Vegetation restoration in South China’s karst region under geological background constraint: Forest or non-forest

Abstract

Over the past two decades, the karst region in southern China has emerged as a critical area for ecological conservation and restoration, attracting global attention for its efforts in ‘greening.’ However, particular areas face challenges transitioning into forests due to the carbonate bedrock constraint, which likely undermines sustainability of greening efforts in karst regions. This study overviews the significant achievements in vegetation restoration in the karst region of southwest China, emphasizing the potential of short-term ecological restoration in effectively sequestering carbon, a valuable approach to achieving carbon neutrality in the region. However, combating rocky desertification remains difficult due to geological limitations that hinder large-scale afforestation. Additionally, variations in soil and rock composition impact the success and quality of vegetation restoration, leading to some areas failing to establish forests. Therefore, establishing the goals for rocky desertification control in different regions (enhancement, degradation, or reduction) should be based on current conditions and the evolutionary characteristics of rocky desertification. To facilitate precise afforestation and grass planting in the karst region, factors such as climate, rock conditions, and human-environment relationships should be considered in regional divisions, including natural restoration zones, artificial afforestation zones, grass planting zones, and protected zones.

Introduction

Karst regions cover approximately 15% of the Earth’s land area and are crucial for supplying drinking water to a quarter of the global population. There are 88 countries with karst landforms over 50,000 km² or more than 20% of their total area (He et al., 2019). Among these countries, China possesses the most extensive karst area, accounting for roughly one-third of its land territory. The karst regions in southern China, specifically in Yunnan, Guizhou, and Guangxi, are the world’s largest and most concentrated contiguous karst area, spanning approximately 540,000 km² and supporting a population of 229 million (Wang et al., 2019). This region features a delicate geological composition consisting of soluble carbonate rocks, developing a dual hydrogeological structure between the surface and underground. As a consequence of this geological background, the region has slow soil formation, a thin soil layer, rapid rainwater drainage, and vegetation sensitive to human activities, which ultimately lead to severe rocky desertification (Yue et al., 2022). It is worth noting that there is a substantial overlap between regions affected by rocky desertification and those struggling with poverty.

The Chinese government has taken significant measures to address the challenges of rocky desertification and poverty in the karst regions of southern China. The project, with a comprehensive ecological construction and poverty, stands as the largest initiative in global karst areas. It aims to control rocky desertification and spans from 2008 to 2035. By 2020, an investment of over 130 billion RMB has been allocated towards this endeavor (Wang et al., 2019). Notably, this approach has yielded significant progress in reducing the extent and severity of rocky desertification, establishing a global ‘greening’ hotspot over the past two decades (Chen et al., 2019. Impressively, the southern China karst region, occupying only 0.36% of the global land area, contributed 5% to the world’s fastest-growing vegetation biomass increase from 2002 to 2017 (Brandt et al., 2018). Furthermore, from 2010 to 2016, it accounted for 32% of China’s terrestrial ecosystem carbon absorption, indicating its remarkable ecological restoration and carbon sequestration achievements (Wang et al., 2020).

Nevertheless, despite the ongoing reduction of rocky desertification and the rapid growth in vegetation coverage, the sustainability of ‘greening’ in the karst region remains uncertain due to its fragile geological background. Ecological conservation and restoration efforts encounter new challenges. While large-scale afforestation initiatives often provide limited and unsustainable ecological services, differences in soil and rock composition within the southern Chinese karst area impact the success and quality of vegetation restoration (Callahan et al., 2022; Jiang et al., 2019; McCormick et al., 2021). Consequently, some afforestation efforts fail to transform into forests, and the natural restoration of shrubs and grasses struggles to achieve forest-like conditions within a short timeframe. In 2012, the Chinese government strategically committed to promoting the establishment of an ecological civilization, highlighting the importance of green development in realizing sustainability. This commitment encompasses planting trees for future generations to ‘enjoy the shade‘ and aims to leave behind ecological assets instead of regrets. Furthermore, with the pressing demand generated by the comprehensive plan for protecting and restoring important ecosystems in China from 2021 to 2035, ensuring adaptive ecological restoration and achieving precise afforestation and grass planting in the southwestern karst region of China serve as urgent and essential objectives. Furthermore, due to the launch of the giant plan to protect and restore important Chinese ecosystems (2021–2035), it has become an urgent and essential objective to ensure adaptive ecological restoration and achieve precise afforestation and grass in the karst regions of southern China.

