## 1 Introduction  
Since the threshold of 2 \({}^{\circ}\)C warming was first proposed by [PERSON] [1], it has gradually been accepted as the important greenhouse gas reduction target. Human beings, especially the residents of small island states, are looking for a new long-term target, which is lower than 2 \({}^{\circ}\)C warming [2]. The Paris Agreement sets a new global goal that aims at holding the increase of the global average temperature to well below 2 \({}^{\circ}\)C above pre-industrial, and pursuing efforts to limit the temperature increase to 1.5 \({}^{\circ}\)C [3]. The Intergovernmental Panel on Climate Change (IPCC) special report on 1.5 \({}^{\circ}\)Cwarming indicated that limiting global warming to 1.5 \({}^{\circ}\)C rather than 2 \({}^{\circ}\)C would be expected to reduce the risks in terrestrial and wetland ecosystems [4; 5]. However, large uncertainties still remain about the responses of the terrestrial carbon cycle to 1.5 \({}^{\circ}\)C and 2 \({}^{\circ}\)C warming, especially 1.5 \({}^{\circ}\)C warming. Previous studies have usually focused on terrestrial carbon cycle change during specific periods, such as the middle or the end of the 21 st century [6; 7; 8; 9]. Although the enhanced global warming target has been proposed, the assessment of associated impacts on terrestrial ecosystems is very rare in China. Therefore, it is crucial to explore the potential risks of terrestrial ecosystems under the 1.5 \({}^{\circ}\)C and 2 \({}^{\circ}\)C warming scenarios for accommodating regional and national strategies for climate mitigation and adaptation. Previous modeling studies have shown that the net primary productivity (NPP) will increase in response to future global warming [9; 10]. However, the simulated net ecosystem productivity (NEP) in response to future warming is highly variable [11; 12; 13; 14]. Some studies indicated that the land biosphere would turn into a net CO\({}_{2}\) source in the 21 st century [6; 15; 16]. Recent studies using the Coupled Model Intercomparison Project Phase 5 (CMIP5) model projections suggested that the rate of net terrestrial carbon uptake would decrease during the 21 st century in most scenarios due to enhanced soil organic matter decomposition, water deficit, climatic extreme events, land use change, etc. [2; 17; 18]. In China, the mean air temperature has increased by 0.9-1.5\({}^{\circ}\)C during 1909-2011 as compared to the 1971-2000 period; all observations and model estimations showed a higher warming rate in China than the global average [19]. The CMIP5 model simulations showed that the mean air temperature over Asia would increase 2.3 \({}^{\circ}\)C in response to 1.5 \({}^{\circ}\)C global warming, and 3.0 \({}^{\circ}\)C in response to 2 \({}^{\circ}\)C global warming, respectively [20]. Most of the research focused on mean and extreme climate change [21; 22] and the impacts of future 2 \({}^{\circ}\)C warming on the hydrological cycle and water resource in China [23; 24; 22]. A few recent studies in China suggested the high ecological risks in the Yangtze River Basin and northern China [25]. Some studies indicated that grassland productivity would increase under the 1.5\({}^{\circ}\)C and 2\({}^{\circ}\)C warming scenarios due to the increased precipitation and atmospheric CO\({}_{2}\) concentration [26]. Changing temperature and precipitation had large temporal and spatial variations because of the vast area and varied terrain in China [21; 22], which resulted in diverse responses of terrestrial ecosystems to climate change [8; 27; 28]. Therefore, future variations in terrestrial ecosystems might be more uncertain under the 1.5 \({}^{\circ}\)C and 2 \({}^{\circ}\)C global warming scenarios [24]. It is imperative to conduct a comprehensive assessment of the spatial patterns of NPP and NEP responses in China under the threshold of 1.5 \({}^{\circ}\)C and 2 \({}^{\circ}\)C warming scenarios. CMIP5 provides the global scale simulations of multiple Earth system models (ESMs), which were widely used in large-scale climate impact analyses [29].