Ecosystem service (ES) has been defined as the benefits for human populations derived directly or indirectly from ecosystem functions [1], which provide essential material and non-material conditions for human existence and development. However, ES and biodiversity have been severely degraded in recent years ^[2][3][2,3]. The Millennium Ecosystem Assessment (MA) evaluated 24 ESs globally and suggested that 60% were degrading. In addition, according to the report of the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services (IPBES), the average abundance of species in most important terrestrial communities has declined by at least 20%; 14 of the 18 services assessed were declining; and the global species extinction rate was tens or even hundreds of times faster than the average for the past 10 million years [4]. These conclusions indicated that biodiversity loss might lead to a severely negative impact on ES supply ^[5][6][5,6]. Climate change, ecosystem structure, and land use change were the main driving factors of ES change at the macro scale ^[7][8][9][7,8,9], with plant functional traits, species invasion, and microbial diversity acting at the micro and meso scales ^[10][11][10,11]. The impact of global climate change and disturbance mechanisms on mountain areas is greater than that of other biogeographic regions [12] and is expected to intensify further [13]. Human activities are the main factors affecting the significant decline in ecosystem service value and ecological problems, and land use change is the most obvious manifestation of human activities [14]. Climate and land use change will cause changes in the structure and process of different ecosystems and ultimately affect the supply capacity of ES. The functional diversity of plant traits is linked to ecosystem function and biodiversity [15]. Plant communities composed of functionally divergent species or traits contain species combinations that enhance productivity through complementary resource use [16]. The impacts of climate change [17] and land use/cover [18] on ES have been summarized well.

Many researchers have studied the impacts of climate change ^{[19][20][21][22][23]}[19,20,21,22,23], land use ^{[24][25][26][27]}[24,25,26,27], and landscape structure ^{[28][29][30][31][32]}[28,29,30,31,32] on ES in different ecosystems. The results showed that all these drivers have a direct and significant effect on ES. However, climate change, land use, and landscape structure change do not affect ES directly but by controlling community structure, plant functional traits, and species diversity in the forest ecosystem. Studies have found that both negative and positive climatic impacts have only small effects on forest dynamics compared to silvicultural measures. Only for very few water-limited stands did climate change affect forest growth negatively due to pronounced drought stress and mortality [33]. On the contrary, forests can regulate the climate through their own attributes. Forests efficiently recycle water using several plant traits, such as deep rooting systems, high leaf area, and surface roughness that facilitates upward water vapor transport. These conditions, strongly related to the forest structure, increase rainfall over tropical forests compared to grass in grazing lands or soy crops [34]. In addition to carbon sinks and carbon storage services (biogeochemical processes of climate regulation), forests also provide climate regulation ES through biogeophysical processes [35]. Forests are responsible for an atmospheric cooling effect through transpiration, and surface winds can transmit the cold air beyond the forest boundary [36], which plays a role in regulating the regional microclimate. Furthermore, biodiversity has been considered to be either the basis for ecosystem services provisioning or a service in itself ^[2][37][2,37]. As the terrestrial ecosystem with the highest biodiversity, the forest has been widely recognized for its role in biodiversity conservation and ecosystem multifunctionality maintenance ^[38][39][40][38,39,40]. However, many precious studies paid more attention to species diversity and ignored the role of functional diversity. Lyashevska and Farnsworth [41] pointed out that plant diversity consists of multiple dimensions of diversity, including classification (such as species richness), function (such as diversity of wood density), and structure (such as the average height of the community). Different plant diversity indicators can show different relationships with different ecosystem services [42]. For example, forest functional diversity is positively correlated with hydrological regulation services. The increase in land use intensity led to a reduction in niche differentiation of interspecific functional traits, resulting in the degradation of hydrological services in the forest ecosystem [43]. Although some studies have shown the role of functional traits [44], species diversity ^[45][46][45,46], and functional diversity [47] in forest ES, the relationship among them needs further study, and the key scientific issues need to be clarified.

