Supporting SDG 15, Life on Land: Identifying the Main Drivers of Land Degradation in Honghe Prefecture, China, between 2005 and 2015

The essence of the 2030 Agenda for Sustainable Development adopted by the United Nations is described in 17 Sustainable Development Goals (SDGs). SDG 15 focuses on Life on Land, in other words, terrestrial biodiversity and ecosystems, as well as their services. Land degradation is a severe anthropic and natural phenomenon that is affecting land use/cover globally; therefore, a dedicated target of the SDG 15 (the indicator 15.3.1) was proposed. The identification of the areas where land degradation has occurred and the analysis of its drivers allow for the design of solutions to prevent further degradation in the studied areas. We followed the methodology proposed by the United Nations Convention to Combat Desertification (UNCCD) to study the land degradation in the Honghe Prefecture in southwest China between 2005 and 2015. Through spatial analysis, we found that the degraded areas were consistent with the areas of active human activities (such as urban centers), while the impact of natural factors (such as disasters) on land degradation existed in small areas at high altitudes. Land degradation was affected primarily by the loss of land productivity and secondly by land cover changes caused by the growth of artificial areas. Changes in the soil organic carbon were not significant. We concluded that human activity was the main driver of land degradation in Honghe Prefecture. Decision makers should work to find a balance between economic development and environmental protection to restore degraded land and strive to achieve a land degradation-neutral prefecture to defend all ecosystem services.


Introduction
The United Nations (UN) adopted the 2030 Agenda for Sustainable Development in September 2015 [1]. The Sustainable Development Goals (SDGs) are a collection of global goals designed to be a "blueprint to achieve a better and more sustainable future for all" that includes 17 goals, 169 targets, and over 230 indicators. Increasing institutions and scholars have begun to study and evaluate the progress of different targets in different countries, working together to achieve the goals set by the UN

Study Area
The study area was the Honghe Hani and Yi Autonomous Prefecture (Honghe Prefecture), in the Yunnan Province of China ( Figure 1). This area is located between 22 • 26 -24 • 45 N and 101 • 48 -104 • 17 E, in the southeast of the Yunnan Province, which is adjacent to Kunming, Wenshan, and Yuxi, and bordering Vietnam in the south. The area is in a subtropical monsoon climate zone with diverse climates. There are clear dry and rainy seasons in this prefecture. The mean annual precipitation is 2026.5 mm, and the mean annual temperature is 16.3 • C. The altitude ranges between 76.4 and 3074.3 m above sea level. The prefecture government is located in Mengzi City. The Red River flows through Honghe Prefecture from west to east. To the north of the Red River are the karst plateau and the lake-basin plateau, while to the south are the Hengduan Mountains and the Ailao Mountain Canyon. The economic development of the northern areas is better than that of the southern areas.

Study Area
The study area was the Honghe Hani and Yi Autonomous Prefecture (Honghe Prefecture), in the Yunnan Province of China ( Figure 1). This area is located between 22°26′-24°45′ N and 101°48′-104°17′ E, in the southeast of the Yunnan Province, which is adjacent to Kunming, Wenshan, and Yuxi, and bordering Vietnam in the south. The area is in a subtropical monsoon climate zone with diverse climates. There are clear dry and rainy seasons in this prefecture. The mean annual precipitation is 2026.5 mm, and the mean annual temperature is 16.3 °C. The altitude ranges between 76.4 and 3074.3 m above sea level. The prefecture government is located in Mengzi City. The Red River flows through Honghe Prefecture from west to east. To the north of the Red River are the karst plateau and the lake-basin plateau, while to the south are the Hengduan Mountains and the Ailao Mountain Canyon. The economic development of the northern areas is better than that of the southern areas.

Data Sources
The spatial resolution affects the results of land degradation assessment [19][20][21][22][23], particularly in mountainous regions, small island states, and highly fragmented landscapes [5]. Datasets with a finer spatial resolution give more detail and significantly enhance land degradation evaluation and monitoring on a local scale. However, many of these datasets will not be useful for estimating land degradation neutrality (LDN) baselines due to their limited temporal coverage [24]. The selection criteria for land cover and land productivity datasets in this research are that datasets must have global coverage to ensure the consistency, be publicly available at no cost to end users, be able to cover the time range of the study area, and the higher the spatial resolution, the better.

