The concept of landscape function derives from ecosystem function, which refers to the ability of products and servicesclosely related to human well-being to be provided by the landscape (Tang and Ding, 2014). Compared with ecosystem services, landscape services can better reflect the interrelationship between landscape patterns and processes, help integrate multiple disciplines and research methods, and help local stakeholders directly participate in landscape management (Liu et al., 2016). Landscape multi-functionality is a derivative concept and inherent characteristic of the multifunctional landscape, and it reflects the inherent attributes of the multifunctional landscape. Meanwhile, the multifunctional landscape is the external appearance of landscape multi-functionality (Peng et al., 2015). Landscape function is the basic element of the multifunctional landscape, and the multifunctional landscape reflects the integrated effect of all landscape functions or services. Ecosystem services are the research focus of landscape functions. Because of their homology with landscape multi-functionality, the diversity of ecosystem services becomes the material basis of landscape multi-functionality (Lü et al., 2013; Peng et al., 2015; Liu et al., 2016). Landscape multi-functionality builds a bridge between nature and society for ecosystem and landscape management, as well as a suitable platform for spatially integrating or analyzing the impact of multiple environmental stress factors on the landscape, which can facilitate the analysis of trade-offs, synergy and conflicts between landscape functions, and promote the effective dissemination of research results to decision-makers and the public (Fig. 2) At present, the evaluation of landscape multi-functionality and the classification of regional types are hot fields that domestic researchers focus on, and the number of relevant research cases continue to grow (Peng et al., 2015). Evaluation methods for the comprehensive functions of landscape include the use of unified measurement units such as currency, the use of an indicator system for separate accounting, and the use of models (de Groot et al., 2010). However, current research has failed to highlight the differences in the importance of different landscape functions in specific areas, and lacks a unified framework for landscape multifunctionality evaluation. The in-depth study of landscape multi-functionality evaluation models and methods has become an important issue facing the current quantitative research on the multifunctional landscape.

There are numerous methods for the quantitative mapping of landscape functions (Tang and Ding, 2014; Peng et al., 2015). The quantitative identification of hot areas of a multifunctional landscape based on GIS spatial overlay analysis has become a basic research paradigm for multifunctional landscape spatial identification, but the corresponding multifunctional conflict area is less involved in spatial identification (Liu et al., 2016). It remains difficult to integrate it into the planning and management of the multifunctional landscape (Lü et al., 2013; Peng et al., 2015; Liu et al., 2016). Strengthening the understanding of the concept and classification system of landscape functions, the analysis of landscape multifunctional trade-off mechanisms and scenario simulation are the important directions for future research development (Peng et al., 2015).

As an important foundation of landscape multi-functionality, ecosystem service assessment and its policy application have become critical approaches and hotspots for coping with global sustainability challenges. The understanding and scientific application of landscape multi-functionality depends to a large extent on the deepening of research on ecosystem services (Burkhard et al., 2012). Research on the trade-offs of landscape functions mainly relies on the trade- offs of ecosystem services (Qian et al., 2018). Rodríguez et al. (2006) classified ecosystem service trade-offs into four basic categories: spatial trade-offs, temporal trade-offs, reversible trade-offs, and trade-offs among ecosystem services. Since ecosystems and landscapes are hierarchically related, the trade-offs of ecosystem services and the trade-offs of landscape functions have the same homology (Lü et al., 2013). Existing research on ecosystem services mostly focuses on the value estimation of ecosystem services (Kareiva et al., 2011; Xie et al., 2015; Cao et al., 2016), ecosystem function trade-offs (Cao et al., 2016; Fu and Yu, 2016), human well-being relations (Vemuri and Costanza, 2006; Engelbrecht, 2009), and policy design for ecological compensation (Zhang and Liu, 2006; Felipe-Lucia et al., 2015). However, the transformation of ecosystem service functions from awareness to management practice is confronted with severe challenges such as quantitative measurement, trade-offs of multiple services or functions, multi-scale correlation of service functions, and the combination of ecosystem service functions and policy design (Zheng et al., 2013; Fu, 2018).

Research on the trade-offs of landscape functions has attracted enough attention that it has become a hotspot of current research. However, due to the complex trade-offs and synergy between the landscape functions of different ecosystems, the mechanisms of the connections and interactions between the landscape functions are still unclear, the formation process of the trade-offs and synergy is also not clear, and the scale effects of landscape multi-functionality and synergy are not emphasized, so basically the study of the trade-off of landscape multifunction still needs to be strengthened (Fu and Yu, 2016).

At present, there are still many qualitative studies being conducted on ecosystem service trade-off and synergy analysis, and root mean square error and the production possibility boundary are new indicators for quantifying trade-off (Wong et al., 2015). In the future, it will be necessary to strengthen the studies on the non-linear characteristics of interactions between landscape functions, to attach importance to the study of the temporal and spatial heterogeneity of trade-off and synergy, to reveal the multi-scale correlation and dynamic changes of trade-offs and synergy between regions caused by different policies and incentives, and to improve the research methods of trade-off and synergy analysis of landscape multi-functionality (MA, 2005).

