A Social‐Ecological Framework to Enhance Sustainable Reforestation Under Geological Constraints

South China Karst faces the challenge of balancing ecological conservation and regional development, a task made more intricate by the geological limitations of carbonate formations. Large‐scale conservation and restoration initiatives have mitigated rocky desertification and positioned South China Karst as a global hotspot for vegetation regrowth over the past two decades. However, challenges stemming from geological constraints and oversights in recognizing the synergies within social‐ecological systems remain. Here, we propose a social‐ecological framework that integrates the extended timescale of both historical and recent human impacts with the corresponding processes of forest evolution. The framework elucidates the enduring and continuous progression of “forest‐deforestation‐reforestation,” offering applicability for optimizing ecological space across diverse social‐ecological contexts. Moreover, it serves to enhance the sustainability of reforestation efforts when faced with geological constraints.


Introduction
Over the past two decades, China has undertaken the largest series of ecological restoration projects in human history, with an investment exceeding $378.5 billion (in 2015).These efforts have yielded considerable improvements in ecosystem quality, as demonstrated by enhanced vegetation cover and improved ecosystems services (Bryan et al., 2018;Chen et al., 2019;Ouyang et al., 2016).In 2012, the Chinese government introduced an ambitious national strategy for ecological civilization, placing a strong emphasis on ecological conservation and restoration as fundamental tasks.Enhancing the diversity, stability, and sustainability of ecosystems, fostering green development, and promoting a harmonious relationship between humanity and nature have emerged as strategic objectives within China's eco-civilization and ecological security framework.This is notably evident in the implementation of major projects for the protection and restoration of national key ecosystems (2021)(2022)(2023)(2024)(2025)(2026)(2027)(2028)(2029)(2030)(2031)(2032)(2033)(2034)(2035).There is an urgent need for comprehensive territorial space planning and the classification of ecological restoration zones.Such measures would clarify and categorize areas for utilization, conservation, and restoration efforts (Fu, 2021).Central to this endeavor is the adoption of systematic governance that employs a site-specific approach.This involves taking into account the interplay among mountains, rivers, forests, farmlands, lakes, grasslands, and human activities, both at watershed and regional scales.However, most restoration goals were associated with increased habitat area and connectivity, or improved ecological processes and functions.Few restoration initiatives paid attention to the feedback between social and ecological systems (Tedesco et al., 2023).
A deeper understanding of the interaction between humans and nature across various geographical contexts is essential for enabling sustainable ecological restoration efforts (Fu et al., 2023).The South China Karst regions, centered around Yunnan, Guangxi, and Guizhou provinces, constitute the world's largest and most concentrated karst distribution area, spanning approximately 540,000 km 2 .This area surpasses the two other major global karst distributions located in the Eastern United States and the Mediterranean coast of Europe.Moreover, it serves as an ecological barrier, crucial for water resource security related to the upstream area of the Yangtze River and the Pearl River.Due to its distinctive geological features, the region exhibits a dual hydrogeological structure characterized by slow soil formation, shallow and non-continuous soil layers, and rapid precipitation infiltration.The vegetation habitat in the karst regions is lithophytic and drought-prone, rendering it sensitive to human activities and susceptible to rocky desertification-a severe form of land degradation observed in humid and semi-humid areas characterized by prevailing carbonate rocks and human disturbances (Jiang et al., 2014;Wang et al., 2019).
Concurrently, this region contends with the challenges of low socioeconomic development, hosting a population of approximately 229 million people and historically representing one of the largest concentrated impoverished areas in China.The intersection of high population pressure, rocky desertification, and poverty, underscores significant conflicts between ecological conservation and regional development.Furthermore, the carbonate bedrock likely undermines the sustainability of forestation efforts in the karst regions.Some areas face challenges transitioning into forest ecosystems after prolonged human disturbances, in particular deforestation (Yue et al., 2023), and some areas may have a naturally low tree cover and do not require forestation (Parr et al., 2024).However, although being a hotspot of global greening, there has been little attention on the sustainability of reforestation under geological constraints.This paper unfolds in four parts.In the first two parts, we review the natural and social dimensions of rocky desertification controls in South China Karst, respectively.Subsequently, the third section introduces challenges pertaining to forestation under geological constraints.In the final part, we present a social-ecological framework designed to enhance sustainability in the realm of reforestation and rocky desertification controls.

