退耕还林还草工程对黄土高原植被总初级生产力的影响

采用Landsat解译的2000年和2015年土地利用/覆盖数据和VPM模型(Vegetation Photosynthesis Model)模拟的2000～2016年总初级生产力(Gross Primary Productivity, GPP)数据,识别出了近16 a黄土高原退耕还林还草的空间范围,并估算了GPP的年际变化趋势。在此基础上,对比分析了退耕区和未退耕区GPP年际变化的差异,从而揭示退耕还林还草工程对GPP年际变化的影响。结果显示,2000～2015年,黄土高原退耕还林还草面积约3.5万km2,占2000年耕地面积的16.8%。期间,GPP呈增加趋势,GPP显著上升区域占全区面积的67.3%,平均增速24.1 g/(m²?a)(以C计,下同)。虽然退耕区多年平均GPP低于未退耕区,但退耕区GPP年际增速和相对变化率明显高于未退耕区,分别提高了5.9 g/(m²?a)和1.5%。

The Effects of the “Grain for Green” Project on Gross Primary Productivity in the Loess Plateau

The vegetation coverage and carbon sequestration have been reported experiencing a significant increase since the implementation of the “Grain for Green” (GFG) project in the Loess Plateau (LP). However, the assessment of the effects of the project on ecosystem is not timely and accurate due to the limit of data availability. Using the newest land use dataset (2000 and 2015) interpreted by Landsat and the Gross Primary Productivity (GPP) data from 2000 to 2016 based on the Vegetation Photosynthesis Model (VPM), we examined the spatiotemporal pattern of land use changes due to the GTG project and estimates the inter-annual trend of GPP. A systematic comparison of the inter-annual variations of GPP was conducted among the GFG areas and the unconverted cropland areas (CRO), to reveal the effects of the GFG project. The results showed that an area of 3.5×10 ⁴km ² croplands was converted into grasslands and forests. The annual GPP showed a significant increasing trend from 2000 to 2016 for the whole plateau with averaged increasing rate of 24.1 g/(m ²?a), and the area with a significant increasing trend of annual GPP accounted for 67.29% of the whole plateau. Although the multi-year averaged annual GPP in the GFG areas was lower than that in the CRO due to the relatively young ages, the increasing rate of annual GPP in the GFG areas was 5.9 g/(m ²?a) higher than that in the CRO. The relative change ratio of annual GPP showed the same situation, the relative change ratio in GFG areas was 1.5% higher than that in the CRO. The GFG project accelerated the increasing of GPP in GFG areas, and then promoted the rising trend in the whole LP. This study provided an updated and more accurate analysis on the pattern of land use change and its effects on GPP dynamics in the Loess Plateau, which could contribute to the effectiveness assessment of the GFG project, future planning of the project, and decision making for the sustainable development of the eco-socio-economic coupling system in the plateau.