## 4 Discussion
### Policy Implications for Economic Forest of the GGP  
Forest assets are often geographically associated with household poverty [5; 51], especially in the mountainous areas of China. To enhance households' economic benefit, the new policy of the GGP that did not restrict the proportion of economic forest has been implemented since 2015. Driven by the new policy, a large number of economic trees would be planted. As results of the different management goals, economic forests are different from ecological forests in terms of tree density, vegetation coverage, and undergrowth (Figure 6). Our household surveys showed a rather high proportion (77%) of the farmers' preference towards the afforestation of economic tree species, and this result was supported by the statistical data (83%) from the local forestry administration. However, we found that the unrestricted afforestation proportion did not lead to the most desirable level of ecosystem services. The result suggested that the high proportion of economic forest significantly decreased the concerned regulation services, although a high provisioning service was obtained. It should be noted that the tradeoffs were spatially variable. The findings indicated a decline in soil retention and water purification occurred in response to the gradually enhanced proportion of economic forest. The starker tradeoffs between regulating services and provisioning service was observed when planting economic trees in ecological prioritizing areas that can provide high regulating service levels but a low provisioning service. These gainful activities were carried out by the farmers who consider that the economic benefit from an economic forest compared to ecological forest, but neglect to account for the limitations, such as site conditions, management techniques, and market, which probably suppress the expected economic benefit from planting economic trees. Figure 6: Adjacent conversions farmlands to economic forest (**a**) and ecological forest (**b**) under the Grain-for-Green Program in this study area (location: 41\({}^{\circ}\)52\({}^{\prime}\)59\({}^{\prime\prime}\) N, 124\({}^{\circ}\)55\({}^{\prime}\)47\({}^{\prime\prime}\) E). The tree species of economic forest: _Pinus koraiensis_; the tree species of ecological forest: _Larix kaempferi_. Previous studies have suggested that an excessive chase of the supply of several ecosystem services may lead to declines in many other ecosystem services [18; 52; 53; 54]. A key challenge of the GGP is how to convert farmlands to forests to achieve dual goals of improving livelihood and environment. Through the scenario analysis, we found a suitable scenario with proportions (50-60%) of economic forest (Figure 5d) whereby economic forest is mainly allocated in relatively flat terrain with good site conditions. Economic forests in these areas can produce a relatively high seed yield, at the expense of relatively moderate decreases in the regulating services. On the contrary, the conversion of farmlands to economic forests in poor site conditions (e.g., steep-sloped area) may produce a relative low seed yield, and may severely reduce regulating services. The findings showed that the trade-offs of ecosystem services between economic forest and ecological forest closely associated with afforestation types and their spatial pattern. A reasonable planning for the spatial allocation of afforestation types can lessen the tradeoffs between regulating services and provisioning service. Therefore, we suggested that the afforestation pattern of the GGP should be proposed though deliberate efforts, accounting for afforestation types and their spatial allocations. Our findings have important implications for reforestation programs beyond this case study in mountainous areas. The different forest types provide significantly variant ecosystem services. Analyzing the ecosystem service characteristics for different afforestation types (e.g., economic forest with a special tree species) can help to optimize reforestation strategies.