**1. Introduction**

Constituting almost 40% of the terrestrial biosphere, grasslands provide the habitat for a great number of diverse animals and plants and contribute to the livelihoods of more than 1 billion people worldwide [1]. Under the growing impact of global climate change and unreasonable human activities, grasslands are facing major problems that threaten the sustainable development of grassland ecosystems, such as a sharp decline in biodiversity, pasture degradation and reduced supply capacity [2–4]. Therefore, how to promote the healthy development of grassland ecosystems and enhance their service capacity has become an urgent issue [5–7]. Grassland ecosystem services are influenced by multiple factors such as their structure and stability. Furthermore, trade-offs and synergies between ecosystem services often depend on their structural and functional interactions [8]. A reasonable ecosystem structure can improve ecosystem productivity; promote material cycling, energy flow and information transfer; and increase the provision capacity of

**Citation:** He, S.; Xiong, K.; Song, S.; Chi, Y.; Fang, J.; He, C. Research Progress of Grassland Ecosystem Structure and Stability and Inspiration for Improving Its Service Capacity in the Karst Desertification Control. *Plants* **2023**, *12*, 770. https://doi.org/10.3390/ plants12040770

Academic Editors: Bingcheng Xu and Zhongming Wen

Received: 11 January 2023 Revised: 4 February 2023 Accepted: 5 February 2023 Published: 8 February 2023

**Copyright:** © 2023 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/).

ecosystem service [9]. This plays a very important role in the healthy development of ecosystems and in human well-being. Rational allocation of grassland structure is one of the main measures to improve the stability and resilience of grassland ecosystems [10]. However, while the global concept of sustainability continues to spread, irrational structural configurations of grasslands (unreasonable cropping patterns and unscientific pasture management, such as planting density, species or grazing methods, etc.) continue to exist, which not only undermines the gains of ecological restoration and conservation, but also exacerbates the conflict between ecological conservation and economic development [11]. In particular, large-scale cultivation-based rangelands generally have a single planting structure, low biodiversity, high vulnerability and low stability, making it difficult to create a cascade of ecosystem service benefits [12]. As a result, grassland ecosystem structure and structural optimization are gradually becoming a research priority, with a focus on component structure, spatial and temporal structure, nutrient structure and their driving factors [13,14]. Optimizing the structure of grassland ecosystems is an important measure to improve and maintain the service capacity of grassland ecosystems [15,16], to balance grassland ecology and farmers' livelihoods, and to resolve the contradictory issues of grassland ecology and sustainable economic development [17].

Structural changes drive changes in stability in grassland ecosystems [18], which, in turn, change their ecological functions and the provisioning capacity of an ecosystem [19]. At present, there are many studies about revealing the mechanisms of species diversity on grassland stability through controlled experiments in grassland [20–23]. The application of basic principles and methods of biochemistry to quantitatively study nutrient limitation and nutrient balance in forage, as well as the regulation mechanism of water–fertilizer coupling on forage quality, productivity and stability in grassland, which is the focus of the current research [24–26]. At the same time, studies on assessing the stability of grassland ecosystems are gradually emerging [27], such as those using remote sensing data; indicators characterizing ecosystem vitality, resilience and organization; and landscape pattern indices to evaluate ecosystem stability [28,29]. However, there is a lack of research regarding the mechanisms by which the complementarity and diversity of functional traits regulate the stability of grasslands, which results in a structure–process–function– service cascade [30]. What is more, there are different methods for quantifying ecosystem stability indicators, and the evaluation models are uneven; therefore, whether they are universally applicable remains to be verified [31,32]. Biodiversity and species diversity have an important influence on the productivity, stability and nutrient cycling of grasslands and their resistance and resilience to disturbance. Resilience, resistance and restoration are the main elements in determining whether an ecosystem is stable [33]. The study of the role of biodiversity, species diversity and functional traits on stability has been the focus of ecosystem research [34–37], and they are influenced by multiple factors on multiple scales [38]. It has been shown that extreme weather and irrational human activities are the main drivers of structural changes in grassland ecosystems [39], which indirectly alter ecosystem stability, increase the overall vulnerability of grassland ecosystems and reduce their capacity to provide ecosystem services [40]. Therefore, a deep understanding of the relationship between the structure and stability of grassland ecosystems is a major part of maintaining the stability of grassland ecosystems and enhancing their service capacity [41–43].

