Driving factor analysis on the vegetation changes derived from the Landsat TM images in Beijing

The normalized difference vegetation index(NVDI),linearly correlated with density distribution of vegetation,is widely used for investigating macro-scale vegetation coverage and thus vegetation changes at short and long time scales.We calculated the vegetation coverage and NDVI greater than 0.1 by the Landsat TM satellite images on Sept.26,1987 and Sept.22,2009 for Beijing,China.The driving forces for vegetation changes were also analyzed including climate,geomorphology,human activities.In addition,the buffer analysis tool in the ARCGIS was used to derive the vegetation changes along the rivers as well as around the reservoirs and lakes.The quantitative analysis indicated that the land area of extremely low coverage,medium coverage and high coverage decreased by 5.15%,0.54% and 0.03% respectively,while the area of vegetation of low coverage and extremely high coverage increased by 5.71% and 0.01%,respectively from 1987 to 2009 for the whole region of Beijing.The statistical results in NDVI greater than 0.1 showed that the vegetation quality of the whole city had basically improved and improved area reached 919302.3 ha.The ratio of slightly,moderately and extremely improved area was 28.31%,41.33% and 30.36%,respectively.In addition,the degraded area was identified as 326931.12 ha.Considering NDVIs for different sub-regions of the city,the most noticeable vegetation degradation took place within the 5th ring and between the 5th ring and the 6th ring and the percentage of the area of degraded vegetation was 58.17% and 47.38%,respectively for these two 2 regions.Contrarily,the regions with minimum percentage of vegetation degradation included Taihang Mountain and Yanqing Basin and the vegetation degradation area was 13.35% and 17.02%,respectively.Furthermore,the degradation for the Taihang Mountain and Yangqing Basin was basically attributable to slightly moderate degradation.There was no significant correlation between vegetation changes and climate changes both for whole Beijing and its sub-regions.However,human activities were found to be the major driving forces for vegetation changes in Beijing including regional green ecological restoration projects for both mountains and plains),urban landscape and greening projects,agricultural structure amendment for plain regions,and eco-environment restoration efforts for rural regions.In addition,water bodies had a very important effect on the vegetation changes along the river and around the reservoirs and lakes.A massive construction of reservoirs in the mountain region resulted in the no flow phenomena for the rivers in plain regions,thus,water body effects on the vegetation occurred in the mountain area.NDVI and vegetation coverage changes were buffered by 0—50m,50—100m,100—150m,150—200m and 200—250m.We found that the buffer distance can be divided into two spatial levels for NDVI calculation.NDVIs buffered by 0—100m were enhanced over 0.1,but the NDVIs buffered by 100—250m increased by 0.0084—0.0089 from 1987 to 2009.We also found that the 0—250m buffer ranges also can be divided into two levels for vegetation coverage analysis.Compared with 1987,the ratio of vegetation coverage in 2009 was reduced for the buffer of 0—200m and increased for the buffer of 200—250m.Therefore,the changes of river water body caused by precipitation can influence riparian vegetation within 150 m from the average water level,especially within 100 m.