The study selected MODIS land use data and analyzed the spatial and temporal characteristics of forest and grassland after the implementation of the Grain to Green Program, as well as the impact of policy changes on the changes of forest and grassland. The following results are obtained. 1) After the program’s implementation, the overall trend of forest land and “cropland land broadly defined” including orchards in China showed a small upward trend, and the trend of grassland showed a downward trend. 2) Considering that part of the orchards and forest land have not destroyed the arable layer, which can be effectively restored to cropland, there is no tendency to lose the cropland, and the red line of 1.8 billion mu (1.2 million km²) of cropland is not currently under threat. 3) Returning cropland to forest mainly occurs in Yunnan Province, Sichuan Province, and Heilongjiang Province, accounting for 9.72%, 9.28%, and 7.68% respectively of the total area of cropland converted to forestland (109500 km²) nationwide. In contrast, the conversion of forestland to cropland mainly occurs in Sichuan, Hunan, and Guizhou Provinces, accounting for 18.47%, 9.59%, and 9.35% respectively of the area of forestland converted to cropland (104400 km²) nationwide. The simultaneous existence of a large number of both croplands converted to forest and forest converted to cropland in Sichuan and other provinces may be due to benign adjustments in geographic location and land quality, or it may be due to the regional heterogeneity of the policies of “food security” and “environmental protection”, which need to be further studied. 4) Half of the “forest land” after returning cropland to forests has been cultivated again after 18 years, some of it has been preserved as forest land (37%), and a tiny part of it has been degraded to grassland (3%) (taking the forest land cultivated in 2001 and returned to forests in 2002 as an example). 5) Subsidies for the Grain to Green Program can reduce the proportion of “forest land” converted from cropland for replanting crops, but increase the conversion of land from cropland to forest land where the natural conditions are unsuitable for afforestation. 

grain for green program, land-use change, spatial and temporal characteristics, subsidies, replanting; degradation, conservation

选取MODIS土地利用数据, 分析我国退耕还林工程实施后林草用地时空变化特征以及政策变动对林草用地变化的影响, 得到以下结果。1) 退耕还林工程实施后, 我国林地和“广义耕地” (即包含果园的耕地)面积总体上呈现小幅上升趋势, 草地呈下降趋势。2) 部分果园和林地未破坏耕作层, 可恢复为耕地或通过工程措施恢复为耕地, 如果将这些土地考虑在内, 则我国的耕地没有流失的趋势, 18亿亩耕地红线目前并未受到威胁。3) 退耕还林主要发生在云南、四川和黑龙江等省份, 分别占全国耕地转为林地面积(10.95 万 km²)的9.72%, 9.28%和7.68%; 林地转耕地主要发生在四川、湖南和贵州等省份, 分别占全国林地转为耕地面积(10.44 万 km²)的 18.47%, 9.59%和9.35%。四川等省份退耕还林和复耕同时大量存在, 可能是考虑地理位置和土壤质量等因素进行的良性调整, 也可能是因为粮食安全和环境保护政策区域的异质性而导致的无谓变化, 有待进一步研究。4) 以2001年为耕地, 2002年退耕还林的林地为例, 退耕还林后的林地18年后一半复耕(50%), 部分保存(37%), 极小部分退化(3%)。5) 退耕还林补贴能够降低退耕还林的林地复耕比例, 但增加了自然条件不适合造林的土地由耕地转为林地的比例。