1990s - 2000s (MOE, 2014). 
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Table 5 showed that land cover changes from other land cover to cropland in North Korea during two periods of 1980s - 1990s and 1990s - 2000s. About 6700 km2 of croplands were converted from forests while 1700 km2 were converted from grasslands. Considering absolute area, land converted to croplands was found to be larger than land converted from forests. Conclusively, 75% of the area converted from forests became croplands and 69% of land converted to croplands came from forests. These results also confirmed that the deforestation significantly contributed cropland expansion in North Korea.
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Table 5. Land cover changes from other land cover to cropland in North Korea during two periods of 1980s - 1990s and 1990s - 2000s (MOE, 2014). 
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The distribution of deforested areas by elevation showed that the maximum slope in the deforested areas during the 1990s - 2000s was 17.3°, while 20.2° during the 1980s - 1990s. The deforestation in slightly lower elevation during the 1990s - 2000s represented that the deforestation occurred on large size in western provinces where most of the plains were located. Unlike slope, about 80% of deforestation occurred under 800 m during the 1980s - 1990s, but the proportion was almost 90% during the 1990s - 2000s. The decrease in the mean elevation might mean that deforestation was more active at the lower elevation areas, which might be near urban areas or agricultural fields.
Conclusion
North Korea’s terrain that limits agricultural cultivation to less than 15% of the land area is characterized by successive, rugged mountain ranges, comprising about 80% of the country’s landscape, and soil fertility decline developed over a relatively short time span has been a major factor that prevents the national grain production target. Also, soil degradation in North Korea can be characterized by physical and chemical changes caused by rapid loss of nutrients and organic matter due to continuous cropping and deforestation. The land cover and land uses in North Korea for the last three decades are summarized as follows: (1) net forest loss in North Korea was quite small during the 1990s - 2000s although land cover changes were active; (2) land converted by deforestation of forest became mostly agriculture, particularly during the 1990s - 2000s, followed by grassland; (3) expansion of agriculture continued during three decases from 1980s to 2000s, increasing by 42%; and (4) deforestation had been occurred more frequently in areas of slightly lower elevation and gentler slope during the 1990s - 2000s than the 1980s. Thus, increased soil erosion at the expense of deforestation resulted in a drastic change in soil physical and chemical properties that strongly influenced cropland stability and productivity.
Acknowledgement
This research was conducted with the support of a research grant from Chungnam National University.
Authors Information
Chung Doug Young, https://orcid.org/0000-0001-7948-1297
Lee Kyo suk, https://orcid.org/0000-0002-8668-5500
Kim, Sung Chuk, https://orcid.org/0000-0002-2521-6516
Lee, Dong sung, https://orcid.org/0000-0003-2234-6996
Byeong-Deok Hong, https://orcid.org/0000-0003-3026-7681
Il-Hwan Seo, https://orcid.org/0000-0002-8943-140X
Jin-Hee Ryu, National Institute of Crop Science, Rural Development Administration, Research Associate