Regional ecological risk assessment is an important research field for current ecological environmental management (Yin et al., 2009). In terms of research direction, it mainly focuses on pollution, natural disasters and the ecological risks of development activities (Fu and Xu, 2001). In terms of evaluation methods, the relative risk model (RRM) based on factor weights (Zhao et al., 2013), the hierarchical dynamic framework method (Burkhard et al., 2012) and the procedure for ecological tiered assessment of risks (PETA) (Moraes and Molander, 2004) are the most widely used. Such research in China mostly focuses on static ecological risks of natural disasters (Xu et al., 2011) or landscape ecological risks represented by landscape pattern indexes (Kareiva et al., 2011; Burkhard et al., 2012). Insufficient attention has been paid to changes in ecosystem structure, pattern, and function, which makes it difficult to quantify the response of ecological risks to resource development (Sun et al., 2009). It remains a great challenge to effectively connect the ecological process of landscape and improve accuracy and application (Peng et al., 2015), so it is necessary to change the research perspective to reveal the mechanisms of ecological risk formation.

One important direction of current research is the introduction of ecosystem services into ecological risk assessment (Galic et al., 2012). Compared with the regional ecological risk assessment that focuses on the comprehensive overlap of multiple risks, landscape ecological risk assessment emphasizes the impact of landscape patterns on ecological processes or functions, but the lack of research on dynamics, scales and uncertainties restricts the accuracy of landscape ecological risk assessment results. The multi- objective oriented landscape ecological risk not only includes the direct loss to human society, but also pays more attention to the measurement of ecological losses among different levels of systems and the evaluation of the potential impact of these ecological losses on human well-being (Peng et al., 2015). Taking ecosystem services as the terminus of ecological risk assessment can be closely linked to the various structures and processes of the ecosystem, laying a good foundation for subsequent assessment, communication and management (MA, 2005; Burkhard et al., 2012).

The current ecological risk assessment strategies based on ecosystem services can be divided into two categories. One is the risk assessment for a specific ecosystem service function (Faber and van Wensem, 2012); while the other is the comprehensive risk assessment for a variety of ecosystem services or functions (Raudsepp-Hearne et al., 2010; Kang et al., 2016). Most of these studies are retrospective and static evaluations, and the evaluation framework is still in the discussion stage.

Human well-being is the fundamental starting point for research on landscape ecosystem services. Realizing the coordinated development of ecosystem services and human well-being is one of the primary purposes of ecosystem service management. A full understanding of the relationship between landscape multi-functionality and human well-being is an essential foundation for promoting a win-win situation between economic and social development and ecological protection. Efficient landscape multi-functionality can effectively improve regional human well-being (Liu and Fu, 2019). Ecosystem services, the material basis for landscape multi-functionality, are the various benefits that humans obtain from ecosystems (Liu et al., 2018b). With the continuing growth of human material and spiritual needs, the direct and indirect effects of excessive consumption and intervention on landscape functions have been significantly increased. The degradation of landscape multi-functionality endangers the well-being of contemporary human society and also affects the benefits from the ecosystem for subsequent generations (Zhao et al., 2018).

Researchers have conducted various explorations of the relationships between ecosystem services and human well-being, especially on the relationship between ecosystem services supply and consumption and the relationship between ecosystem services and human subjective well- being (Vemuri and Costanza, 2006; Xie et al., 2008; Engelbrecht, 2009). However, there is a lack of systematic research on the mechanism of the impact of landscape multi-functionality changes on human well-being at different scales. Although researchers have fully elucidated the dependence of human well-being on ecosystem services at the global and national scales (Goldstein et al., 2012), our understanding of the policy basis for poverty eradication, improvement of human well-being and ecosystem conservation on a regional scale remains deficient, so regional scale research needs to be strengthened (Peng et al., 2015). At present, there is a lack of systematic research methods for assessing the relationship between regional and large-scale human well-being, as well as a classification framework of ecosystem services connecting multiple levels of human well- being (Kremen, 2005).

The production, utilization and loss of ecosystem services are closely associated with human society and human well-being. The supply and demand for ecosystem services are highly heterogeneous in time and space (Hein et al., 2006). Natural factors determine the basis for the spatial distribution of ecosystem services, while human factors indirectly lead to the spatial differences in the supply and demand for ecosystem services (Liu et al., 2018a; Zhao et al., 2018). Most of the existing research describes the influence of an ecosystem service on human well-being by studying the increase or decrease of that ecosystem service (Xie et al., 2008; Wang and Lu, 2009), but it is impossible to determine the beneficiaries and spatial locations of ecosystem services (Wang et al., 2013). Determining how to balance the utilization of landscape multi-functionality, promote sustainable development between landscape multi-functionality and human well-being, and reduce ecological risks are important directions of multidisciplinary comprehensive research (Carreño et al., 2012).