## Results
### Effect of Afforestation Types and Their Spatial Allocation on Ecosystem Services and Trade-Offs  
We modelled the scenarios for proportional combinations of economic forest and ecological forest under the GGP, with a total simulating afforestation area of 8704.4 ha (the steep-sloped farmlands: 6156.5 ha; the riparian farmlands: 2597.0 ha; the intercepted area of the two types: 49.1 ha). According the rank of site condition, farmlands with a higher site condition were preferentially converted to economic forests (Section 2.5.2), and the remaining farmlands were converted to ecological forests. Table 1 illustrates that the changes in regulating services and product provisioning service along a gradience of increase in afforestation area of economic forest. Not surprisingly, the total amount of the seed product increased with the proportion (from 10% to 100%) of economic forest, whereas all the regulating services showed a decreasing trend. The soil export and N export to stream or river increased from 2.77 t/ha year to 4.59 t/ha year and from 0.37 mg/L to 0.65 mg/L, respectively; flood mitigation and carbon storage decreased from 89.93 mm to 73.14 mm and from 207.32 t/ha to 60.84 t/ha, respectively. We further examined the changes in ecosystem services between the two scenarios, which represents realistic situations under the first round (20% of retired farmlands were converted to economic forests and the rest farmlands were converted to ecological forests) and the second round (77% of farmlands were converted to economic forests and the rest farmlands were converted to ecological forests) of the GGP, respectively. The seed production provision increased (272.2%) under the policy of the second round of the GGP, but at the expense of significant decreases in soil retention (\(-\)25.1%), flood mitigation (\(-\)11.4%), carbon storage (\(-\)48.5%), and water purification (\(-\)36.6%), showing stark trade-offs between the regulating services and provisioning service. Although the total amount of seed product increased with afforestation area of economic forest increasing, the seed product per unit hectare showed a decreasing trend (\(R^{2}\) = 0.81, \(P\) < 0.01) along a gradient of site condition, with a reduction from 325.7 kg ha\({}^{-1}\) to 300.2 kg ha\({}^{-1}\). Meanwhile, we compared the difference in regulating services between economic forest and ecological forest for the specific area that identified by the proportional interval according the rank of site condition (Figure 5). The result indicated that the differences in soil retention between economic forest and ecological forest enhanced with a decrease in site condition (Figure 5a; \(R^{2}\) = 0.73, \(P\) < 0.01). The similar trends were also observed in water purification (Figure 5b; \(R^{2}\) = 0.68, \(P\) < 0.01), whereas the differences in flood mitigation (Figure 5c; \(R^{2}\) = 0.26, \(P\) = 0.14) did not show a significant trend. Furthermore, we found a well-marked turning point (50%). The differences in soil retention and flood mitigation services between economic forest and ecological forest fluctuated with no significant trend before this turning point, whereas dramatic increases were observed after this turning point. On the other hand, the seed product per unit hectare also showed a decline after this turning point. This result indicated that the converting farmland to economic forest beyond an area proportion of 50% was less cost-effective at