The Natural Dimension of Rocky Desertification Control
Evolving from intense human disturbances to large-scale afforestation and natural recovery, South China Karst has emerged as a global leader in vegetation greening.The area of rocky desertification in eight provinces of Southern China has consistently decreased from 129,600 km 2 in 2005 to 67,400 km 2 in 2021 (Yue et al., 2023).Over the past two decades, South China Karst has emerged as a focal point for global vegetation growth, contributing 5% to the world's carbon sink, while accounting for only 0.36% of the global land area (Brandt et al., 2018;Chen et al., 2019).Between 2002 and 2017, 1.76 Pg carbon have been sequestered, offsetting 25% of the region's anthropogenic carbon emissions from fossil fuel burning, which was approximately 7 Pg carbon in 2017 (Tong et al., 2018).Notably, from 2010 to 2016, China's terrestrial ecosystems absorbed remarkable amounts of the nation's anthropogenic carbon emissions, with the southwestern region contributing 32% (Wang et al., 2020), thus playing a crucial role in achieving China's carbon neutrality objective (Piao et al., 2022;Tong et al., 2020;Xu et al., 2023).
However, the feasibility of large-scale artificial afforestation is limited by the distinctive geological characteristics of the karstic region.While situated in a humid to semi-humid climatic zone, variations in rock and soil composition impact the type and quality of vegetation restoration.The formation of karsts has given rise to a dual hydrogeological structure, distinguishing between surface and subsurface water.This structure is marked by rapid hydrological processes, resulting in a limited water storage capacity.In dolomite areas with underdeveloped bedrock crevices, the soil depth typically does not exceed 20 cm.This shallow soil layer is conducive to the growth of herbaceous plants with shallow root systems but not suitable for the deeper roots of woody vegetation (Figure 1).Additionally, the area exhibits a scarcity of soil mass, characterized by thin soil layers with low insoluble acid content, leading to a deficiency of mineral nutrients in the soil-vegetation ecosystem (Zhang & Wang, 2011).Consequently, the effectiveness of vegetation restoration in the karst area exhibits significant spatial heterogeneity, with some regions encountering challenges in achieving successful forestation (Yue et al., 2023).

