Ecosystem services, as the term of a scientific period, began in 1970 with the publication of the UN University’s report on “Human Impact on the Global Environment” [1]. Since then, Costanza [2] has proposed that ecosystem services are benefits derived directly or indirectly by human beings from the ecosystem, which are used to maintain the natural environmental conditions and utilities on which human beings depend for survival and development. Trade-offs, as a fundamental concept, first appeared in economics and were defined as opportunity costs. Where resources are scarce, an individual or group must give up a certain amount of additional scarce resources to obtain more of the scarce resources. Karst desertification refers to the land degradation phenomenon caused by the disturbance and destruction of unreasonable human social and economic activities under the fragile karst environment in the subtropical zone, which is manifested by soil erosion, gradual rock exposure, land productivity degradation, and a desert-like landscape on the surface. The results of management over the years show that forests, as the main provider of ecosystem services, have the functions of water conservation, soil and water conservation, carbon sequestration, and climate change mitigation. In particular, they play an irreplaceable role in managing karst desertification.
1. Ecosystem Service Trade-Off/Synergy
1.1. Spatial and temporal characteristics of ecosystem service trade-off/synergy
The trade-off/synergy among ecosystem services is spatial and temporal scale-dependent and nonlinear, and the trade-off and synergistic relationships of ecosystem services may change at different spatial and temporal scales [
17]. Current studies on the time scale of the trade-offs and synergies of ecosystem services rarely use continuous time series and usually choose years with certain time intervals. For example, Niu et al. [
18] analyzed the trade-off/synergistic relationships of ecosystem services in the Songhua River basin in China during 2000–2018. They concluded that there was a synergistic relationship between water harvesting, soil conservation, biodiversity, and carbon sequestration, and a trade-off relationship between broadleaf forests, coniferous forests, and crops. Schroder et al. [
19] found a short-term trade-off and long-term synergistic relationship between national forest fire risk management (logging) and owl habitat protection and water quality regulation in the United States. Chen et al. [
20] found an increasing trend in net primary productivity, water production, and soil retention capacity from 2000 to 2018 during their study in a karst watershed. Still, the ecosystem synergy in the region was poor, and the trade-offs were particularly pronounced in karst stone desertification areas.
At the spatial scale, Zhang et al. analyzed the trade-off/synergistic effects of forest ecosystem services in the Fuyu Mountain region. They found that the synergistic relationship between services was better on the south slope than on the north slope, and that the best synergistic relationship between services was found in the middle mountain deciduous broadleaf forest belt on the south slope. The worst synergistic relationship was found in the low mountain deciduous broadleaf forest belt on the north slope [
21]. Han et al. studied the trade-off and synergistic relationships between ecosystem services and land use change in the karst region response, finding synergistic relationships between soil water yield and soil retention, soil water yield and carbon storage, and soil retention and carbon storage. In contrast, most of the relationships between other crops were trade-offs [
22]. In summary, the trade-off/synergistic relationships among services showed significant differences at different temporal and spatial scales due to the natural recovery status and the selectivity of human use of the services.
1.2. Drivers of ecosystem service trade-off/synergy
The drivers of changes in the ecosystem service trade-offs and synergies are mainly divided into anthropogenic and natural factors [
23]. As one of the typical fragile ecosystems in the world, the generation of trade-off/synergistic relationships between services in karst ecosystems mainly comes from the coupling effect of two significant factors: anthropogenic and natural. Scholars have studied the causes of the changes in the synergistic relationships of service trade-offs in karst ecosystems. For example, Han et al. suggested that climate is the leading natural factor for the transformation of service trade-off/synergistic relationships, which changes the temperature and precipitation intensity in karst regions through climate change, thus affecting the distribution pattern of plants, causing competition for species’ ecological niches, and indirectly changing the relationship between services [
24]. Chen et al. found that lithology is also one of the factors influencing the trade-off/synergistic relationships, and that karst areas dominated by dolomite and limestone, where limestone is more susceptible to dissolution by running water, have a poorer soil retention capacity. Therefore, the trade-off relationship between soil conservation and services such as water production is more significant in this region [
25]. In addition, some scholars also believe that factors such as soil erosion [
10], vegetation degradation [
9], and reduced biodiversity [
26] have a more significant impact on the trade-off/synergistic relationship between services. With the rapid development of society, the economy, and urbanization, many karst ecosystems such as agricultural land, forests, and wetlands have been occupied by human activities. This approach has not only changed the land use pattern and disrupted the material and energy balance of the karst soil–vegetation system [
27], but also induced the reverse evolution of the soil–vegetation system. In addition, the supply capacity of karst ecosystem services will also be reduced, directly affecting the relationship between the benefits of the karst ecosystem [
28]. In addition, to meet the needs of human survival and development, a large amount of scrub was reclaimed for cultivation, which contributed to the loss of organic carbon from the surface layer of karst limestone soils and the intensification of surface erosion, resulting in the weakening of soil carbon and nitrogen sequestration capacity, which affected the relationship between the services to some extent [
29]. Thus, it is clear that human activities are a key factor in the structural and functional changes of karst ecosystems.
1.3. Synergistic relationship of forest ecosystem service trade-offs
Forest ecosystems provide vital ecological services to the Earth’s ecological environment, and the conservation of biodiversity and the development of forest industries are necessary ecological safeguards [
12]. Karst forests are a special type of forest ecosystem developed on karst landscapes in the context of forest climate conditions [
30]. However, long-term anthropogenic disturbances have exacerbated the problem of stone desertification in karst regions, reducing the stability of existing forest structures and leaving them in a state of fragmentation or secondary degradation succession. This significantly reduces the capacity of karst forest ecosystems in terms of water containment, soil conservation, biodiversity, and ecological product supply [
31], creating trade-offs or synergistic relationships among services (
Figure 4).
