延安市退耕过程植被覆盖度变化及其影响因子分析

Analysis of Fractional Vegetation Coverage Changes and Its Influence Factors during Farmland Returned to Forest Period in Yan'an City

基于像元二分模型,利用2000—2012年的MODIS植被指数资料分析了延安市退耕还林过程中植被覆盖度的变化及影响因子。研究结果表明,2000—2012年延安市植被覆盖度呈极显著增长趋势,由43.4%增加到66.8%,增幅达53.9%。延安市各区县的植被覆盖度在波动中均呈现增长趋势,增幅较大的为子长县、延川县和延长县,分别为212.8%、134.2%和127.8%,增幅较小的为黄龙县和黄陵县,分别为28.9%和29.2%。植被覆盖状况总体呈现稳定的改善状态,改善极显著和显著的县区面积占研究区总面积的74.04%,改善不明显的县区面积占研究区总面积的23.91%,退化显著和极显著的县区面积仅占研究区总面积的2.05%。研究区高覆盖度植被和中覆盖度植被面积增加,低覆盖度植被面积减少,退耕初期由大到小表现为:低覆盖度植被面积、高覆盖度植被面积、中覆盖度植被面积,退耕后期由大到小分别为:高覆盖度植被面积、中覆盖度植被面积、低覆盖度植被面积,表明该区域植被覆盖度结构转好。植被覆盖度变化受6—8月份的降水量影响不大,相关性较低,但和累计退耕还林面积有较强的正相关性。说明退耕还林工程的实施增加了地面植被覆盖度,改善了植被覆盖的结构,植物应对环境变化的能力越来越强,降水量的年际变化对植被覆盖度造成的影响越来越小。

Based on the dimidiate pixel model, the changes of fractional vegetation coverage and its influence factors were analyzed by using the MODIS vegetation index in 2000--2012 during the period of returning farmland to forest in Yan＇an City. The results indicated that the fractional vegetation coverage in Yan＇an City was increased by 53.9% coverage in the districts and counties , which was changed from 43. of Yan＇an City was increasing 4% to 66. 8%. Fractional vegetation in fluctuation, and those of Zichang County, Yanchuan County and Yanchang County were increased greatly by 212. 8%, 134. 2% and 127.8% , and those of Huanglong County and Huangling County were increased by 28.9% and 29.2% , respectively. The fractional vegetation coverage was generally improved steadily and remarkably, and the area with remarkable improvements took 74.04% of the total research area, inconspicuous improvements took 23.91% and remarkable degenerations took 2.05%; the area with high coverage and moderate coverage in the research area were increased while the area with low coverage was decreased. In the early stage, the area of returning farmland to forest can be listed in descending order as follow： low coverage area, high coverage area, and moderate coverage area, while in the later stage the descending order are： high coverage area, moderate coverage area and low coverage area, which showed that the coverage structure in the research area was improving. The changes of coverage were influenced little by precipitation amount from June to August, and it had strong positive correlation with the accumulated conversion area of farmland to forest. Therefore, it can be concluded that the implementation of returning farmland to forest increased the vegetation coverage and improved the structure of fractional vegetation coverage. As plants are more and more capable of adjusting to the environmental changes, the interannual variations of precipitation have decreasing influences on the fractional vegetation coverage.