The Social Dimension of Rocky Desertification Control
The pivotal strategy in mitigating rocky desertification lies in addressing human-land conflicts.With the advent of large-scale ecological restoration, the impact of human activities on karstic ecosystems has undergone a transformative shift from traditional deforestation and intense cultivation to conservation and restoration efforts, Earth's Future resulting in a substantial reduction in anthropogenic disturbances.Tangible evidence of this transformation is observed in the phased changes in the rural population across eight provinces of South China Karst from 1953 to 2020.Notably, there was continuous population growth from 1953 to 1982, followed by substantial decreases starting from 1990, resulting in a reduction of the rural population by 107.4 million people by 2020 (Figure 2a).Especially within the context of rapid urbanization over the past two decades, the rural population decreased by 102.11 million people from 2000 to 2020, representing 95% of total reduction of the rural population.The urban population surpassed the rural population for the first time in 2015 (Figure 2b).Furthermore, the considerable shift in urban-rural migration patterns due to migrant work has markedly alleviated actual population pressure in rural areas.This shift has transitioned human-land relationships from a state of tension to a more balanced stage (Zhang, 2022;Zhang et al., 2022).Taken together, these results suggest that rocky desertification control is entering a new phase, marked by an upcoming inflection point in the relationship between humans and the land.It is imperative to focus on the interaction and coupling processes between ecological and social systems in South China Karst for sustainable conservation and restoration efforts.Furthermore, South China Karst is characterized by the complex distribution of karstic landforms such as karst peak cluster depressions, graben basins, karst plateaus, karst trough valleys, karst canyons, mediumhigh mountains, peak forest plains, and dissolved hill depressions.The regions with different distributions of these karst landforms exhibit substantial disparities in socio-economic development levels, mainly driven by the dynamics of social-ecological transformation.The trajectory of rural construction in China has been going through different processes, beginning with the assurance of basic living needs in the late 1970s, progressing to the building of a well-off society since 2005, and culminating in the achievement of prosperity for all people after 2020 (Guo & Liu, 2021).These rural development processes align with the evolutionary stages of ecological restoration, transitioning from the initial phase of coordinating the territorial layout to the current ongoing phase of systematic governance.Ultimately, the aim is to achieve green development and foster harmony between humanity and nature within the coupled social-ecological systems (Figure 3).
To foster sustainable ecological restoration, a critical step is to understand the relationship between humans and the diverse karst landforms (Fu et al., 2023;Tedesco et al., 2023).Building on the foundation laid by the initial Karst Rocky Desertification Treatment Program (2006Program ( -2015) ) and the comprehensive control initiative outlined in the 13th 5-Year Plan (2016-2020), rocky desertification control has achieved a continuous net reduction in affected areas and considerable improvements in the extent of degradation.Presently, it faces the challenge of transitioning into a phase of governance transformation (Yue et al., 2022).However, current research on ecological restoration in South China mainly focuses on the changes in ecosystem structure and function, while the essence of rocky desertification control lies in mitigating conflicts between human demands and environmental capacities.Thus, there is an urgent need to redirect attention toward the focal points of regulation for rocky desertification control and human-induced changes.This requires a shift in focus from the natural ecosystem to the coupled social-ecological systems and their feedbacks.

Building Upon the Initial Greening Following Human Disturbances, Considerations Arise Regarding Suitable Locations for Afforestation or Reforestation With Diverse Geological Contexts
The majority of South China Karst is situated in humid and semi-humid climates, experiencing an annual average precipitation of more than 1,000 mm.During the early stage of restoration, vegetation growth tends to rapid.However, due to the constraints from the carbonate geological background, the positive succession of shrubs and grasses slows considerably during natural recovery, often resulting in stable shrubland or grassland for several decades (Jiang et al., 2020;Liu et al., 2019).For tree plantation initiatives, the composition of soil and rock plays a pivotal role in determining the types and quality of the plants that can be restored.Subsequently, reforestation efforts have failed in certain areas, particularly in limestone regions with minimal soil and in dolomite areas with undeveloped bedrock crevices.Therefore, uncertainties persist in the feasibility of reforestation in some areas of South China Karst following human disturbances (Yue et al., 2023).
Understanding historical contexts not only informs contemporary ecosystem management but also provides valuable opportunities for anticipating how ecosystems respond to both climate and human disturbances (Fordham et al., 2020;Vilà-Cabrera et al., 2023).Historical records and documents reveal that prior to the Ming Dynasty, human disturbances had minimal impact on the natural karst mountains, primarily because of limited transportation options (Zheng, 2013).Only a small number of low-lying regions were converted to farmland, leaving the forest vegetation largely undisturbed.However, significant ecological transformation occurred in Southwest China during the Ming and Qing Dynasties, spanning the last 500 years, particularly with the large scale migration of Han immigrants (Han, 2015;Zheng, 2013).A pivotal factor in this shift was the introduction of drought-tolerant yet high-yield crops, such as maize, suitable for the karst mountainous areas.This accelerated deforestation, transforming the primary forests into agricultural landscapes and contributing to the onset of rocky desertification (Zhang et al., 2020;Zheng, 2013).Given this historical context, addressing rocky desertification and sustainable ecological restoration becomes urgent.The critical questions that arise include: Was an area currently deemed difficult for afforestation ever a forest in history?Would it be possible for it to become a forest in the future, and if so, what might be the timeline for such transformation?