Outstanding achievements in vegetation restoration in the karst region of southern China

1. There have been three notable achievements in vegetation restoration in the karst region of southern China. First, the extent of rocky desertification in the karst region has remarkably decreased from 129,600 km² in 2005 to 67,400 km² in 2021. The ecological structure and service functions of the karst ecosystems in the eight southwestern provinces have also greatly improved. Moreover, the control of rocky desertification has made the karst region in southwest China one of the hotspots for global ‘greening,’ with its vegetation biomass increase being the fastest-growing and contributing 5% to the global total, despite covering only 0.36% of the land area (Brandt et al., 2018; Tong et al., 2018; Chen et al., 2019). In 2018, Nature journal highly praised the positive effects of rocky desertification control and ecological restoration in the karst region of southwest China, affirming that ‘China is advancing in greening efforts’ (Macias-Fauria, 2018, 553: 411-413).

2. Second, through long-term scientific research and innovative practices, replicable and scalable techniques and models have been developed for controlling rocky desertification in karst areas. A ‘Chinese governance paradigm’ in karst ecological management has consequently formed, characterized by government leadership, technological support, and public participation. It has become a crucial initiative for China to fulfill the United Nations Convention to Combat Desertification and contribute to global governance in addressing climate change (Wang et al., 2019). Furthermore, the integration of rocky desertification control with ecological industries has significantly alleviated absolute poverty for approximately 29 million people in the karst regions of the eight southwestern provinces between 2010 and 2020. The poverty reduction efforts in the karst areas of Yunnan, Guangxi, and Guizhou have been remarkable, making a substantial contribution to achieving United Nations Sustainable Development Goal 1 (No Poverty).

3. Third, the karst region in southwest China has demonstrated its potential for rapid carbon sequestration in ecological restoration, making significant contributions and leading the region’s efforts towards carbon neutrality. Between 2002 and 2017, vegetation restoration in the southwestern region sequestered 1.76 PgC (petagrams of carbon), offsetting approximately 25% of the anthropogenic carbon emissions (around 7 PgC) during the same period (Tong et al., 2020). From 2010 to 2016, China’s terrestrial ecosystems absorbed about 1.11 billion tons of carbon annually, accounting for 45% of the concurrent anthropogenic carbon emissions. Notably, the southwestern region alone accounted for 32% of the total carbon absorption, significantly enhancing its carbon sink capacity and substantially accelerating local carbon neutrality efforts (Wang et al., 2020).

Problems and challenges in vegetation restoration in karst areas

Due to the geological composition of carbonate rocks and alterations in human disturbance patterns, the restoration of vegetation in karst areas in southern China faces new challenges and problems in the contemporary era. These changes include a shift from deforestation and cultivation disturbances to widespread ecological protection and restoration efforts, significantly reducing disturbances. Additionally, changes in human-land relations, such as urbanization and migration for work, have contributed to a more harmonious relationship between humans and the land, marking a turning point.

1. The task of addressing rocky desertification in karst areas remains challenging and carries significant risks. The Fourth National Monitoring Bulletin on Rocky Desertification (2021) reveals that 72,200 km² of rocky desertification persist, predominantly localized in the upstream areas of the Yangtze River and Pearl River systems, thereby severely impacting the ecological stability of these two rivers. Furthermore, the potential area of land susceptible to rocky desertification is estimated at 176,800 km² (2021), accounting for 36.5% of the karst land area and underscoring the considerable risk of a resurgence in rocky desertification. To combat this issue, the National Major Ecological System Protection and Restoration Project (2021–2035) has devised a comprehensive plan to address 39,400 km² of rocky desertification, indicating the formidable nature of this task.