Data Sources
The spatial resolution affects the results of land degradation assessment [19][20][21][22][23], particularly in mountainous regions, small island states, and highly fragmented landscapes [5]. Datasets with a finer spatial resolution give more detail and significantly enhance land degradation evaluation and monitoring on a local scale. However, many of these datasets will not be useful for estimating land degradation neutrality (LDN) baselines due to their limited temporal coverage [24]. The selection criteria for land cover and land productivity datasets in this research are that datasets must have global coverage to ensure the consistency, be publicly available at no cost to end users, be able to cover the time range of the study area, and the higher the spatial resolution, the better.
The CCI-LC dataset is a time series of consistent global LC maps with a 300 m spatial resolution on an annual basis from 1992 to 2018. MOD13Q1 Collection-6 is a Level 3 product with a 250 m spatial resolution generated every 16 days [26]. The MOD13Q1 product provides the Normalized Difference Vegetation Index (NDVI) layer and the Enhanced Vegetation Index (EVI) layer used in land productivity calculation. The resolution of the Soil Type data is 250 m. Soil Grids Data are global soil property maps at six standard depth intervals at a spatial resolution of 250 m used as the reference values for the soil organic carbon (SOC) calculations.

Methodology
Following the Good Practice Guidance of UNCCD [10], the SDG 15.3.1 indicator is composed of three sub-indicators: (1) land cover and land cover change, (2) land productivity, and (3) soil organic carbon stocks. It follows the principle of "One Out, All Out", which means that the degradation of any sub-indicator for a land unit leads to its classification as degraded.

Land Cover and Land Cover Change
Of the three sub-indicators, the land cover change is the easiest to calculate. First, we obtain the land cover data of a base year and a target year. Second, we use the default land cover transition matrix from [10,27], which determines, for this region, whether transitions between classes correspond to degradations or improvements (no change indicates stable), as shown in Table 1. Third, we calculate the sub-indicator of each unit, we map and create a statistical table, and then use spatial analysis tools to identify the driving factors of this change.

Land Productivity
Land productivity is the biological productive capacity of the land, the source of the food, fiber, and fuel that sustains humans [10]. Land productivity can be measured across large areas using observations of the Net Primary Productivity (NPP), which is the net amount of carbon assimilated after photosynthesis and autotrophic respiration over a given period time.
Two of the most commonly used proxies of NPP are NDVI and EVI. Both of them are internationally comparable. Users can find free global datasets with them easily. EVI has two versions: the three-channel EVI (EVI-3) [28], and the two-channel EVI (EVI-2) [29]. EVI-3 exhibits high uncertainties in green vegetated areas due to the disturbance of blue band reflectance [23,30]. Researchers [31] showed that the EVI-3 performed poorly when compared to the NDVI and the EVI-2 at all scales in a VI test. Data of more than 2000 points selected from the world show a very high degree of similarity between NDVI and EVI-2, which results from a greater dynamic range of NDVI than EVI-2 [23,31]. Based on numerous and rigorous studies that have identified a strong relationship between NDVI and NPP [32][33][34], UNCCD and many researchers recommend using NDVI as an indicator of changes in land productivity [10,12,23,35].
Three sub-indicators can be calculated from remotely sensed estimates of land productivity [10,11]: 1.
Trend: We compute a linear regression at the pixel level to identify areas experiencing changes in primary productivity. A Mann-Kendall non-parametric significance test is then applied to statistically assess if there is a monotonic upward or downward trend [36,37], considering only significant changes that show a p-value ≤ 0.05. If significant, a positive slope of the fitted line indicates improvement, while a negative slope shows degradation. If not significant, the result is stable.

2.
State: With 2005-2012 as the baseline period, we compute the frequency distribution of the NDVI for each pixel. According to the distribution, we divide the NDVI data into 10 classes, calculate the mean value of each point, and determine the class. We use the mean NDVI from 2013 to 2015 as the value of the comparison period and determine the class of each pixel. If a difference in class between the baseline and the comparison period is smaller than −2, then this pixel could be degraded. If the difference is greater than 2, this pixel is improved. Pixels with small changes between −2 and 2 are considered stable [27].