Improving Tree Plantations to Increase Productivity and Ecosystem Services
In ancient times, especially before the Qing and Ming dynasties, South China Karst was predominately covered by lush evergreen and deciduous broad-leaved mixed forests, experiencing minimal human disturbances (Zheng, 2007).Contrastingly, in the present-day South China Karst, mature forests accounts for only 9.3% of the forested area, whereas planted forests constitute nearly 40% of the forests, although they exhibit limited ecological functions.The forested area across the eight karst provinces spans 92.36 million hectares, constituting 41.9% of China's total forested expanse.Although forest coverage has increased from 23.3% in 1975 to 47.9%, the composition of forest ages is imbalanced.A total of 73.2% of the area consists of young and middle-aged stages.The planted forests predominantly in the form of pure artificial forest.Southwest China, especially Guangxi, is also a major contributor to China's timber production, accounting for 38.6% of the nation's output in 2021.Despite the rapid growth, considerable carbon sequestration and economic benefits of monoculture forests, they are characterized by low biodiversity, underdeveloped food chain structures, diminished soil fertility and productivity, as well as relatively weak water retention and soil conservation capabilities (Peng et al., 2023).Forest growth in harvested forest areas contributed 16% to the carbon sink in Southern China during 2002-2017, while timber harvest tripled and soil moisture showed an overall decline, with a statistically significant decrease (p < 0.05) observed in 8% of the area (Tong et al., 2020).Therefore, beyond enhancing short-term productivity, a desired feature of tree plantations should be resilience to disturbances and environmental stochasticity, particularly in the face of climate change.Given the current context of rocky desertification control and increasing demand for timber, there is an urgent need to improve tree plantations in Southern China.The focus should be on species that contribute to biodiversity and socio-ecological systems in South China Karst.

Balancing Ecological Greening With Economic Benefits
The southwestern karst regions, once the largest concentrated area of poverty in China, achieved considerable strides in poverty eradication, lifting approximately 29 million people out of absolute poverty from 2010 to 2020.Having transitioned beyond absolute poverty, the region has become a focal point for consolidating and expanding the gains of poverty alleviation, aligning with China's rural revitalization strategy during the "14th 5-Year Plan" period.The southwestern region is pivotal, contributing 96 out of the 160 key poverty-stricken counties for national rural revitalization assistance.However, the interplay between ecological greening and economic growth presents new challenges, particularly in rocky desertification areas that need to develop characteristic industries for sustainable rural revitalization.Amid poverty alleviation and the development of characteristic industries, unsustainable practices, such as shrub cultivation and imprudent breeding have resulted in new human disturbances, deforestation, and localized rocky desertification.As a result, the prevalence of mild rocky desertification has increased from 27.5% in 2005 to 40.8% in 2021.Therefore, it is important to recognize that restoring terrestrial ecosystems goes beyond a simplistic "the greener, the better" approach.It requires more than afforestation and the implementation of restoration projects.A comprehensive analysis of natural environments and corresponding socioeconomic conditions is imperative to establish appropriate restoration goals and methodologies for effective rocky desertification control (Fu et al., 2023).

