## Materials and Methods  
### Subregions  
The GGP started with a pilot program in three provinces in 1999 and was formally launched in 2002. It expanded to a total of 1897 county units across 25 provinces (autonomous regions and municipalities) and the Xinjiang Production and Construction Corps--a wide area with huge ecological and economic heterogeneity, covering 76.77% of China--and involved tens of millions of rural households [16]. Following the principle of hazard prevention, the GGP implementation area was divided into 10 subregions by China's State Forestry Administration (SFA) according to the risk of soil erosion, wind erosion, and desertification; local hydrothermal conditions; and landform characteristics: the Alpines-Gorges Region in Southwestern China (AGRSC), the Mountains-Hills Region in Sichuan, Chongqing, Hubei and Hunan Province (MHRSCHH), the Low Mountains-Hills Region in Middle and Lower Yangtze Plain (LMIRRMLYP), the Yunnan-Guizhou Plateau Region (YGPR), the Mountains-Hills Region in Hainan and Guangxi Province (MHRHG), the Alpine Steppe and Meadow Region in the source of Yangtze River and Yellow River (ASMRYY), the Arid Desert Region in Xinjiang Province (ADRX), the Hills-Gullies Region in Loess Plateau (HGRLP), the Arid and Semi-arid Region in Northern China (ASRNC), and the Mountains-Sands Region in Northeastern China (MSRNC) (Figure 1). By taking different conservation measures in each subregion (Table 1), the government hoped to recover and enhance natural ecosystems more efficiently [27]. Figure 1: Distribution of subregions involved in the Grain for Green Program (GGP). For the full names and description of each region, see Table 1.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Classification** & **Subregions** & **Natural Characteristics** & **Restoration Measures** \\ \hline \hline \multirow{8}{*}{Mountains-Sands Region in Northern China (ASRNC)} & & Strive to develop artificial forest mainly composed of shelterbells and timber tree on the basis of protection of natural forests. \\  & Transits from continental monsoon zone in the east to temperate continental semi-arid zone in the west. Mountains, hills and plains are interspaced. Land use is a mix of agriculture, pasture and forest. Grassland degradation is severe in the west. & Strive to develop artificial forest mainly composed of shelterbells and timber tree on the basis of protection of natural forests. \\  & Transits from continental monsoon zone in the east to temperate continental & Afforestation for firewood and national defense as appropriate. Develop economic forest forest and medical herbs by 0.1 ha per capita on gently sloping land where the erosion risk is low. Emphasize the regulation of ecological forests, taking account also of economic, forage and firewood forests and high-quality pasture. \\ \hline \multirow{8}{*}{Arid and Semi-arid Region in Northern China (ASRNC)} & Transits from continental monsoon zone in the east to temperate continental semi-arid zone in the west, with annual average precipitation ranging from 550 mm to 150 mm from southeast to northwest. Relatively flat terrain consists of plateau and plain. & Carry out artificial forestation and grassland planting, mainly by shrub-grass, on returned farmland, develop and model herds by 0.1 ha per capita and gently sloping land where the erosion risk is low. Return farmland by 0.1 ha per capita on sloping and hummock land. Emphasize replanting of ecological forest, taking account also of forage and firewood forests. The proportion of forest and grassland for forage, forest for firewood must be kept under 30\% and 10\%, respectively. \\ \hline \multirow{8}{*}{Northern subregions} & & Return and revegetate all frontier farmland suffering severe desertification. Establish windbreak and sand-fixation forest systems with proper combination of trees, shrubs and herbs. Control sand movement and fix dunes by fencing, laying barriers and planting shrubber belt on the windbar slope of dunes within 100 m of their upwind direction.