Figure 4. Trade-offs/synergies between forest ecosystem services.
2. Optimization of Ecosystem Service Functions
2.1. Ecological compensation
Ecological compensation and payment are differentiated regulation strategies for the interaction between ecosystem services and human welfare, which should be adapted to the current ecological environment and social and economic system. As a mechanism to stimulate ecological construction and environmental protection, ecological compensation can realize the effect of internalizing the external ecological benefits [35]. Through the analysis of the spatial flow of supply, demand, and trade-offs, the key benefits and losses of ecosystem services are obtained, and the conditional payment transaction from service users to service providers is facilitated [36]. In general, a combination of remote sensing data and location-based observations is used to measure the profit and loss of supply, regulation, support, and cultural services. Complementary compensation strategies have been developed based on matching supply, demand, and trade-offs/synergies of ecosystem services. For example, Sun analyzed ecosystem service processes in crucial ecological function areas in Xinjiang. According to the total value of ecosystem services, all counties in the autonomous region can be divided into three compensation levels: priority compensation, secondary compensation, and potential compensation areas [37]. Thus, the efficiency of the ecological compensation mechanism can be improved. It was more difficult to compensate for the particular habitat of the karst desert region, and further funds were used. However, certain shortcomings in financial compensation can result in poor compensation outcomes. As a result, alternative ways to precisely refine and structure economic compensation methods exist such as improving the ecosystem structure stabilization and enhancing ecosystem service functions.
2.2. Enhance forest ecosystem service functions based on site conditions
A fragile ecological environment, fragmented surface morphology, and severe soil erosion and desertification characterize karst regions. The essence is the destruction of the ecosystem structure, which leads to a decline in and loss of ecosystem functions [
38]. Given the uneven spatial distribution of forest ecosystem services and the selectivity of human use, the ecosystem service cascade framework was developed to increase the stability of the forest ecosystem structure, improve the function of forest ecosystem services, and promote regional synergies (
Figure 5). According to the trade-off/synergy differentiation of the services, some scholars have proposed that plant communities can be adjusted and configured through functional groups to enhance the overall ecosystem service capacity [
39]. Chen [
40] offered optimal control strategies for the forest, irrigation, and grass communities in different rock desertification classes, considering the growth traits, adaptation strategies, ecological niches, and functional group types of tree species. Zhang [
12] constructed a model for the ecological restoration of forests of different stone desertification classes with species adaptation strategies and ecological service functions. Zhang [
41] proposed plant community optimization techniques by studying the plant species diversity, leaf functional traits of established species in plant communities, spatial patterns of plant communities, and interspecific correlations in the rocky desertification succession of different grades. Yu [
39] took the upper reaches of the Chishui River as the study area, divided 32 water resource conservation forest tree species into seven water resource conservation function groups, and proposed optimization and adjustment strategies for the different function groups on this basis. Therefore, the conservation strategies for forest ecosystems in karst areas should be more targeted and specific.
Figure 5. Forest ecosystem services.
3. Summary
Good social, economic, and ecological benefits have been achieved in participating forest ecosystems in managing karst rock desertification [
61]. However, the fragility of the karst rock desertification ecological environment [
62] makes the forest ecosystem in this area less able to resist external disturbances and less stable [
63]. Once unreasonable human activities occur and exacerbate the fragility of stone desertification habitats, they will change the forest ecosystem’s structural configuration and species composition [
64] and affect the service provisioning capacity and trade-off/synergistic relationships.
For the ecological structure–function, the karst stone desertification area has a shallow soil layer, discontinuous soil, high rock exposure rate, and rich soil calcium and magnesium content [
56]. The suitability for small habitats should be considered when selecting restoration species, and drought-tolerant, calcium-loving, rocky, fast-growing, widely applicable, ecologically and economically valuable trees, bushes, vines, and grasses should be selected for ecological restoration such as species of any bean, cedar, and lady’s mantle [
65]. Second, it should adjust the cultivation methods of natural forests and plantations, which are sound ecological construction systems and natural gene pools with strong regulation and restoration abilities. The conservation of natural forests is mainly based on “no access to protected forest areas” measures to preserve their species and genetic diversity and maintain the community structure and function [
66]. For natural secondary forests in poor health, people adopted the measure of “managing the forest closed and adjusting the structure of the trees”, and promoted better plant growth through artificial support measures such as replanting, nurturing, and inter-logging. For the planted plantations in natural forest areas, measures that support a “change of management approaches while protecting the forest in culturing” were adopted. Through the nurturing of mixed conifer and broadleaf forests, and the regular replanting of native species, it can induce succession in native forests and optimize the community structure and function [
31]. Regarding the trade-offs between services, the trade-offs between forest ecosystem services in karst stone desertification areas resulted from a combination of natural and human factors.
Due to the poor soil in this area, plants compete with other species spatially to occupy more ecological niches during growth, leading to trade-off relationships between some services such as water connotation and soil conservation, nutrient netting, etc. [
67]. Natural restoration or scientific fertilization can enhance soil quality in the stone desertification area and promote the balance of nutrient supply and demand. In addition, the appropriate stand density and planting depth need to be determined during planting to provide a wider space for plants to grow. The competition between plants for ecological niches should be reduced and the service capacity of water connotation and carbon sequestration and oxygen release should be improved while optimizing their trade-off relationship. The most important point is to reduce human interference in the forest ecosystem in stone desertification areas. The impact on the service function can be minimized by prohibiting deforestation, stepping on the protected forest area, reclaiming scrubland for cultivation, and planning the land for urban construction and highway construction in a reasonable way, in order to maintain the original state of the service function and reduce the formation of trade-off relationships.
This entry is adapted from the peer-reviewed paper 10.3390/f14010088
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