A Social-Ecological Framework to Enhance Sustainable Forestation Under Geological Constraint
To resolve the above-mentioned challenges for effective rocky desertification control and fostering sustainability greening in South China Karst demands integrated, multi-scale approaches.The critical issues of carbonate bedrock constraints and human disturbances impact deforestation and reforestation efforts in karst regions.Given the historical occurrences of deforestation and rocky desertification in the Ming and Qing Dynasties, adapting a long-term perspective that integrates historical, modern and contemporary periods is crucial to anticipating the future of forest.Here, we propose a social-ecological framework designed to seamlessly integrate the extensive transformation of human disturbances over long time and the corresponding evolution of forests (Figure 4).
The framework aims to optimize ecological space for future environmental policies, considering diverse socialecological contexts.It builds on the interactions between human activities and elements of diverse ecosystems that constitute social-ecological systems (Ostrom, 2009).These interactions give rise to patterns and dynamics that provide feedback on the processes that generated them in a continuously evolving manner over a long time period (Folke et al., 2016;Schlüter et al., 2019).Common frameworks that capture the links between interactions and emerging patterns and processes of social-ecological systems often overlook historical changes, which are crucial for informing present-day ecosystem management and formulating conservation policies to adapt to climate change (Aleman et al., 2018;Fordham et al., 2020).We address this gap by defining a continuous timescale within the framework, integrating long-term human disturbances and forest evolution.Specifically, the key periods for human disturbance transformation in South China Karst encompass the beginning of intense human disturbances (Ming and Qing Dynasties, 1400-1920 AD), subsequent decades of intense human disturbances (since the 1920s), contemporary conservation and restoration efforts (in recent decades, since around 2000), and the anticipated optimization of ecological space in next century (until 2100).The framework centers on forest development, emphasizing the interconnected relationships between "natural forest-deforestationreforestation" since the Ming and Qing Dynasties.It provides a comprehensive and forward-looking strategy to address the intricate dynamics of social-ecological systems in South China Karst.To further analyze the intricate interplay between human impacts and forest evolution processes, we propose specific operational methods for each key period: 1. Intense human disturbances to forests in Ming and Qing Dynasties: (a) Analysis of low-lying depressions: Given the widespread distribution of low-lying depressions in the karst region, intense human disturbances increase soil erosion and the depositional process in these depressions.Analyzing the stratified changes of sediments within depressions allows for a comprehensive understanding of historical surface processes and the causes of human disturbances (Zhang et al., 2020).(b) Isotopic dating and paleo-proxies analysis: The 14 C isotopic dating can reveal the ages of stratified sediments formation.The alteration and types of forest and nonforest throughout historical periods can be inferred by paleo-proxies (pollen, phytolith, and δ 13 C) changes in different sediment layers (Aleman et al., 2018;Fordham et al., 2020).Combining these proxies with historical records establishes connections between human disturbances, deforestation, and rocky desertification.(Tong et al., 2020).High spatial resolution satellite images and machine learning accurately identify complex forest types under geological constraints (Li et al., 2023).4. Ecological space optimization and sustainable reforestation in the next 100 years: (a) Continuous Evolution Analysis: Integrating human disturbances and forest evolution across history and recent decades provides a comprehensive understanding of the continuous evolution of "forest-deforestation-reforestation."(b) Optimization strategies: Clarifying and optimizing ecological space involves reforestation, natural regeneration, grass plantation for herbivorous animal husbandry development, and nature reserves.Anticipating climax forests under geological constraints is paramount.(c) Ecological intensification: Enhancing productivity and ecosystem services in reforestation involves the concept of ecologically intensified tree plantation.This entails integrating genetic, functional, and demographic diversity across heterogeneous landscapes, utilizing biodiversity as a tool to increase productivity, timber production, and ecosystem services while minimizing ecological impacts (Gómez-González et al., 2023;Vilà-Cabrera et al., 2023).It is anticipated to achieve the balance of ecological greening with economic benefits.
Based on this multi-time scale and interdisciplinary framework, we gain profound insights into the extensive processes of human disturbance transformation and forest evolution (Table 1).Unrevealing the continuous chain of "natural forest-deforestation-reforestation" under human disturbances and different geological contexts becomes achievable.This framework allows us to address pivotal questions: Which areas are suitable for afforestation or reforestation under different geological conditions?What is the natural restoration timeline for areas to transition into forests?How can existing tree plantations be improved?What areas are suitable for natural restoration, tree plantations, grassland, or nature reserve under different human-land relationships in South China Karst?Ultimately, by integrating climate, lithology, landforms and human-land relations, it is possible to project the development of climax forests within various eco-social contexts and apply appropriate measures (e.g., natural regeneration, forestation, grass plantation, nature reserves) to specific locations, guided by a careful balance between social demands and the provision of ecosystem services.
It should be highlighted that not all karst regions are suited for afforestation, because prolonged human impact has created alternative stable states with eroded/shallow soils that cannot be reverted into forests (Pausas & Bond, 2020;Yue et al., 2023).This scientific foundation not only contributes to China's concept of eco-civilization, but also informs the implementation of major projects for the protection and restoration of national key ecosystems (2021)(2022)(2023)(2024)(2025)(2026)(2027)(2028)(2029)(2030)(2031)(2032)(2033)(2034)(2035).The versatility of this framework extends to its application in restoring ecologically vulnerable areas under other social-ecological contexts.Incorporating both historical and recent data on forest dynamics and deforestation will enhance our comprehension of the changes in forest structure and function, whether driven by human activities or climate shifts.This approach will enable the testing of conservation strategies and reforestation practices at detailed spatial and temporal scales.It provides a robust evidence foundation to evaluate the degree to which areas identified for forest restoration genuinely necessitate such interventions.
Earth's Future

