## 1 Introduction  
Nature supplies food and recreation that human beings rely on. Unfortunately, ecosystems worldwide are threatened by anthropological activities and climate change [1]. Under such pressures, maintaining a healthy ecosystem is essential for supplying stable and sustainable goods and services for human societies [2]. Assessing and monitoring ecosystem health can not only provide early warning of environmental degradation, but also identify the cause of an existing problem [3]. It thus is an important and initial step for ecological conservation and ecological service assessment. Ecosystem health assessment (EHA) as a part of environmental management that began in the late 1980s. Ecosystem health merged the concept of ecosystem with health science [4] and integrated social and physical science [5]. The early definition of ecosystem health was simply animal health or plant health [6]. However, the definition of ecosystem health should consider the ecosystem as a complex system to emphasize the connection between community process and physical environment [7]. Early ecosystem health research evaluated ecosystem health using keystone species [8]. However, keystone species evaluation cannot present the energy flux, nutrient cycle, productivity, diversity or response capacity to disturbance, although it may indirectly present the interaction among keystone species, other species, or the physical environment in the ecosystem. In 1999, [PERSON] and [PERSON] defined ecosystem health as \"a comprehensive, multi-scale, dynamic, hierarchical measure of system resilience, organization, and vigor\" [9]. Based on this definition, health condition of one specific ecosystem can be assessed by measuring the integrated ecosystem attributes: vigor, organization, and resilience [9], although it is impossible to set up a number of specific health indictors for all ecosystems to assess health status [10]. Traditionally EHA was conducted based on the field ecological data and (or) models driven by such field data, which cannot be widely applied at a large spatial scale [11] and has difficulty in providing spatially and temporally explicit assessment [12]. Nevertheless, there is an urgent need to understand and monitor the spatial heterogeneity of ecosystem health [13] for the purpose of optimum conversation [14]. Remote sensing data have potential for assessing and monitoring ecosystem health at different temporal and spatial scales across extensive areas with a broad extent [12,15]. They can be used for directly detailing ecological health indictors, such as productivity, species richness, and resilience after natural and human-induced disturbances [12] and for indirectly providing inputs for spatially explicit ecological process modeling [16]. To date, the application of remote sensing on EHA or monitoring has been focused on single ecosystem attribute, such as vigor (productivity [17, 18, 19], or species invasion [20,21]) or resilience (response to stress (fire [22], or climate change [23,24])). These studies are beneficial to relevant ecological studies using remote sensing in terms of their methods and conclusions. However, it is impossible to understand a complex ecosystem through only one ecosystem attribute [9]. A comprehensive and dynamic ecosystem health assessment with the integration of ecosystem vigor,organization, and resilience is urgently needed. Establishing such a spatially explicit EHA and monitoring system needs the close collaboration of both remote sensing specialists and ecologists [25]. Reviews have surveyed remote sensing of land cover, biodiversity, and carbon flux related, water flux related, and soil-based ecosystem services [26,27]. There are also summaries on the role of remote sensing in ecological applications [12,28]. However, to our knowledge, none of the reviews has focused on the opportunities and challenges of developing a comprehensive remote sensing-based spatially explicit EHA and monitoring system.