2. The viability of large-scale artificial afforestation faces limitations due to the unique geological characteristics of the karst region. Despite the karst area being located in a humid to semi-humid climatic zone, the karst formation has resulted in a dual hydrogeological structure between surface and subsurface water, characterized by swift hydrological processes. Furthermore, the area exhibits a scarcity of soil quantity, featuring thin soil layers with low insoluble acid content and inadequate soil mineral nutrient supply. Consequently, the area’s vegetation is susceptible to droughts and water scarcity due to its poor water-holding capacity (Liu et al., 2019). In January 2020, a study published in Nature Communications confirmed that nearly 50% of the southwestern karst areas experienced a decline in soil moisture content as a result of large-scale afforestation, particularly in regions with concentrated artificial afforestation. This decline significantly impacted the long-term sustainability of afforestation efforts (Tong et al., 2020).

3. The effectiveness of vegetation restoration in the karst area exhibits significant spatial heterogeneity, with certain regions encountering challenges restoring forest landscapes. Despite the karst area gaining recognition as a ‘hotspot’ for global greening, the vegetation cover during the growing season has experienced a rise from 69% in 1999 to 81% in 2017 (Brandt et al., 2018), however, 21% of the vegetation demonstrates a consistent and alarming degradation trend. Differences in rock and soil composition influence the type and quality of vegetation restoration, resulting in unsuccessful afforestation in specific areas, particularly those with dense and undeveloped fissures within the dolomite zone, ultimately leading to low tree survival rates (Zhang and Wang, 2009; Liu et al., 2019). Additionally, the vegetation in dolomite or limestone areas has persisted predominantly as low shrubs or grasses for nearly two decades under natural restoration conditions. Moreover, the southwestern region has witnessed a continuous reduction in annual rainfall at a rate of 11.4 mm per decade over the past 60 years, thereby increasing the frequency of extreme drought and flood events, which further impedes the restoration of forest landscapes (Liu et al., 2014).

Spatial optimization of ecological construction in southern China’s karst area

The governance of rocky desertification in southern China’s karst area should be based on the principles of ‘zone-based classification and targeted implementation,’ in line with the national goals of achieving carbon neutrality and building a beautiful China (by 2035 and 2050), the strategic needs for rural revitalization, and the goals outlined in the General Plan for the Protection and Restoration of Important National Ecological Systems (2021–2035). This approach builds upon the ‘basic zoning and integrated governance’ in the rocky desertification comprehensive management project (2006–2015) and the governance priorities highlighted in the 13th Five-Year Plan (2016–2020). Different regions should establish specific vegetation restoration goals based on their current status and the different evolutionary characteristics of rocky desertification (Zhang et al., 2021). Efforts should focus on increasing efficiency in areas where rocky desertification is continuously improving and enhancing the ecological service functions and comprehensive benefits of governance in rocky desertification areas. Relevant strategies include improving the quality of single-species plantations and shrubs on hilly slopes, integrating short-term and long-term benefits, developing characteristic ecological derivative industries, and promoting rural revitalization (Wang et al., 2020). For the regions where rocky desertification remains severe and challenging to control, the objective should be to decrease the area and intensity of rocky desertification.

Additionally, significant differences in vegetation restoration occur in different rock backgrounds within the karst region due to the development of rock fissures and variations in the chemical characteristics of bedrocks. In limestone areas, the soil on the slopes commonly form a pitted pattern within dissolution holes and channels. Weathering profiles in the region exhibit the development of vertical fissures with widths ranging from several centimeters to tens of centimeters, and depths ranging from tens of centimeters to meters. The surface soils on the slopes consist of black or yellow lime soils, whereas lower parts of some fissures contain yellow soil or even iron-manganese nodules (Liu et al., 2019; Zhang and Wang, 2009). Vegetation with deep root systems can penetrate the fissures extensively, making it suitable for restoring woody plants. By comparison, in dolomite areas, the rock particles are denser, and the soils on the slopes are frequently distributed in continuous or discontinuous thin layers with a thickness ranging from several centimeters to over ten centimeters. The absence of fissures in the weathering profiles makes it suitable for restoring shallow-rooted herbaceous plants. Therefore, the governance of rocky desertification should consider the regional differentiation characteristics of climate-rock type and human-land relations in the future. Afforestation or grassland construction should be carried out based on local conditions, along with necessary soil and water conservation measures (Zhang et al., 2021). In areas where artificial vegetation recovery is challenging, such as soilless karst hills and slopes, rocky desertification that remains difficult to control below a mild level after governance, and uninhabited areas affected by rocky desertification, natural restoration or protected closure should be implemented.