Figure 1 .
Figure 1.The rock-soil profiles in limestone (a) and dolomite (b) slopes, and landscape view of low shrubland (limestone) and grassland (dolomite) under natural restoration with more than 20 years.Photographs were taken in 2019.

Figure 2 .
Figure 2. (a) Urban and rural population changes during 1953-2020, and (b) the spatial-temporal pattern of urban-rural migration in eight provinces of karst landforms distributed, South China.Mobile phone location data from 2017 is derived from Zhang et al. (2022).We calculated the difference in the population density maps between the first day during spring festival (28 January) and after the spring festival (28 February) to calculated the urban-rural migration before/after the holiday.

Figure 3 .
Figure 3. Social-ecological transformation along with China's rural construction and ecological restoration evolution processes.

Figure 4 .
Figure 4.A framework to integrate human disturbance transformation and forest evolution for sustainable reforestation.
2. Intense human disturbances to forests over the past 100 years: (a) Dating techniques: Utilizing dating techniques such as 137 Cs and 210 Pb ex determines the year of stratified sediment formation over the past century.(b) Identification through erosion rates: Changes in soil erosion rate and modulus, coupled with local chronicles, help identify intense human disturbances.Dynamics of forest and deforestation can be deduced through pollen and phytolith identification (e.g., Yue et al., 2024).3. Forest conservation and restoration in recent decades: (a) Ecological restoration projects: The Chinese government's ecological restoration projects for rocky desertification control since around 2000 can be assessed at the regional scale.(b) Remote sensing and machine learning: Time-series of remote sensing images spanning 40 years unveil trends in reforestation

Forest
for slash-and-burn cultivation and timber trade throughout the period Mainly primary forest, forest area declinedZheng (2007),Chen and Kung (2016), Yang et al. (2018), and Yue et al. (2024) b.Institutional change, population migration and rapid growth, require resources c.Maize introduction (since the 1600s) and plantation in steep slopes, exacerbating ecological degradation The past 100 years (since the 1920s) a.The Anti-Japanese War (1937-1945) and the Civil War (1920-1949) Mainly primary forest, forest area declined Li et al. (2019), Zhang et al. (2020, 2023) b.Heavy deforestation in the Great Leap Forward (1958-1960) and Refining Iron and Steel in 1958 c.Closing mountains to facilitate afforestation (1964-1966) d.Afforestation projects (since the 1980s) Recent decades (since 2000) a. National restoration projects (e.g., The Grain to Green Program since 2002

Table 1
Analysis of the Proposed Social-Ecological Framework to Identify Historical and Recent Human Disturbances and Forest Development in South China KarstTimescale