3.
Performance: We calculate the mean NDVI of each pixel for the entire period. We define units having the same combination of land cover and soil type and compute the frequency distribution of all the mean NDVI in each of them. Taking the value representing the 90th percentile as the maximum NDVI for each unit, we calculate the mean NDVI/NDVI max for each pixel. If the value is less than 0.5, then the pixel is marked as degraded.
The three productivity sub-indicators are then combined, and the possible combinations of results are indicated in Table 2. Table 2. Aggregation of the productivity sub-indicators.

Soil Organic Carbon
The first 30 cm of the soil profile was used as the reference for the calculation of changes in the SOC content. Data for evaluating changes in the SOC for Asia are typically insufficient as countries often do not monitor SOC stock and its changes [38]. We used a method for monitoring the SOC based on land cover changes between different land classes [25]. The conversion coefficients (Table 3) were derived from [12,27]. As suggested by [10], a loss in SOC greater than 10% indicates degradation, and a gain of larger than 10% shows improvement.  Figure 2 shows the land degradation caused by the land cover conversion, and Table 4 shows the conversion area between the different classes of the land cover from 2005 to 2015. Table 5 shows the change in the area covered by each class of the land cover. According to the transition matrix (Table 1), Table 6 gives a summary of the change in land cover degradation.     Comparing the data from 2005 and 2015, the areas with the highest growth rate were the wetlands and the artificial areas ( Table 5). The increase in the wetlands mainly came from the water bodies and the forests, while the increase in the artificial areas mainly came from the croplands and the grasslands. The increase in the artificial areas resulted in a total degradation area of 92.9 km 2 (Table 4), accounting for 90.3% of the total land cover degradation area (102.9 km 2 , Table 6). Figure 3 shows the land degradation caused by land productivity, and Table 7 gives a summary of the change in land productivity degradation. of the change in land productivity degradation. From Figure 3, the improvement and the degradation of the land productivity were widely distributed in the counties of Honghe Prefecture, while the areas with severe land degradation were concentrated in Mile, as well as the junction of Kaiyuan, Gejiu, and Mengzi. As seen from Table 7, the land productivity changes in Honghe Prefecture were essentially stable, maintaining a stable status of 48.7%, an increase of 36.2%, and a decrease of only 14.8%.  From Figure 3, the improvement and the degradation of the land productivity were widely distributed in the counties of Honghe Prefecture, while the areas with severe land degradation were concentrated in Mile, as well as the junction of Kaiyuan, Gejiu, and Mengzi. As seen from Table 7, the land productivity changes in Honghe Prefecture were essentially stable, maintaining a stable status of 48.7%, an increase of 36.2%, and a decrease of only 14.8%. Figure 4 shows the land degradation caused by the SOC changes, and Table 8 gives a summary of the changes in SOC degradation. Land cover change occurs with an associated change in SOC: loss can be significant in a short period while accumulation occurs at a slower rate. It takes more years for the SOC to be restored to the original level than for its reduction [39,40]. From Figure 4 and Table 8, the SOC of most land remained stable. It can be seen from Table 8 that, compared with 2005, the total area of SOC improvement and SOC degradation in Honghe Prefecture in 2015 accounted for less than 0.3%. Combining the land cover changes (Table 4) with the coefficients (Table 3), we can see that the improvement of the SOC occurred in the conversion from croplands to forests and to grasslands, while the degradation of the SOC mainly occurred in the conversions from croplands and the grasslands to the artificial areas, and from the forests and the grasslands to the croplands. However, as the evaluation threshold was limited to 10%, the actual improvement/degradation area was less than the total area above (Table 8).  Combining the land cover changes (Table 4) with the coefficients (Table 3), we can see that the improvement of the SOC occurred in the conversion from croplands to forests and to grasslands, while the degradation of the SOC mainly occurred in the conversions from croplands and the grasslands to the artificial areas, and from the forests and the grasslands to the croplands. However, as the evaluation threshold was limited to 10%, the actual improvement/degradation area was less than the total area above (Table 8). Figure 5 shows the land degradation of Honghe Prefecture, and Table 9 gives a summary of the SDG 15.3.1 indicator.  Figure 5 shows the land degradation of Honghe Prefecture, and Table 9 gives a summary of the SDG 15.3.1 indicator.

