3.2.1. Changes under Different Elevations

Under different levels of elevation, the forest area in three altitude ranges, 1000–1500, 1500–2000 and 2000–2500 m, decreased from 1944 to 2013 (Table 2). Specifically, the forest distributed in 2000–2500 m reached the highest value of 1103.26 km2 at the beginning of the study, with the largest decrease of 458.77 km2 in 1986, and then gradually recovered. Only the forest area in the range of 2500–3000 m above sea level shows an increasing trend, which may be related to fewer human activities in high altitude areas [60].

The percentage of forest change area in different elevations in the total forest area can be seen more intuitively from Figure 7a. Among them, the biggest change area percentage was from 1944 to 1977, and the change area percentage of the forest with 2000–2500 m was −26.45%. Secondly, from 1986 to 2000, the change area percentage of 2000–2500 m forest was 23.26%.

#### 3.2.2. Changes under Different Slopes

Table 2 also shows the forest area under different slopes, from 1944 to 2013, only the forest area with a slope >25◦ increased, while the other three slopes included a decrease in the forest areas with a slope ≤6◦, 6–15◦ and 15–25◦. Among them, the reduced area is mostly distributed in the range of slope 6–15◦, which decreased from 813.51 km<sup>2</sup> in 1944 to only 681.41 km<sup>2</sup> in 2013, indicating that forests with gentle slope in this range is more likely to be occupied by cultivated land or other land types. Forests with steep slopes, such as those with a slope of more than 25◦, are more likely to be preserved, because they are less damaged [61,62].

The difference in forest changes area percentage between different periods and slope is shown in Figure 7b. The period 1986 to 2000 presented a relatively large change area percentage, accounting for 48.26% at 6◦–15◦ and 23.90% at 15◦–25◦, with a total of more than 72%. This result is significantly associated with the "grain for green" policy during this period. For the periods of 1944–1977 and 1977–1986, the changes were mainly negative, and the gradients mainly focused at 6◦–15◦ and 15◦–25◦, respectively. Therefore, the region with the gradient 6◦–25◦ was frequently interrupted by human activities [63].

#### 3.2.3. Changes under Different Soil Types

The forest area of four different soil types (other soil, rock soil, red soil and yellow soil) in the study area showed a decreasing trend from 1944 to 2013 (Table 2). Among them, the biggest decrease in forest was in rock soil, which dropped sharply from 168.49 km<sup>2</sup> in 1944 to 22.86 km<sup>2</sup> in 1986, and the ecological restoration after that only increased to 59.19 km2 in 2013. The largest forest distribution area is red soil, which was 966.61 km2 in 1944, and decreased sharply to 386.99 km<sup>2</sup> in 1986. After that, the forest coverage gradually recovered to 943.19 km<sup>2</sup> in 2013. Therefore, once the vegetation of karst rock soil type is destroyed, it is much more difficult to restore it than other soil types [64].

The changes in forests area percentage in different soil types in different periods are shown in Figure 7c, among which the biggest change area percentage is that the positive growth rate of forest distributed on red soil was 55.89%, from 1986–2000. However, before this, the negative percentage of change in this soil type were almost offset. Specifically, during the periods of 1944–1977 and 1977–1986, the reduction area percentage of forests distributed on red soil was −24.11% and −26.57%, respectively. However, the change area percentage of forests distributed in karst soil is −7.40%, which means that forests distributed in laterite is easier to recover after being destroyed than that in karst.

#### 3.2.4. Changes under Different Lithologies

The distribution and change characteristics of forests under different lithology generally show that the reduced area of forests in karst lithology (including dolomite mixed with limestone) is obviously larger than that of non-karst lithology (Table 2). Among them, limestone is the largest forest decrease, which decreased sharply from 427.23 km<sup>2</sup> in 1944 to 86.07 km2 in 1986, and then recovered to only 239.53 km<sup>2</sup> in 2013, only recovering to nearly half of the forest area at the early stage. However, the forests distributed in non-karst lithology decreased by half from 1944 to 1986, and gradually increased to 656.72 km2 in 2013 in the process of restoration, which is similar to the forest area in 1944.

As shown in Figure 7d, the rapid growth of forest area percentage, that is, from 2000 to 2007, it was about 51.78% in non-karst areas and 26.11% for limestone areas, which may have a great relationship with the implementation of natural forest protection projects during this period. On the whole, the limestone area had the biggest negative area percentage from 1944 to 2013, which was −12.72%, and the dolomite area had −4.73%. It also shows that vegetation restoration in karst areas is difficult for non-karst areas [65,66].

#### **4. Discussion**

#### *4.1. Comparison with Other Studies*

This part of the discussion is mainly aimed at the comparative analysis of other studies that use historical maps combined with remote sensing images to reveal the temporal and spatial changes in long-time series forest cover. At the same time, the comparison is mainly carried out from two aspects: one is to cover all or part of the study period (1944–2013), and the other is to include or relate to the study area (typical karst area in southwest China).

He et al. [67] revealed the trend and main process of forest dynamics from 1700 to 1998 by using historical documents, modern surveys and statistical data, and the results of previous studies. Among them, during the rapid decline from 1700 to 1949, the northeast, southwest and southeast regions suffered the most serious decline, and the coverage rate of most provinces fell by more than 20%. During the recovery period from 1949 to 1998, the western provinces (including Yunnan) increased by over 5%. In addition, another article by the researcher [11] shows that from 1700 to the 1960s, deforestation mainly occurred in southwest China. Judging from the changing trend and the general turning point, the trend of first worsening and then recovering is consistent. In addition, other studies using historical maps and remote sensing images to reveal the long-term changes in forests do not include or involve the study area of this paper, such as Taiwan Province Province [68], Hainan Island [69], Heilongjiang Province [70], etc.