In the carbonate region, especially in the karst desertification area, the soil layer is shallow and forms slowly because of the binary (aboveground and underground) hydrological structure and process. Serious soil erosion limits vegetation growth, and strong human disturbance leads to the occurrence of karst desertification eventually ^[48][49][48,49]. Further, karst desertification intensifies the frequency of droughts, floods, and other disasters, which seriously damage ecological functions and restrict regional sustainability [50]. For example, water and wind can erode the topsoil easily after vegetation degradation, which reduces the water conservation and nutrient supply capacity [51]. Moreover, the decline in plant coverage also degrades regulation services such as carbon sequestration, oxygen release, and hydrological regulation [52]. Water flow takes nutrients into underground spaces, which limits the growth of ground flora and reduces product provision services [51]. Restoring damaged ecosystems rapidly and effectively and improving ES supply capacity are the primary tasks of the eco-degradation area. According to a study by Duarte et al. [53], landscape composition and configuration significantly affect ES. Hodder et al. [54] believed that conservation, management, and interventions at the landscape scale might enhance the supply of a series of ESs (carbon storage, fiber and food, etc.). Laughlin [55] proposed a quantitative model for ES recovery using trait values (e.g., selecting species with high wood density and low specific leaf area to improve community resistance to drought). Biodiversity and ES loss not only directly affect the livelihood of poor populations but may also further exacerbate a decline in human well-being [2]. The ecosystem structure and landscape pattern lend support to this theory at the macro scale, while tree structure, functional trait, species diversity, and functional diversity at the meso and micro scale can provide specific community construction and species configuration schemes. Although the combination of the two scales can effectively optimize the functions of degraded forest ecosystems, there are few existing studies that have been reported.

2. A Systematic Review of ES Drivers for Forest Ecosystem Improvement in Karst Desertification

(1)

The optimization of stand spatial structure helps improve the quality of the ecosystem

The regulation of forest spatial structure and distribution pattern by manual measures is conducive to promoting interspecific interaction and improving ecosystem functions. Pruning significantly increases the light intensity, temperature, understory biomass, and Shannon Weiner index of species ^[56][112]. The intercropping of forest and grass can promote the regulation of soil quality and microclimate and increase forest products ^[57][113]. This shows that reasonable management measures can not only improve the forest structure and productivity but improve the carbon fixation capacity of vegetation. Thinning and replanting can significantly improve the forest layer index and mixing degree. The increase in individual differences between trees can expand the growth space of young and middle-aged forests, which reduces the competitive pressure between trees and significantly improves the stand structure ^[58][59][114,115]. There are more problems in the spatial structure of the karst desertification forest ecosystem compared with non-karst desertification. Common situations include sparse understory vegetation and incomplete hierarchical structure of arbor, shrub, and grass; broken patches and a lack of a large, connected forest landscape; and a single species disposition, which is not conducive to resisting diseases and pests. Therefore, implementing artificial management in this area to optimize the forest spatial structure is conducive to improving the forest ecosystem function and service supply capacity. According to the characteristics of karst desertification forest, the optimization of stand spatial structure can be carried out horizontally and vertically. In the horizontal space, natural forests can be thinned, replanted, and renewed manually. Intercropping of trees and cereal/grass and mixed plantation can be implemented to regulate the composition and the proportion in the artificial forest. In the vertical space, trees can be pruned and reshaped to control their height and crown width ^[60][116] so that the lower trees can fully absorb solar energy and improve the community’ productivity. In short, during planting and ecological restoration, the layers of trees, shrubs and herbs should be intact to improve the self-regulation ability of the ecosystem in terms of nutrient decomposition and circulation ^[61][117].

(2)

Building plant functional groups based on functional characteristics and environmental conditions is conducive to improving ecological functions

Plant functional types combine a series of plants with certain plant functional traits, which are the basic units for studying the dynamic changes of vegetation along with the environment ^[62][63][118,119]. They link plant morphology, community science, and ecological processes, and are a very useful tool for studying the dynamic changes between climate and vegetation. Environmental heterogeneity (such as soil, light, and terrain) shapes the characteristics of individual plants to a certain extent, it also affects the interactions between species and their proportions in different spatial ranges. The aim of community structure optimization can be achieved by making full use of the adaptability of species characteristics to the environment to dispose of species ^[64][65][120,121]. In karst desertification areas, due to exposed bedrock and a lack of surface water, adaptive plants are usually drought resistant, lithophytic, and calciphilous ^[66][122]. The existing research found that the soil enzyme activity, soil nutrients, and microbial community diversity index of forest grass intercropping in karst mountain areas were significantly higher than in wasteland and farmland returning to grassland ^[67][123]. Therefore, since few species are included in afforestation for karst desertification control, multi-species interplanting can play an important role in promoting ecological restoration. In addition, in forest gaps with enough sunlight, short-lived and shade-intolerant species have higher growth rates than long-lived and shade-tolerant species ^[68][124]. In forests with high canopy density and insufficient light, shade tolerant species can be selected for planting, which can enhance the integrity of the stand structure. The characteristics of herbaceous plants are mostly similar—weaker than most trees and shrubs, but they can also be appropriately added to enhance the overall stability of the community.