SDG 15.3.1 Indicator
Through the overlay of the land productivity changes and land degradation status in 2005-2015, we conclude that the land productivity degradation had a great impact on the land degradation. Figure 6 further verifies that land productivity degradation was the main cause of land degradation.  Land area with no data 94.7 0.3% Through the overlay of the land productivity changes and land degradation status in 2005-2015, we conclude that the land productivity degradation had a great impact on the land degradation. Figure 6 further verifies that land productivity degradation was the main cause of land degradation.

Land Cover Change Analysis
From Figure 7, the degradation was mainly located in the areas near the administrative center of the prefecture government Mengzi and the administrative centers of the counties north of the Red River. The NOAA VIIRS night light data from May 2015 was used to overlay with the degradation data, which clearly shows the correlation between the degradation and the urban expansion. In the densely populated urban center areas, the vegetation cover of the cultivated land decreased significantly. This is inseparable from the urbanization of Honghe Prefecture.

Land Cover Change Analysis
From Figure 7, the degradation was mainly located in the areas near the administrative center of the prefecture government Mengzi and the administrative centers of the counties north of the Red River. The NOAA VIIRS night light data from May 2015 was used to overlay with the degradation data, which clearly shows the correlation between the degradation and the urban expansion. In the densely populated urban center areas, the vegetation cover of the cultivated land decreased significantly. This is inseparable from the urbanization of Honghe Prefecture.