Although there is a lack of research on forest evolution in southwest karst area by using historical maps and remote sensing images, much research that only uses remote sensing data to reveal forest or vegetation cover changes in karst area can also provide a reference for the second period of this study (the forest restoration stage after 1986). For example, Tong et al. [71] used the gimms-3 g Normalized Difference Vegetation Index (NDVI) from the period 1982–2012 to evaluate the effect of ecological engineering vegetation restoration in Yunnan, Guizhou and Guangxi. It was found that although the whole vegetation area was afforested, the restoration rates were different in different areas. On this foundation, Zhang et al. [72] also used the gimms-3 g NDVI from 1982 to 2016 to study the trend of vegetation change in Guizhou, Guangxi and Yunnan, and they found that the trend of vegetation greening in karst areas was strengthened from 1982 to 2016, and ecological engineering was the main reason for the increase in vegetation in karst areas, while the climate was the main driving factor for the decrease in vegetation in non-karst areas. This is consistent with the trend of forest restoration after 1986 in this research. Similarly, using NDVI data, Xu et al. [73] examined the vegetation mutation in Southwest China from 1982 to 2015, and found that the mutation point appeared in 2001, and the trend of NDVI changed from no significant increase to significant increase after the mutation point. For the above researches on forest or vegetation cover in karst areas of southwest China, the time scale mainly concentrated after 1982, and the forest cover showed a consistent increasing trend.

In the last 30 years, there is still a consensus that vegetation will turn green, whether in China [74–76] or in the region [77,78]. The research at the China national level before the 1970s shows that southwest China is a region with significant reduction in vegetation cover [11,67], but the research on forests in southwest karst area before 1970s is very scarce. It may be limited by the difficulty in obtaining remote sensing image data, and it also highlights the advantages of this research in combining historical maps with remote sensing images to deal with this problem.

#### *4.2. Events and Factors That Dominate Forest Cover Changes in Different Periods*

Forest deterioration from 1944 to 1986: Many incidents occurred during this period, including World War II (also called the "Anti-Japanese War" in China), the civil war between the Chinese Nationalist Party and the Chinese Communist Party from 1945 to 1950 (the Liberation War), the founding of the People's Republic of China in 1949, the shifting in the national system from capitalism to socialism, the land reform movement in 1950 (i.e., the transformation from feudal land ownership to private land ownership for

peasants), the rural cooperative movement in 1953 (i.e., the transformation from private land ownership for peasants to collectivization and socialization of agricultural land), the population policy (i.e., "many hands make work easy") in 1958, the "Great Leap Forward," the smelting of steel, and the movement to establish communes for rural residents in 1958. World War II, the Chinese Civil War, the change in the national system, the change in land ownership, or the smelting of steel might have resulted in the sharp deterioration and even the loss of forest vegetation. The policy on family contract business was implemented, particularly the transfer of the collective operation of lands and forests to families or individual corporations, the implementation of a "system of fixed output for households, work contracted to households, and mountain contracted to households," and the policy to divide privately farmed hilly lands and forest lands among individuals. Farmers were afraid of the change in ownership of such lands, Thus, they engaged in large-scale firewood gathering and logging, which might have damaged forest vegetation.

Forest recovery from 1986 to 2013: Construction projects were conducted to protect the forest system of the Yangtze River Basin in 1989 and the Pearl River Basin in 1996. A project to return grain plots to forests was also implemented in 1999. This project involved ecological construction engineering with the strongest policy, largest investment, widest coverage, and the highest extent of public engagement in China. The project was also the largest one that supports and benefits farmers, with funds of more than CNY4.3 trillion provided by the central government, thereby becoming the largest ecological construction project in the world. The implementation of various projects effectively promoted the increase in forest coverage rate and the reduction in soil erosion incidents. China formally launched conservation programs for natural forest resources in 2000 to strictly manage and protect ecological public welfare forests, strongly develop forestation, and adjust and optimize the ecological structure of forest zones. These programs greatly improved the regional ecological environment and reduced water and soil erosion areas. The comprehensive termination of the stony desertification project was implemented in 2006. In this project, the drainage basin was considered as a unit, the damaged natural ecological system was gradually recovered by increasing the vegetation land cover and conserving water and soil, and the extent of karst rocky desertification was effectively reduced.

The change in forest cover is affected by both natural and human factors [79,80], but the dominant factors are different in different time periods [81]. Although the past forest destruction has brought about the deterioration of the ecological environment, fortunately, a series of ecological projects have made great contributions to the restoration of forest vegetation [82,83].

#### *4.3. Limitations and Future Research Prospects*

First of all, due to the limitation of data sources, there was a long period between 1944 and 1977, and only the historical map of 1944 was used, which caused uncertainty of forest change trend analysis at present. In future research, we can increase the number of historical maps obtained as reasonably as possible before the 1970s, or update the latest year to the latest year. Secondly, in the correlation analysis of influencing factors, this study only considered four factors: elevation, slope, soil types and lithology. Then, on the time scale of several decades, these four factors will not change much. In future research, climate factors such as temperature and precipitation can be considered for analysis.
