## 1 Introduction  
The expansion of agricultural land use and urbanization processes are the primary drivers of deforestation [1; 2; 3]. Facing great pressure in food security, as well as the challenge of land scarcity caused by accelerated urbanization in China, more available lands were converted to farmlands, and agricultural intensification was developed to satisfy the increases in demand for food provision [4]. Nevertheless, the pursuit of maximizing agricultural production was realized at the indirect expense of the regulating ecosystem services (e.g., soil conservation, flood mitigation, wind, and fixing sands) [5; 6; 7], leading to ecological degradation and catastrophe, such as severe flooding along the Yangtze River in 1998. In around 2000, China's government implemented several national-scale ecological programs through vegetation restoration, aiming to maintain environmental sustainability and improve livelihoods. The Grain for Green Program (GGP) was known as the largest ecological restoration project in China due to its ambitious goals, great spatial range, huge government investment, and broad participation degree [8; 9]. The GGP aims to alleviate soil erosion through converting slope or sand farmlands into forests or grasslands [10]. According to the report from the National Forestry and Grassland Administration, 9.27 million ha of farmlands had been converted to forests in the first round of the GGP (1999 to 2014). Approximate 32 million households participated in the GGP, and 405.7 billion Yuan (US$66.4 billion) was cumulatively invested as subsidies for their loss of benefit from agricultural activity, as well as their efforts in the GGP [11]. As expected, the implementation of GGP has improved the regulating services, such as a reduction in soil erosion [10; 12; 13], and improvements in carbon storages and carbon sequestration potentials [14; 15]. Although great achievements have been accomplished by the GGP in the first round, some problems have been reported. Given the GGP primarily aims to improve regulating services, the aforestation types under the GGP was restricted by a fixed area proportion of afforestation types at a county level: more than 80% of ecological forests (primarily aims to improve environmental conditions) area, while less than 20% of economic forests (primarily aims to provide non-timber forest products, e.g., fruit and nuts). Despite the fact that ecological forests (e.g., _Larix_ spp.) can produce timber as well, the long rotation period (>40 years) suppresses economic benefit from selling timber, depressing farmers' enthusiasm of participation in the GGP, in particular when the GGP's subsidies end. This problem has been recognized by policymakers. To achieve the dual goals of improvements in the environment and in farms' livelihoods, China's government announced not to limit the proportion of afforestation area of economic forest in the second round of the GGP since 2015. Driven by the sustainable benefit from an economic forest, farmers are supposed to plant economic trees preferentially. It should be noted that an economic forest is usually managed with a lower tree density, a lower vegetation coverage, a lower tree height, and a sparse undergrowth, which are entirely different from those characteristics of an ecological forest. The stand structure of an economic forest aiming to enhance non-timber forest products, as well as to facilitate management, may lead to a decline in their regulating services. An assessment of the change in ecosystem services under the second round of the GGP can reveal the benefits and deficiencies of the new policy, and can inform optimum strategies and policies of the GGP. The GGP is implemented through remarkable land cover change, which is a typical and practical topic for ecosystem service management across landscapes, thus its influences on multiple ecosystem services have interested ecological scholars. Being limited by biophysical, economic, and social factors, however, it is difficult for multiple needs to be met simultaneously [16].