Land Cover Change Analysis
From Figure 7, the degradation was mainly located in the areas near the administrative center of the prefecture government Mengzi and the administrative centers of the counties north of the Red River. The NOAA VIIRS night light data from May 2015 was used to overlay with the degradation data, which clearly shows the correlation between the degradation and the urban expansion. In the densely populated urban center areas, the vegetation cover of the cultivated land decreased significantly. This is inseparable from the urbanization of Honghe Prefecture.  The urbanization process of Honghe Prefecture is in synchronization with China's rapid development. Rural people increasingly work in urban centers. When they earn enough money, they buy real estate from where they originated, which requires a great deal of land to build more apartments around the county center. The investments in real estate development and the urban infrastructure construction caused a sharp reduction in the cultivated land resources around the center, which is one of the important causes of land degradation in Honghe Prefecture, which is driven by the population growth and migration.
The fastest changing area is at the junction of Mengzi and Gejiu, which may be due to two reasons. First, the Honghe Prefecture government began to relocate in 2003, and there were a large number of factories and enterprises moving with the government agencies. Therefore, the demand for the land near the new administrative center of the government increased dramatically. Second, Gejiu is the tin capital of China, and Datun is one of the main producing areas of tin. With strong financial support, the government has planned the Datun New District. This area is changing rapidly with the two benefits of being close to the prefecture government and establishing a new district.
Google Earth was used to validate the land cover change. Figure 8 shows the image near G326 (China National Highway) at the junction of Mengzi and Gejiu on 10 November 2006, from which we can see that most of the area is cropland. In contrast, Figure 9 shows that man-made buildings became the main type of land cover in this area on 23 January 2015.
Gejiu is the tin capital of China, and Datun is one of the main producing areas of tin. With strong financial support, the government has planned the Datun New District. This area is changing rapidly with the two benefits of being close to the prefecture government and establishing a new district.
Google Earth was used to validate the land cover change. Figure 8 shows the image near G326 (China National Highway) at the junction of Mengzi and Gejiu on 10 November 2006, from which we can see that most of the area is cropland. In contrast, Figure 9 shows that man-made buildings became the main type of land cover in this area on 23 January 2015.  The urban expansion and the construction of the new functional areas directly and indirectly occupy a large number of land resources, which have become important factors in the land resources degradation in Honghe Prefecture. The driver of the land degradation in Chenggong District, Kunming City, Yunnan Province is similar to that of this region [41]. The urban expansion of Honghe Prefecture mainly occupied croplands and grasslands around urban areas, a rule that also applies to the farming-pastoral ecotone of northern China [42]. In the process of industrialization and urbanization of the Shazand Watershed in Iran, a large number of grasslands and rain-fed agricultural lands were converted into artificial areas [43].
With the reduction of croplands and the continuous population growth, cutting shrubs and reclaiming grasslands became a measure to compensate for the reduction of croplands [44]. A total of 6.47 km 2 of forests and 2.23 km 2 of grasslands in Honghe Prefecture were converted into croplands in 10 years. Unlike the practice of felling wood as fuel in Ethiopia [44,45], coal is the main fuel in Honghe Prefecture. The deforestation in Tanzania is due to the energy demand for curing tobacco leaves [46]. Honghe Prefecture is the second main tobacco producing area in Yunnan, where coal and electricity are mainly used for flue-cured tobacco.
Another area with significant changes is in the middle of Mile. It is distributed along a stripe pattern, which is consistent with the direction and location of the G80 National Highway, which was The urban expansion and the construction of the new functional areas directly and indirectly occupy a large number of land resources, which have become important factors in the land resources degradation in Honghe Prefecture. The driver of the land degradation in Chenggong District, Kunming City, Yunnan Province is similar to that of this region [41]. The urban expansion of Honghe Prefecture mainly occupied croplands and grasslands around urban areas, a rule that also applies to the farming-pastoral ecotone of northern China [42]. In the process of industrialization and urbanization of the Shazand Watershed in Iran, a large number of grasslands and rain-fed agricultural lands were converted into artificial areas [43].
With the reduction of croplands and the continuous population growth, cutting shrubs and reclaiming grasslands became a measure to compensate for the reduction of croplands [44]. A total of 6.47 km 2 of forests and 2.23 km 2 of grasslands in Honghe Prefecture were converted into croplands in 10 years. Unlike the practice of felling wood as fuel in Ethiopia [44,45], coal is the main fuel in Honghe Prefecture. The deforestation in Tanzania is due to the energy demand for curing tobacco leaves [46]. Honghe Prefecture is the second main tobacco producing area in Yunnan, where coal and electricity are mainly used for flue-cured tobacco.
Another area with significant changes is in the middle of Mile. It is distributed along a stripe pattern, which is consistent with the direction and location of the G80 National Highway, which was completed in 2012. Based on the above reasons, human land use is a dominant driver of the land degradation in Honghe Prefecture.
The largest increase in land improvement in the land cover change is from the conversion of grasslands to forests, with an area of 227.58 km 2 , accounting for 94.27% of the total improvement area of 241.42 km 2 , mainly distributed in Honghe (county), Lvchun, Shiping, Jianshui, and the south of Jinping. Most of these forests occur in remote mountainous areas in each county, which may be related to the Grain for Green Program (GGP) launched in China in 1999.
The GGP is the largest ecological restoration project in China and is closely related to land use and cover change (LUCC) [47]. The project accelerated the process of national afforestation, and the forest coverage rates in the project areas increased by an average of more than four percentage points [48]. An article of Nature Sustainability stated that China alone accounted for 25% of the global net increase in leaf area with only 6.6% of the global vegetated area from 2000 to 2017. The greening in China is from forests (42%) and croplands (32%) [49]. This program underlines that government policy can play an important role in the modification of landscapes [45].
Land expropriation is a significant source of revenue for municipal governments. To improve the government's fiscal performance, the newly amended Land Administration Law grants greater autonomy to local governments over land conversions and farmland acquisitions. The new land policy will accelerate the conversion of good-quality farmlands into artificial areas, and the lost farmlands will be made up of grasslands and forests, aggravating land degradation and destroying the local ecosystem. The Honghe prefecture government should reduce the scale of conversion from cultivated land to construction land year by year, to protect high-quality arable land. They should also strengthen the water conservancy construction to improve the quality of land, use resources scientifically, stabilize the scale of the city, and cut down on the demand from the natural environment.