(3)

Biodiversity conservation is the foundation for maintaining EMF

Biodiversity determines ecosystem functions and processes ^[69][70][125,126]. Higher biodiversity can produce higher levels of ecosystem functions ^[71][72][127,128]. The plants are more abundant in karst habitats than in non-karst habitats in South China karst (accounting for 30%–40% of the total local species). Many species are rare, endangered, protected groups, and endemic species (10% are endemic karst species, and 20%–30% are characteristic karst species) ^[73][129]. However, the change and degradation in the ecological environment in karst desertification have led to the fragile karst ecosystem becoming more unstable and biodiversity declining ^[74][130]. The degradation of plant communities led to reduced biomass and soil organic matter, which affected microbial diversity. The evolution of habitat toward drought accelerated the decomposition rate of soil organic matter ^[75][131]. As a result, the content of soil organic matter and water permeability decreased, and finally, a fragile ecosystem with poor ecological structure and functions was formed. The assessment and protection of biodiversity loss should be one of the core tasks in this area. Unfortunately, few researchers have carried out assessments and proposed feasible protection plans so far. Under the threat of climate change, Hylander et al. ^[76][132] proposed two forest biodiversity conservation tools (Resistance and Transformation) at the landscape scale, including eight specific implementation measures. In addition, Lindenmayer ^[77][133] also proposed four general principles from the perspective of natural forest restoration.

(4)

The combination of macro-scale landscape structure optimization and micro-scale biodiversity improvement can effectively increase the supply of ES

As a result of ecological degradation, the landscape in karst desertification areas shows high heterogeneity and fragmentation ^[78][134]. Meanwhile, the loss of plant and microbial diversity has also caused great damage to the EMF at the micro scale ^[79][80][81,85]. However, the relationships of services have obvious spatial scale dependence ^[81][135]. Research on a single scale may miss or even distort the interaction rules between ESs, which is not conducive to a comprehensive and objective understanding of ESs ^[82][136]. The fragmentation of landscape in karst desertification areas not only leads to a loss of biodiversity but also reduces the sustainability of land use ^[83][137]. Research showed that after implementing a series of afforestation and forest cultivation measures, the landscape diversity increased by 8%, and fragmentation decreased by 25% ^[84][138]. Therefore, it is necessary to adjust the type, number, and spatial distribution pattern of landscape components and patches at the macro scale so as to make each component harmonious and orderly, ultimately restoring the damaged ecosystem and achieving regional sustainable development ^[85][139]. For example, according to the ecological vulnerability characteristics of karst desertification areas, steep slopes and gentle slopes can be planned as forests/grasslands and farmland, which can improve soil and water conservation capacity and make full use of soil nutrients ^[86][87][140,141]. The configuration of patches (such as forest land, grassland, and water) around farmland can help to increase landscape diversity and biodiversity, improving the EMF ^[88][142]. The coupling of water and fertilizer in poor soil regions can help to increase the content of soil organic matter and the number of microorganisms, improving plant productivity ^[89][143]. Therefore, at the micro scale, plant and microbial diversity can be increased by artificial afforestation ^[90][144] and organic matter addition.

3. Key Scientific Issues to Be Solved and Prospects

(1)

How do ecosystem functions respond to structural changes? Research on interspecific relationships and functional differences in ecosystems with different structures can be carried out.

Understanding the response of ES and functions to the change in ecosystem structure is crucial for the efficient allocation of environmental resources and rational formulation of environmental policies ^[91][145]. An unreasonable landscape structure will lead to an overall decline in ESs and functions ^[92][146]. In the stand structure, interspecific interactions not only directly affect the flow and circulation of matter and energy among different components of the ecosystem but also affect the process of community construction, making the network structure closely related to ecosystem functions and community stability ^[93][147]. Mixed forests and multi-storied forest have stronger disease and pest resistance than monoculture forests and single-storied forests; natural forest have a better stand structure and biodiversity than artificial forests, as well as stronger overall ecological function ^[94][95][148,149]. Although more and more evidence show that landscape structure and stand structure are crucial to the supply of services, there are still some important questions to answer about the mechanism and process behind this role, including the key question about how to configure them to improve the ecosystem function. There are detailed results on the impact of a certain ecosystem structure on services, but few researchers have focused on the interspecific relationships and ecosystem functions driven by different ecosystem structures. In the future, research on biodiversity and ecosystem function differences within different structures should be strengthened, and the role of structures in ecological processes, functions, and services should be revealed.