Due to their fragile attributes, ecologically fragile areas are prone to high ecosystem sensitivity and structural vulnerability under climate change and irrational human activities, which, in turn, reduces the stability of their ecosystems and changes their ability to supply services [44]. Therefore, the mutual interaction between ecosystem structure and stability should be deeply understood to enhance their resistance to human disturbances and environmental changes [45,46]. However, in ecologically fragile areas, irrational land use reduces the diversity of grassland species and productivity stability and affects the sustainable development of the region [47]. Especially in the environmentally fragile areas of karst, unreasonable human activities have led to the degradation of vegetation, increased

soil erosion, gradual exposure of rocks and degradation of land productivity, with the surface showing a visual evolution similar to that of a desert landscape [48,49]. For this reason, the first task in the comprehensive control of rocky desertification is to restore and re-establish vegetation. In order to solve the ecological problem of karst desertification, the Chinese government has carried out a lot of work on the issue of karst desertification in southwest China since 1989, such as grain for green and closing the land for reforestation (grass), etc. The area of rocky desertification generally exhibits a trend of "continuous net reduction" [50], which provides a Chinese solution to global greening [51–54]. However, the KDC ecosystem still suffers from its simple structure, incomplete system function, high ecosystem sensitivity, lagging ecosystem service capacity and difficulty in maintaining the results of rocky-desertification management [55]. The rugged and fragmented surface in the karst areas, coupled with the constraints of rocky desertification, has affected the biodiversity of grassland ecosystems, resulting in a homogeneous structure and low productivity for the grassland [56]. Therefore, how to maintain ecological stability and optimize the structure of the system in the grassland ecosystems of KDC has become a key issue to consolidate the achievements of rocky-desertification management, ensure the smooth flow of service supply and demand, and enhance the well-being of local people [57,58]. Therefore, the structure and stability of grassland ecosystems and their interactions are not only a global concern [59–61], but also an important element that needs attention in KDC [62,63]. In the natural vegetation succession, grasses are the pioneer plants for vegetation restoration and ecological improvement [64]. Meanwhile, artificial grass breeding can enrich grassland species diversity, improve grassland ecosystem stability, provide high-quality forage, reduce the risk of surface erosion, improve soil nutrient composition and provide multiple ecosystem services for humans [51,65,66]. Optimizing the spatial configuration of systems is an important means of improving the stability and resilience of grassland ecosystems [67]. Therefore, optimizing the structure of grassland ecosystems for KDC and enhancing their stability are of great significance in enhancing the service capacity of grassland ecosystems and promoting the sustainable development of the regional ecological environment.

Thus far, research on the structure and stability of grassland ecosystems is increasing and breakthroughs have been achieved. The structure and stability of grassland is an essential element to maintain the sustainable and healthy operation of grassland in KDC, which is an important part of its ecosystem. However, there is a lack of relevant studies on the structure and stability of grassland ecosystems in KDC. In view of this, based on the perspective of grassland-ecosystem pattern change and its relationship with ecosystem processes, and the systematic review method, this study systematically reviewed the main research progress and landmark results on the structure and stability of global grassland ecosystems, and summarized the key scientific issues on structure optimization, stability enhancement and the interaction between structure and stability, aiming to provide some insights into grassland-ecosystem structure optimization and stability enhancement in KDC. In this way, it can enhance the supply of grassland ecosystem services and promote the sustainable development of the regional ecological environment and social economy in KDC.