Land Productivity Analysis
According to our results, the changes in land productivity occurred mainly in the forests, the grasslands, and the croplands. Degradation or improvement mainly occurred within the same land class. The overall trend showed that, regardless of the land cover class, the area of improvement was greater than the area of degradation (Table 10), which indicates that the land productivity improved over time. This is consistent with the changes in the characteristics of the three land cover classes in China during the same period as derived by Wang Zhen et al. [50]. In recent years, the Chinese government attached great importance to environmental protection. Through returning farmland to forests, returning grazing land to grasslands, and the development and use of desert areas, the vegetation coverage indices of all land cover classes increased significantly. Although there is a lag in the response of the NDVI to the temperature and precipitation in the Red River Basin [51], the lag time is less than 1 year. The lag effect can be reduced or eliminated by calculating the average annual NDVI each year from 2005 to 2012 and the 2013-2015 average value. However, geological disasters caused by rainfall may cause sharp changes in the local NDVI value. The degradation of the land productivity in Jinping mainly occurred on the top of the mountain over 2000 m above sea level. Disasters caused by rainfall may be the main cause of ecological degradation in high-altitude areas [52]. The special geographical environment and complex geological structure of Honghe Prefecture and even Yunnan Province, coupled with the unreasonable exploitation and utilization by human beings, brought more frequent natural disasters. In particular, since the 1990s, the collapses, landslides, and debris flows in Yunnan Province have shown an increasing trend, causing huge losses [53].
As the area distribution of land productivity degradation north of the Red River overlapped very well with the night light data, we infer that this is related to the distribution of the county centers. These areas also have densely distributed agricultural land and populations [54]. In addition, the degradation of southeast Mile is also significant. Since 2005, we used remote sensing data to monitor the planting distribution of the cash crops north of the Red River. We found that the cash crops tended to shift from low to high altitudes, and from the basins to the slopes and mountains.
Cash crops with a smaller average area, in particular Panax notoginseng and the greenhouse vegetables, might be the cause of land productivity degradation in the grasslands, the forests, and the croplands. Panax notoginseng prefers shaded environments, distributed in the forests or the grasslands, and is often covered with black plastic nets ( Figure 10). Figures 11 and 12 show images of the same area in Luxi County before and after planting Panax notoginseng in Google Earth. Greenhouse vegetables are mainly distributed in the fields and covered with polyethylene films, which affect the NDVI acquired from the satellite images. Panax notoginseng and vegetables are cultivated in small fields covered with thin films and shade nets, which will lead to a lower assessment of the land productivity. In addition, climate change and geological factors can also result in the NPP degradation of irrigated farmland [43].

Soil Organic Carbon Analysis
Land cover conversions strongly influence ecosystem services, such as the soil organic carbon, for climate mitigation [56,57]. Articles [47,58] demonstrated that the GGP in China led to greatly increased soil organic carbon. The SOC accumulation rates were significantly higher under the Conversion of Croplands to Forest (CCF) compared with under the Conversion of Croplands to Grasslands (CCG) at depths of 0-20 cm and 20-40 cm, which implies that CCF is a more efficient way to accumulate carbon in the soil compared with CCG [47].
The SOC is central to soil health, fertility, quality, and productivity [59]. A long time is needed to significantly increase the SOC after the conversion of cultivated land to forest or grassland. Our results show only a 10-year change, while it typically takes 14 to 30 years to significantly increase the SOC value as the accumulation speed is affected by the SOC content before conversion [47]. Another article on the Yunnan Province showed that the value of SOC had significant relationships with the rainfall, temperature, and altitude [60].

SDG 15.3.1 Indicator Land Degradation
According to the principle of "One Out, All Out", the result of the SDG 15.3.1 land degradation The NDVI decreased in areas with high economic development near administrative centers. In contrast, the NDVI increased significantly in the areas with low human activities [55]. Benefiting from the implementation of the GGP, forests and grasslands in the remote areas were well protected. An upward trend of NDVI indicates that the implementation of ecological projects effectively improved the regional environment [54].