(2)

Species diversity or functional diversity contributing more to ESs; comparative studies on species and functional traits of different communities are needed.

Functional traits determine ecosystem functions, and species are considered a collection of functional traits ^[96][150]. Species provide many material products for human beings, and functional traits can affect regulating services such as the water cycle. They are indispensable carriers of ES. However, it is unclear who contributes more to ES supply between species and functional diversity. At present, research on the driving mechanism of biodiversity to ES is mostly focused on a single scale or dimension, and different conclusions will be drawn in different ecosystems ^[97][98][151,152]. Thus, the impact of species diversity and functional diversity on ES change needs to be further studied. In addition, aboveground and underground biodiversity, as well as their comprehensive impact in different scales or dimensions, can effectively explain more variations in EMF ^[99][100][153,154]. Researchers should pay more attention to the synergistic effect of above- and below-ground biodiversity in the future, extending the field observation period, enriching the community survey content, and selecting representative functional indicators to construct a long-term, multi-spatial, and multi-dimensional biodiversity-EMF database ^[101][155].

(3)

The application of relationships between biodiversity and ecosystem service is insufficient. The practical application of existing research results should be strengthened.

Although some studies have paid attention to the interaction between biodiversity and ES, most focused on the impact of biodiversity on ESs. Few studies have talked about how to apply the relationship between biodiversity and ES at the practical level, and there is a lack of effective ways to realize relevant cognition. An important research direction is thus to explore ways to improve ESs according to the relevant knowledge of biodiversity and ESs, as well as diminishing the leap from theory to the application of the biodiversity ES relationship. In the face of the continuous impact of human interference and environmental change on ES, maintaining and improving ESs of oceans, forests, grasslands, and agriculture has become a practical problem that many regions must address ^[102][156]. Theoretically, it is possible to formulate management measures to improve and restore ES from the perspective of biodiversity, and the implementation of ecosystem management measures can, in turn, improve biodiversity and ESs. For example, forest restoration can increase species diversity and ecosystem productivity at the same time ^[103][157]. In practice, some studies have explored and verified the feasibility of applying the knowledge of biodiversity-ES relationships to policy-making and natural reserve management, forest ecosystem management, degraded ecosystem restoration, and agricultural ecosystem improvement, but the application of existing research results still needs to be strengthened in future studies.

(4)

Few pieces of research integrate multiple driving factors of ES change; the research on the co-influence of natural and human factors should be strengthened.

In addition to climate change and human activities, ES changes are also affected by various drivers, such as ecosystem structure, biodiversity, and landscape. There are complex interactions between these driving factors ^[104][158]. Most existing research mainly focuses on the role of a single driver, while research on the synergy of multiple factors and their contribution rate is scarce. In the future, researchers should not only continue to deeply explore the mechanism of impact of climate change and land use on services but also strengthen the driving force of population, economy, policy, culture, and other social factors, as well as natural factors, such as ecosystem structure, biodiversity, landscape pattern, regional differentiation, and the interaction of multiple factors on service change. Meanwhile, it is necessary to reveal the contribution rate of different driving factors on the service change to manage the environment in the development, utilization, and protection of ecological resources. This will provide scientific guidance for ecological restoration in ecologically vulnerable areas and promote the realization of sustainable development goals.

(5)

There is no case study on improving ES through landscape pattern optimization. Long time-series sample plot monitoring should be carried out to explore the optimal landscape pattern.

The landscape composition and configuration affect the ecological process. Understanding how landscape composition and configuration affect the supply of ES is the key to improving landscape management ^[105][67]. Most researchers have focused on the response of ESs to landscape structure changes, there are few reports on how to improve ES via landscape composition and configuration. Thus, effectively configuring the landscape to promote ESs and function is a difficult problem for landscape ecology, especially in areas with high spatial heterogeneity and changing land cover. Core area and grid size are important determinants of ecosystem service trade-offs and synergies, which affect ES interactions ^[106][159]. Future research needs to include a long-term dynamic observation of the field landscape configuration to determine an optimal landscape composition and configuration scheme and provide scientific basis for improving regional ecosystem functions and services.