**1. Introduction**

Forests play an important role in maintaining the ecological balance of the earth [1,2]. The forest is a significant symbol of the ecological environment [3], and it is the material base for forestry production [4,5]. Forest can not only regulate atmospheric circulation and water cycle, but also affect climate change, and play an important role in protecting water and soil resources and preventing wind and sand [6–8]. The rapid change in forest cover is causing the loss of habitat, biodiversity, and climate change [9,10]. Social and economic conditions in different periods have different impacts on land vegetation cover [11]. The spatial reconstruction of historical forests is helpful for a better understanding of the changes human beings have made to the surface and their impacts on the environment [12]. Therefore, long-term and spatial dynamic changes in forest cover have been an important concern for global ecologists, environmentalists, and so on [13–16].

**Citation:** Chen, F.; Bai, X.; Liu, F.; Luo, G.; Tian, Y.; Qin, L.; Li, Y.; Xu, Y.; Wang, J.; Wu, L.; et al. Analysis Long-Term and Spatial Changes of Forest Cover in Typical Karst Areas of China. *Land* **2022**, *11*, 1349. https:// doi.org/10.3390/land11081349

Academic Editor: Francisco Manzano Agugliaro

Received: 7 June 2022 Accepted: 15 August 2022 Published: 18 August 2022

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

Remote sensing data can provide the continuous change in surface elements in time and space, and it plays an irreplaceable role in regional ecological environment monitoring [17]. However, during the period when satellites were not yet launched, satellite images were unavailable, and the surface of the Earth and its features could not be located accurately [18]. Thus, studies on the spatial framework of an ecological environment are limited to the years after 1972 [19]. This issue has always been a challenge for some ecologists [20]. With openness and sharing of data, an increasing number of available historical maps, especially historical military maps, can be obtained easily on the Internet or in libraries [21,22]. Thus, historical maps are an important data source for scientists to reveal the changes in the surface landscape before remote sensing images. Some internationally well-known scholars have conducted a considerable amount of valuable research with the use of these maps [23–26].

Because of the important ecological and economic functions of forests, it is very important to monitor long-term changes by using old maps and written sources, because this method enables people to monitor trends from different time dimensions and find the reasons for the current situation [16,27]. For example, Skaloš et al. [28] carried out a comparison of landscape developments in Sweden and the Czech Republic by using old maps. Pelorosso et al. [29] used historical maps and recent remote sensing-derived maps to reduce misleading changes and to assess spatial aggregation errors based on a data integration procedure using landscape metrics in Italy. Skaloš et al. [16] analyzed longterm land-cover changes in central Bohemia and contributed to a better understanding of the dynamics of forest land using old military survey maps and orthophotograph maps, covering more than 250 years. Furthermore, researchers of some countries have used military maps to study past land cover changes in Slovenia [30,31], Germany [32], Sweden [33], Norway [34,35], and the Czech Republic [28,36,37].

On the other hand, many researchers have studied the long-term land cover in China. He et al. [11] reconstructed the forest cover in China from 1700–2000, and found that the deforestation mainly occurred in southwest China, the hilly regions of south China, the southeast of Gansu province, and northeast China from 1700 to the 1960s. Yang et al. [38] evaluated the reliability of global historical land-use scenarios for forest data in China and pointed out that these global historical land-use scenarios could not accurately reveal the spatial and temporal pattern of China's forests due to differences in data sources, reconstruction methods and spatial scales. Li et al. [39] used the historical forest area allocation model to reconstruct forest cover between 1780 and 1940 in Northeast China. Liu et al. [40] synthesized historical maps and aerial images to describe long-term land-use change and landscape dynamics for a region near Chancellorsville, USA, from 1867 to 2014. In a word, many researchers use historical maps combined with remote sensing images to reveal long-term changes in forest/land cover and proved that climate change and human activities are the main influencing factors, but mainly on the national and regional scales. Aiming at the change in long-time series forest cover in karst areas, Tanacs et al. [41] used an integrated GIS of historical data (18th–19th century military maps, old forest management plans, aerial imagery, etc.) to describe the example of the Haragistya karst plateau and how the forests of the Aggtelek karst region were used in the last few centuries and to what extent they were affected by anthropogenic activity. However, there is still a lack of research on long-time series forest cover in typical karst areas of China.

The southwest China karst area is located in the center of East Asia karst area, and it is one of the three largest karst areas in the world [42]. Karst ecosystem is a fragile ecosystem, which is affected and restricted by the special geological background [43,44]. Because of the special geological and climatic conditions, the bedrock in this area is exposed, which has the basic characteristics of little soil reserves, discontinuous soil distribution, complex and diverse micro-landforms, etc. [45,46]. These characteristics have caused some problems in karst areas, such as high rock exposure rate, slow soil formation speed, easy loss, and weak soil water and fertilizer retention capacity [47,48]. Among them, karst rocky desertification with a large scale and high frequency of change is the most serious

ecological disaster in Southwest China [49,50]. In the past, intense human activities have significantly changed the structure of karst ecosystems, especially the change in land cover [51–53]. The whole karst ecosystem is more sensitive to changes in environmental conditions [54]. Taking the vegetation in the karst area as an example, the research shows that the geochemistry of karst bedrock can affect the growth of vegetation by adjusting the water-holding capacity of the weathered layer, which makes the vegetation productivity in the karst area more susceptible to drought [55]. The fragility and stability of karst ecology and the fragile natural recovery ability have brought challenges to local economic development and environmental protection [56,57]. It is of great significance for karst ecosystems to cope with global changes and achieve sustainable development by exploring the long-time sequence of forest cover changes in karst areas [58].

This study mainly reveals the dynamic changes in forest cover in a long time series from 1944 to 2013 in typical karst areas of Southwest China through historical maps and remote sensing image data and analyzes the different spatial distribution patterns of forest cover from four aspects: altitude, slope, soil type, and lithology. Furthermore, we try to explore the possible influencing factors of forest cover increase or decrease in different change periods. It is hoped that our research results can fully understand the dynamics and temporal and spatial differences in the evolution process and trend of forest cover in recent decades, provide theoretical reference for comprehensive control of rocky desertification in karst mountain areas and rural revitalization, and provide the certain theoretical basis for promoting the sustainable development of karst areas in southwest China.

#### **2. Materials and Methods**