Soil Organic Carbon Analysis
Land cover conversions strongly influence ecosystem services, such as the soil organic carbon, for climate mitigation [56,57]. Articles [47,58] demonstrated that the GGP in China led to greatly increased soil organic carbon. The SOC accumulation rates were significantly higher under the Conversion of Croplands to Forest (CCF) compared with under the Conversion of Croplands to Grasslands (CCG) at depths of 0-20 cm and 20-40 cm, which implies that CCF is a more efficient way to accumulate carbon in the soil compared with CCG [47].
The SOC is central to soil health, fertility, quality, and productivity [59]. A long time is needed to significantly increase the SOC after the conversion of cultivated land to forest or grassland. Our results show only a 10-year change, while it typically takes 14 to 30 years to significantly increase the SOC value as the accumulation speed is affected by the SOC content before conversion [47]. Another article on the Yunnan Province showed that the value of SOC had significant relationships with the rainfall, temperature, and altitude [60].

SDG 15.3.1 Indicator Land Degradation
According to the principle of "One Out, All Out", the result of the SDG 15.3.1 land degradation can be extracted by merging the degradation areas of the three sub-indicators. From these data tables (Tables 6-9), we concluded that there was an overlap among the degradation areas of the sub-indicators. Among the three sub-indicators, the area of land productivity degradation was the largest, which accounted for 98.7% of the total SDG 15.3.1 land degradation area. Land productivity degradation mainly occurs internally, which indicates the need for good plans to protect the forests and the grasslands, as well as crop planting programs to prevent further degradation.
As one of the important factors of human activities, urbanization is the dominant driver of land cover degradation in Honghe Prefecture, and road construction and mining development have directly occupied croplands, grasslands, and forests. Mining development also consumes a great deal of water resources, which indirectly leads to a change in vegetation [61]. According to our field survey, urbanization and population migration in Honghe Prefecture did not cause land abandonment, which was observed in Srpska [62]. Despite the residents' great demand for croplands, they still responded to the national policies and converted part of the croplands into grasslands and forests.
Land degradation shows obvious urban-rural differences. Urban land degradation is serious; however, the land in rural areas demonstrated a clear trend of improvement. Serious land degradation occurred in Mengzi, Gejiu, Kaiyuan, Mile, and Jianshui, which are in the top 100 counties of west China with better economic development, while the area of land improvement in the national-level poverty-stricken counties, such as Yuanyang, Honghe, and Jinping, is limited.
Comparing the land degradation results is difficult due to the different methods and evaluative indicators employed and the lack of uniform standards [63]. The guidance provided by UNCCD and the SDG implementation of Trends.Earth are conducive to the unification of methods and parameters among different countries and regions. This will improve the consistency of assessing land degradation for reporting on SDG indicator 15.3.1. We used the recommend datasets in Honghe Prefecture. LUCC datasets with the highest possible precision, high-spatial-resolution NDVI datasets, and detailed SOC datasets are encouraged to be used for land degradation as they will improve the accuracy of reports on SDG indicator 15.3.1.

Conclusions
We analyzed the land degradation in Honghe Prefecture from three aspects and found that human activities, such as urbanization, road construction, and mining development directly led to changes in the land cover, and the population growth accelerated this process [64,65]. Frequent human activities led to significant degradation of the land productivity in the radiation area from the city center, while unreasonable farming plans and cropland management were important causes of cropland degradation. The large-scale cultivation of cash crops affected our assessment of land productivity using the earth observation data, but this does not hinder the conclusion that human activities are the main cause of land degradation in Honghe Prefecture.
Research has confirmed in many countries and regions that human activities are the main driver of land degradation, but studies are also showing that climate change is the key factor affecting land degradation in certain areas [66]. National-level data and world-scale data cannot accurately reflect the real situation in certain areas. There are comparatively distinct land degradation differences between urban and rural areas. As different regions will produce different results, we should produce corresponding strategies according to different situations. The policymakers should formulate new sustainable development measures based on the population and the land carrying capacity to ease the effect of human activities, particularly urbanization [61]. The aim should be to achieve a land-degradation-neutral world while ensuring economic development and the improvement of people's living conditions.
Two limitations need to be considered. First, we used moderate-resolution geospatial datasets in this paper, and the effect of the high-resolution earth observation data on the results requires further verification. Second, only by fully understanding how human activities affect the productivity degradation of forests, grasslands, and croplands can policies be better formulated, which will be the focus of further research.